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Showing content with the highest reputation on 14/01/13 in all areas

  1. 7 points
    I'd offer a 'tour of the night sky with an experienced astronomer' in exchange for a packet of Jaffa Cakes
  2. 5 points
    Wow. What an idea!! I'm just off to post an add for all serious petrol heads. A sit in my vw polo for £300 or a quick run to sainsburys and back £400.
  3. 5 points
    Looks like a few mates got together for an imaging session , But only one of them knew how to Polar Align so they used his mount . . . . Steve.
  4. 4 points
    He he - its official! I am now in post as the new Observatory manager for the Scottish Dark Sky Observatory in the Galloway Dark Sky Park. I started on Monday and am getting to know the place and sort out how to use the equipment etc. Its my job to make sure this place is a success, to get bums on seats, to host and give talks and to do outreach to local schools etc. Its a fabulous facility in gorgeous surroundings - you can find out about it at www.scottishdarkskyobservatory.co.uk but heres a few piccies to whet the appetite.......... You are all cordially invited to visit...... Of course, I'll be posting more as I get an ID for the SDSO but I just wanted to share my exciting news.. Best regards Robert Ince
  5. 4 points
    Get the wife out there to clear it away......
  6. 3 points
    OK, let's be clear that I couldn't see a witch's head. But, but... snuggled down in the lounger in my arctic gear I spent a nice long time looking to the west of Rigel and I'm confident that I was picking up that suspicious milkiness and reduced star count that whisper 'nebulosity.' This was in our 8x42 Leica bins. The Witch is utterly enormous and totally dwarfs M42 so I reckon I had a whiff of it. Nice. Olly
  7. 3 points
    Screwdriver nerd alert !!!!!!! I'd say the small x-heads that Tal use are more like the 'Frearson' type, rather than the phillips. As I mentioned previously, a phillips will work though. Different sizes of slot headed types. Anything different means they've probably been added by a previous user. Andy.
  8. 3 points
    Cant you put the boys in the boot - surely the scope takes priority
  9. 3 points
  10. 3 points
    Oh the weather outside is frightful But the fire will be so delightful And since we've no place to go Let It Blow! Let It Blow! Let It BLOW! It doesn't show signs of stopping And I've bought some corn for popping The lights are turned way down low LET IT BLOW! LET IT BLOW! LET IT BLOW! Yes it's snowing.
  11. 2 points
    Hiya and welcome - the only limit is how much cash you have. Mine are listed in my signature at the bottom of this post. Its a good place to look and see what other people are using.
  12. 2 points
    he's cut his price down to £350 or best offer [i have emailed him offering a pack of jaffa cakes and a hot mug of tea hope he accepts it] although his telephone number was on it earlier seems to have been removed
  13. 2 points
    drilled two more sets of holes in the mirror box on Sunday, allowing variable alt bearing positions and as a result the scope now balances with no movement even when changing at horizon level from a BGO to a 26mm Nagler. Also tonight I tried out the scope to have a final check on the balance and focus points in action. it's all go for final prep and finish! I'll be going with the walnut colour and possibly a black stain with two clear overcoats inside the mirror box.
  14. 2 points
    I know where the 'tour' button is on the handset. Doesn't that count
  15. 2 points
    It does make me wonder about the success of the human race when common sense seems to allude so many. Sent from my GT-I9100 using Tapatalk 2
  16. 2 points
    Had a touch of advice from Olly, and have a small update to this....
  17. 2 points
    Yesterday I waited and waited but just had milky cloud all day but I did catch a plane transit 12th Jan 2013 Plane transit by Alexandra's Astronomy, on Flickr Today, despite the forecast was the same again I was just going to pack up when I had a sudden clearance and brightening of the sky just long enough to capture a quick Hydrogen alpha and Calcium K with the PST's and DMK41 2013-01-13 by Alexandra's Astronomy, on Flickr and black and white for those who prefer Calcium K 2013-01-13 12-11-07 by Alexandra's Astronomy, on Flickr Hydrogen alpha 2013-01-13 12-06-19 by Alexandra's Astronomy, on Flickr It was definitely a miracle! Regards Alexandra
  18. 2 points
    Globs are what gave me aperture fever I find that the 8SE gives nice views of M13,92 and friends, but bigger is better Also, given their new prices, the CPC is a much more stable platform than the SE for the extra. Putting your Dob on a trolley could save you a lot of money though!
  19. 2 points
    Hi Chris. If you need any further help/info regarding the setting up/general use of the scope etc, don't be afraid to ask. No such thing as a silly question, here on SGL !!
  20. 2 points
    Just got to flog the 4se now or the wife will most definitely kill me Its starting to look like the FLO showroom in my house! Oooh now that needs a photo doesn't it
  21. 2 points
    How about a roof rack and a roof box and put the scope in that ?
  22. 2 points
    I wonder if people buying them from the states is one of the reasons why they are pulling out of the UK? More likely that they just don't sell enough of them to justify having a presence here, maybe they would sell more if they sold them at a more competitive price compared to other countries
  23. 2 points
    I bet Macdonalds would sponsor it.
  24. 2 points
    Just had my first stargazing session with my xmas pressie bargain scope. It was hosted by Sunderland Astro Soc whos members were very welcoming, helpful and knowledgable. There were some interesting talks too, before proceeding outside to set up my new toy. At first I felt a bit inadequate next to all the super massive equipment others had. This soon changed however, when the skies were kind and the stars came out to play. I had an excellent view of Jupiter and 4 moons, the nebula in Orion and the seven sisters. And even when I looked through other peoples super-duper scopes, they really couldn't see any better than I could. I am really pleased with my telescope, thrilled to have seen some amazing things and glad to have met some really lovely, like minded people. I think I may be hooked!
  25. 2 points
    ahhh.... i was getting all besides myself....
  26. 2 points
    Last week I had clear nights nearly every night for a change! So this is my first proper image through the new equipment, Vixen ED80 and HEQ5. Exposure was 8.5 Hours (51 x 10 minute subs) over 4 nights with the QHY10 at -15C, fully calibrated. Hope you like it! Erik
  27. 2 points
    The evolution is now complete... For now
  28. 2 points
    I guess good seeing conditions override problems with LP, and assuming collimation is spot on, you'll also find that on average, Jupiter doesn't really need much more than about 140x - 160x. My 4" frac can easily make out the north and south equatorial belts, great red spot, the equatorial band and the north and south polar regions. If you stick with him, gradually, after 15 minutes or so, Jupiter reveals even more subtler markings especially in the north and south temperate belts and larger markings in the north and south tropical zones. The entire image in the eyepiece is really no bigger than a large pea in the palm of your hand. Just to add more food for thought, try to view Jupiter as close to the zenith as possible and bear in mind that as a general rule of thumb the brightness of an object will decline as you up the magnification. If I up the mag twofold, say, I'm reducing the image brightness by a factor of four. If I keep on doing this eventually details just disappear. On the other hand, increasing the mag does make detail more apparent, so, as you can appreciate, we're now at a trade-off: will increasing magnification gain more detail even though I'm making the object fainter? I've found that playing around with this trade-off - dependent on the evening's seeing (as said, I've found that LP doesn't really affect planets) - does make a difference. Even as little as 1mm increase or decrease in the mag - about 10% to 15% difference of magnification - can be quite surprising. You'll probably find that on a decent night your sweet spot is around 140x to 160x on viewing Jupiter and you will probably only be able to push 200x plus on the most excellent of nights. You've got a wonderful 8" telescope reaching out across the universe some 675,000,000 kilometers and I'm sure - on another better evening - you will be able to see the Great Red Spot, those delicate reddish-brown belts, a darker, greyer hue to the Polar regions, and so on. You'll be able to trace the movement of the Jovian moons and observe their play of shadows over Jupiter in times of transit or of their eclipses by Jupiter's own shadow. From time to time, if you want to enhance that colour of the giant maybe a light blue filter will work nicely, or a Wratten 11 or 12. If you can, try to sit with Jupiter for a peaceful twenty or thirty minutes or so on your next observation session and I'm certain they'll be moments of great clarity and seeing. I've been following Jupiter almost every night since late July and you do notice that with practice more detail can be tweaked from the planet. Stay with it and as the weeks go by you will notice quite a difference.
  29. 1 point
    For anyone interested, revelation now sell an adapter similar to the Antares and Orion self-centering eyepiece adapters, only it costs a lot less at £19.99 http://www.telescopehouse.com/acatalog/Revelation-Self-Center-adapter-1.25-.html
  30. 1 point
    Hi to all, my first time on SGL, hopefully not the last. As my nickname suggests, my name is Mare and my surname is Prah (which means dust in english, maybe a stellar kind of dust, you never know:)). Dind't know how to begin with an introduction of myself, and hey, telling my name first is one way of doing it:) I come from Slovenia, a small country caught in between of Italy, Austria, Hungary and Croatia (hopefully it's not a problem not being from the UK) Please excuse me for my english is not perfect, as I think your Slovene isn't either:P Been looking trough SGL for quite a while now and I find it most helpfull for the beginers like myself. I began with amateur astronomy a year ago, from that time on, a day has not passed when I wouldn't wanna look above. I am also a member of a few Slovenian forums, but ofcourse also always opened for new suggestions on other forums etc. CS, Mare
  31. 1 point
    Just thought I'd say a quick hello. I've been finding my way around the forum for the past few days and I have to say what a great resource. I have found some really great advice on here already. I've always been fascinated by the night sky but never owned a telescope or really learned what it is I'm looking at except for the obvious stuff like Orion and The Plough. That feeling of awe you get from looking up on a clear night is what it's all about for me. The realization that you are a miniscule cog in a infinate machine and all the things you might be worrying about actually make absolutely no difference in the grand scheme of things. I'm still looking for my weapon of chioce and I'm sure I will be asking for help and advice at some point, but in the meantime I'm looking forward to the next clear night to just get out there with my old binoculars and start learning. Martin
  32. 1 point
    Imaged on many different nights over the last two months or so. Processing has been really tough and ended up dumping about 6hrs of Lum data that was giving me strange results, I used only 10hrs of 16 collected (I will return to this, one day), Star colour is a little bit strange on some of the smaller stars if you look closely (might not be apparent in the scaled down image). I keep getting pink and purple stars no matter what way I combine them (RGB) or process them. One other thing I noticed was that some of the blue stars were already clipped even before I began processing the RGB, not sure why, the sub lenght was 10mins, which seems to be a pretty normal sub exposure for RGB. I am quite pleased with the final result even though the lum is not as obvious as I hoped it would be (that is probably my processing) One thing is for sure, I need to get a reducer for the scope. Details: total 28hrs 10mins Ha - 38 x 1200s L - 40 x 900s R - 12 x 600s B - 12 x 600s G - 9 x 600s (discarded 3 due to clouds) Thanks for looking and any advice or comments are always welcome. Johnny
  33. 1 point
    I personally love posts like this! Educational Amusing Factual Relevant And it makes me realise I'm not the only wally in here
  34. 1 point
    The Orion Atlas is an NEQ6 with a different paint job. The Orion Sirius is the same as the HEQ5. They are both made by Synta Corp. Orion (Synta) is mainly a US brand and SkyWatcher the equivalent in Europe. Celestron are also now a Synta brand and some there is some significant cross over to their range as well. Just to confuse matters there is an Orion Optics UK, who mainly specialise in Optical Tube Assemblies OTA = the telescope bit. They are very good, but are a step up in price from Synta.
  35. 1 point
    I would agree that the 8se is a good choice for what you want it's got enough apparture to be pretty good on globular clusters it's pretty good on planets and very good on the moon. it has a tour function and is fairly portable it's a lot more portable than the cpc series not as good as your dob visually but at least you don't have to be afraid to put it on a wet lawn
  36. 1 point
    As is said the best scope is the one you use the most. It sounds like you may be best avoiding an EQ mount and may be better getting an alt/az if you want quick set up, if you want goto the 8SE maybe the one. I have never used one so I can not tell you about the set up time. The other alternative maybe to put one on a Skytee alt/az mount..
  37. 1 point
    just had a peek at that amazon 10% deal - this might make a cheapish good guide scope: http://www.amazon.co.uk/Celestron-21035-Travel-Scope-Telescope/dp/B001TI9Y2M/ref=sr_1_2?m=A3P5ROKL5A1OLE&s=electronics&ie=UTF8&qid=1358174024&sr=1-2
  38. 1 point
    Evening fellow space peeps I had my first go at trying to find the orion nebula through my skywatcher 150p. After about 20 mins I managed to find something which looked like 3 stars close together in a triangle covered by what looked like a finger print mark or smudge. Also within the eye piece i saw 3 stars in a near straight line, right next to it. Was it the orion nebula or just a random assortment of stars? Many thanks
  39. 1 point
    I went to watch this at my folks. I thought it was an excellent interview and highlighted how Sir Patrick was true to himself and never tried to be anything different. His acceptance of things that happen in life and reluctance to moan and just get on with it highlighted how much he cherished life.
  40. 1 point
    You'll have no problems nailing it with your 10" dob, I've seen it regularily enough and from typical surburban light polluted skies, the hardest aspect I find is having the seeing steady enough as the F star has a habit of blurring with others if the image jumps up and down quite a bit. Go relatively high powered (ie 9mm) and you'll have this 'in the bag'. I found this site quite useful: http://www.laughton.com/paul/rfo/trap/trap.html
  41. 1 point
    hi welcome to SGL from peter in bedfordshire
  42. 1 point
    Just picked up an EQ5 syncscan ... Now I just have to read the manual
  43. 1 point
    Hi and welcome to the forum. In order to find some of the great objects up in the night sky or to identify objects you discover along the way, you might want to consider downloading some free planetarium software called "Stellarium" which you can read about here. LOts of great features to keep you entertained, especially when the clouds come in and it can be configured from your precise viewing location to help create an accurate simulated sky identical to the one that can see with your scope. Clear skies for now and hope you enjoy the forum. James
  44. 1 point
  45. 1 point
  46. 1 point
    Many people initially confuse Go To with guiding. Go To simply finds the target. The mount is then motorized to unwind the rotation of the earth and keep the object centred and still on your chip. This it will do with moderate but imperfect success. You therefore have a parallel scope and camera taking successive short exposures (seconds long) and if a chosen star on the guide camera's chip starts to drift the mount will be given a correction to put it back where it started. The tracking precision that can be achieved (when it works!) is extraordinary, in the order of an average error of maybe a third of a pixel. As long as errors remain less than a pixel you'll have an effectively perfect result. I stress - that's when you get it working!!! As for choice of scope, you first need to decide what you want to photograph. Is a telephoto better than a fisheye? No, they do different jobs. If your target is small you need a long focal length. If it is large you need a short one or it won't fit on the chip. But if you want to use a long focal length this long FL will 'magnify' guiding errors as well as the object so you need your mount and guider to work better. If you were to get serious about imaging then, assuming your mount is delivering the accuracy needed, you would upgrade from a DSLR to an astronomical CCD camera. This would give a vastly greater improvement than would upgrading the optics and sticking with a DSLR. Priority order is Mount, camera, optics. (In my view.) Olly http://ollypenrice.s...39556&k=FGgG233
  47. 1 point
    Mate, just look after yourself and be careful....... "We wants it, we needs it, Must have my precious"....(excerpt from The Lord of the Ring Nebula).
  48. 1 point
    Well done. Sky is fantastic tonight, been out there 90 mins. Cold but happy. Sketched Orion. Saw M31 through my binoculars. Below that was an impressive bright blue star, think it's in Pegasus.
  49. 1 point
  50. 1 point
    absolute magnitude - (n.) The magnitude that a star would appear to have if it were at a distance of ten parsecs from us. accretion disk - (n.) A rotating disk of gas surrounding a compact object (such as a neutron star or black hole), formed by material falling inward. albedo - (n.) Fraction of incident electromagnetic radiation reflected by a body such as a planet, star, or cloud. alpha particle - (n.) A cluster of two protons and two neutrons; a helium nucleus. Andromeda galaxy - (n.) The large spiral galaxy located some 700,000 parsecs from the sun; the most distant object visible to the unaided eye. Angstrom (A) - (n.) Unit of length convenient for measuring wavelengths of electromagnetic radiation: 1 A = 10-10m. angular momentum - (n.) A measure of the mass, radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object in circular orbit, the angular momentum is equal to the mass of the object times its distance from the center of the orbit times its orbital speed. antimatter - (n.) A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and certain other properties to a corresponding particle (proton, neutron, etc.) of the same mass of the ordinary type of matter from which the solar system is made. Particles of antimatter are known to exist, but it is not known why matter is dominant in this region of the universe or whether regions exist in which antimatter is common. apastron - (n.) For an orbit around a star, the farthest point from that star. aperture - (n.) In the case of a telescope, an area open to the Universe; signifies either the maximum physical or the effective capture cross section of a telescope or radio antenna; often stated in terms of the diameter or an equivalent diameter if the aperture is noncircular. aphelion - (n.) The point in the orbit of a solar-system object where it is farthest from the sun. Apollo asteroid - (n.) An asteroid whose orbit brings it closer than 1 astronomical unit to the sun. arcsecond (arcsec) - (n.) Measurement of angular separation: a 1-inch stick would subtend an angle of 1 arcsec at a viewer's eye at a distance of about 6.5 miles. Arecibo - (n.) Short name for the National Astronomy and Ionospheric Center (NAIC) at Arecibo, Puerto Rico; often refers only to the NAIC 1000-ft (305-m) zenith (?°) antenna, the world's largest radio astronomy collector. asteroid - (n.) One of thousands of small planetlike bodies orbiting between Mars and Jupiter having diameters from a fraction of a kilometer to about 1,000 km. astrobiology - (n.) The study of life throughout the Universe, its origin, evolution, ecology and destiny. astrology - (n.) A non-scientific system based on superstition, that purports to explain or predict human actions by study of celestial positions. astrometry - (n.) Branch of astronomy that focuses on measurements, especially those relating to positions and movements. astronomical unit (AU) - (n.) Mean Earth-Sun distance: 1 AU = 1.496x10" m = 8.31 light minutes; a convenient unit for measuring distances between planets and their stars. atmosphere : Gaseous mass enveloping a planet or star. atmosphere - (n.) Gaseous mass enveloping a planet or star. atom - (n.) The smallest possible unit of a chemical element. When an atom is subdivided the parts no longer have properties of any chemical element. An atom consists of a nucleus with orbiting electrons. atom epoch - (n.) Fourth epoch in the history of the Universe, lasting from about 100 sec to 106 yr, in which matter came to dominate radiation as the principal constituent of the Universe. atomic mass unit - (n.) Convenient unit for measuring the mass of an atom or molecule: 1 atomic mass unit is defined as I /12 the atomic mass of the most abundant carbon isotope, 12C. A=12 for 12C. atomic nucleus - (n.) Concentrated, positively charged matter at the center of an atom; composed of protons and neutrons. atomic number - (n.) The number of protons in the nucleus of an element. It is the atomic number that defines the identity of an element. Z=6 for 12C. atomic theory - (n.) A model that offers a logical explanation for the law of multiple proportions and the law of constant composition by stating that all elements are composed of atoms, all atoms of a given element are identical, but the atoms of one element differ from the atoms of any other element; that atoms of different elements can combine to form compounds and a chemical reaction involves a change not in the atoms themselves, but in the way atoms are combined to form compounds. atomic weight - (n.) The number of protons and neutrons in an atom, averaged over the abundances of the different isotopes. AU - (n.) See astronomical unit. aurora - (n.) Glowing lights visible in the sky, resulting from processes in the earth's upper atmosphere. azimuth - (n.) The angular distance, around the horizon from the northern direction, usually expressed in angular measure from 0° for an object in the northern direction, to 180° for an object in the southern direction, around to 360°. Together, the altitude and the azimuth define the direction to an object. Big Bang - (n.) The initial singularity that started the space and time of our Universe, now thought to have occurred 13 to 15 billion years ago. big bang theory - (n.) A cosmological model, in which the universe was once compressed to infinite density and has been expanding ever since. Originally a term used by unbelieving scoffers, now as widely accepted as the model itself. billion - (n.) One-thousand million, 109 , in the USA only; elsewhere a million million, or 1012. binary system - (n.) Two neighboring stars that revolve around their common center of gravity; the fainter of the two stars is called the companion. binding energy - (n.) Energy derived from the conversion of mass to energy when neutrons and protons are combined to form nuclei. blackbody - (n.) Body capable of absorbing energy of all wavelengths falling on it; it is also capable of radiating all frequencies in a particular ratio to its absorbing properties. The value of the ratio depends only on the temperature of the body. black dwarf - (n.) A non-radiating ball of gas that results either when a white dwarf radiates all its energy or when gas contracts gravitationally but contains too little mass to begin nuclear fusion. black hole - (n.) An object that has collapsed under its own gravitation to such a small radius that its gravitational force traps photons of light. Bode's law - (n.) The physical laws describing the properties of a numerical scheme that roughly gives the radii of the orbits of the seven innermost planets and the radius of the asteroid belt. More numerology than science. Bohr atom - (n.) Nils Bohr's model of the hydrogen atom, in which the energy levels are depicted as concentric circles of radii that increase as (level number)2. bolide - (n.) An asteroid or comet that crashes onto the Earth or another body in the Solar System, generating a huge fire-ball. bolometric magnitude - (n.) The magnitude of a celestial object corrected to take account of the radiation in parts of the spectrum other than the visible. brown dwarf - (n.) An object substantially (~13 x) larger than Jupiter but with a mass no more than 40 percent that of the Sun. These objects are not big enough for gravitational collapse to heat them to the point that nuclear reactions can be triggered. Brown dwarfs may be very common in the universe and could even have planets in a habitable zone. burster - (n.) A sporadic source of intense X rays, probably consisting of a neutron star onto which new matter falls at irregular intervals. carbon-14 (14C) - (n.) A radioactive isotope of carbon produced in the upper atmosphere and present in living plants and animals that can be used in carbon-14 dating because it decays to nitrogen (14N) and a beta ray with a half-life of about 5,730 years. carbonaceous chondrite - (n.) A meteorite containing controls, with a high abundance of carbon and other volatile elements. carbon cycle - (n.) A chain of nuclear reactions, involving carbon at its intermediate stages, that transforms four hydrogen atoms into one helium atom with a resulting release in energy. The carbon cycle is only important in stars hotter than the sun. cassegrain (telescope or focus) - (n.) Some telescopes, particularly at microwave and shorter wavelengths, have a second reflector near the focus of the larger, primary mirror. This translates the focal point to a position near the apex of the primary where it is more accessible, and where practical antenna feeds are less responsive to radiations arriving from very wide angles relative to the nominal pointing direction. celestial sphere - (n.) The visible, seemingly spherical surface that appears to surround Earth and to be centered at the observer. Celsius scale - (n.) A temperature scale on which water freezes at 0 ° and boils at 100°. Cepheid variable - (n.) A type of supergiant star that oscillates in brightness in a manner similar to the star 8 Cephei. The periods of Cepheid variables, which are between 1 and 100 days, are linked to the absolute magnitude of the stars by known relationships; this allows the distances to Cepheids to be found. CETI - (n.) Acronym for communication with extraterrestrial intelligence. Sometimes pronounced with a long e and short i, to distinguish it from SETI which is favored with two short vowels. chain reaction - (n.) A self-sustaining change in which one or more products of one event cause one or more new events. chaos - (n.) Hypothetical first epoch in the history of the Universe, lasting 10" sec: a period about which we cannot yet even speculate. Charles' law - (n.) For a given mass of gas at constant pressure, the volume varies directly with the temperature (on the absolute scale). charm - (n.) An arbitrary name that corresponds to a property that distinguishes certain elementary particles, including types of quarks, from each other. chemically peculiar stars - (n.) Stars manifesting anomalies in the relative abundances of elements, which may arise from mechanical rather than nuclear effects; so-called manganese stars, for example, show a great overabundance of manganese and gallium, usually accompanied by excess mercury. chondrite - (n.) A type of stony meteorite that contains numerous small spherules of silicate (silica, silicon dioxide) minerals. A subset of this type of meteorites, the carbonaceous chondrites, contains several per cent organic carbon. closed universe - (n.) A possible state of the universe. In this state, the expansion of the universe will eventually be reversed; it is characterized by positive curvature, being finite in extent but having no boundaries. Recent observations indicate that this is unlikely to be the true state of our universe. cluster (astronomical) - (n.) Group of stars numbering from a few to hundreds of thousands of stars. Galactic clusters, sometimes called open clusters, contain up to a few hundred members and occur rather close to the plane of the Galaxy. Globular clusters contain tens of thousands of stars distributed about their center in a spherical manner and are found far from the plane of the Galaxy as well as in it toward the center of the Galaxy. CNO cycle - (n.) A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars on the upper main sequence. Same as carbon cycle. coherent radiation - (n.) Radiation in which the phases of waves at different locations in a cross-section of radiation have a definite relation to each other; in non-coherent radiation, the phases are random. Only coherent radiation shows interference. color - (n.) (a.) Of an object, a visual property that depends on wavelength; ( an arbitrary name assigned to a property that distinguishes three kinds of quarks. color index - (n.) The difference B-V between the blue ( and visual (V) magnitudes of a star. If B is less than V (that is, the star is brighter in blue than in visual light), then the star has a negative color index, and is a relatively hot star. If B is greater than V, the color index is positive, and the star is relatively cool. color-magnitude diagram - (n.) A Hertzsprung-Russell diagram in which the temperature on the horizontal axis is expressed in terms of color index. coma - (n.) The extended, glowing region that surrounds the nucleus of a comet. comet - (n.) An interplanetary body, composed of loosely bound rocky and icy material, that forms a glowing head (coma) and extended tail when it enters the inner solar system. comparison spectrum - (n.) A spectrum of known elements on earth usually photographed on the same photographic plate as a stellar spectrum in order to provide a known set of wavelengths or zero Doppler shift. conservation law - (n.) A statement that the total amount of some property (angular momentum, energy, etc.) of a body or set of bodies does not change. constellation - (n.) A prominent pattern of bright stars, historically associated with mythological figures. In modern usage, each constellation incorporates a precisely defined region of the sky. continuously habitable zone (CHZ) - (n.) Region around a star in which a planet can maintain appropriate conditions for the existence of life (including the retention of a significant amount of liquid water) for a period sufficient to allow the emergence of life. continuous spectrum - (n.) A spectrum with radiation at all wavelengths but with neither absorption nor emission lines. continuum - (n.) The continuous spectrum that we would measure from a body if no spectral lines were present. corona - (n.) The very hot, extended outer atmosphere of the sun and other cool main-sequence stars. The high temperature in the corona (l-2x 106 K) is probably caused by the dissipation of mechanical energy from the convective zone just below the photosphere. coronagraph - (n.) A type of telescope with which the corona can be seen at times other than that of a total solar eclipse. cosmic background radiation - (n.) The primordial radiation field that fills the universe. It was created in the form of gamma rays at the time of the big bang, but has since cooled so that today its temperature is 3 K and its peak wavelength is near 1.1 millimeters (in the microwave portion of the spectrum). Also known as the 3-degree background radiation. Also called cosmic microwave background radiation, CMBR. cosmic ray - (n.) A rapidly moving atomic nucleus from space. Some cosmic rays are produced in the sun, whereas others come from interstellar space and probably originate in supernova explosions. cosmogony - (n.) The study of the origin of the universe, usually applied in particular to the origin of the solar system. cosmological constant - (n.) A term added to the field equations by Einstein in order to allow solutions in which the universe was static; that is, neither expanding nor contracting. Although the need for the term disappeared when it was discovered that the universe is expanding, the cosmological constant is retained in the field equations by modern cosmologists, but is usually assigned the value zero. cosmological principle - (n.) The postulate, put forth by most cosmologists, that the universe is both homogeneous and isotropic; it is sometimes stated that the universe looks the same to all observers everywhere. cosmological redshift - (n.) A Doppler shift toward longer wavelengths that is caused by a galaxy's motion of recession, which in turn is caused by the expansion of the universe. cosmology - (n.) The study of the universe as a whole. critical mass - (n.) The mass of an isotope above which a self-sustaining chain reaction can occur. daughter isotopes - (n.) Isotopes formed by the radioactive decay of another isotope. dB - (n.) decibel; A unit of power ratio; the gain or loss in power in dB is equal to 10 times the logarithm of the power ratio. 1dB is approximately the smallest change in volume of sound which a normal ear can detect. decay constant - (n.) For an atom that undergoes radioactive decay, the decay constant is the proportionality factor between the time rate of decay and the total number of atoms present; it is the inverse of the mean lifetime of an atom. deceleration parameter (q0) - (n.) A particular measure of the rate at which the expansion of the universe is slowing down. declination - (n.) Celestial latitude, measured in degrees north or south of the celestial equator. degenerate gas - (n.) A gas in which either free electrons or free neutrons are as densely spaced as allowed by laws of quantum mechanics. Such a gas has extraordinarily high density, and its pressure is not dependent on temperature, as it is in an ordinary gas. Degenerate electron gas provides the pressure that supports white dwarfs against collapse, and degenerate neutron gas similarly supports neutron stars. density-wave theory - (n.) The explanation of spiral structure of galaxies as the effect of a wave of compression that rotates around the center of the galaxy and causes the formation of stars in the compressed region. detection - (n.) In electromagnetics, an operation converting the vector electromagnetic wave to a scalar time series proportional to either the amplitude or the power of the wave, with or without an accompanying angular time series. A crucial aspect of detection is: the signal-to-noise ratio after detection is the square of the signal-to-noise ratio before detection. determinism - (n.) The doctrine according to which like causes always produce like effects and, conversely, events are entirely explainable by their antecedent causes. deuterium - (n.) An isotope of hydrogen with a proton and a neutron in the nucleus (mass of 2 amu). differential gravitational force - (n.) A gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation feel a stronger force than the portions farther away. Such a force, also known as a tidal force, acts to deform or disrupt the object, and is responsible for many phenomena, ranging from synchronous rotation of moons or double stars to planetary ring systems to the disruption of galaxies in clusters. differentiation - (n.) The sinking of relatively heavy elements into the core of a planet or other body. Differentiation can occur only in fluid bodies, so any planet that has undergone this process must once have been at least partially molten. Also, a process whereby a stem cell acquires the characteristic features of a given cell type. diffraction grating - (n.) A very closely ruled series of lines that, through their diffraction of light, provide a spectrum of radiation that falls on it. disk - (n.) (a.) Of a galaxy, the disk-like flat portion, as opposed to the nucleus or the halo; ( Of a star or planet, the two-dimensional projection of its surface. distance modulus - (n.) The difference m-M between the apparent and absolute magnitudes for a given star. This difference, which must be corrected for the effects of interstellar extinction, is a direct measure of the distance to the star. D lines - (n.) A pair of lines from sodium that appear in the yellow part of the spectrum. Doppler effect - (n.) The shift in wavelength of light that is caused by relative motion between the source of light and the observer. The Doppler shift, Deltagamma, is defined as the difference between the observed and rest (laboratory) wavelengths for a given spectral line. Doppler shift - (n.) A change in frequency resulting from relative motion along the line between the transmitter and the receiver. If the source and the receiver are approaching each other, the frequency received is higher than the frequency transmitted by a factor, depending on the actual relative velocity. Knowledge of this shift is used to determine the relative velocity. double bond - (n.) Two shared pairs of electrons. Drake equation - (n.) An approach to estimating some of the factors in guessing at the number of communicating civilizations in the galaxy. drifting (signal) - (n.) Refers to a signal with an apparent time rate of change in its typical frequency. All signals drift to some extent. In a SETI system, the dominant drift should be largely the result of only the time rate of change in the Doppler shift (q.v.). dwarf elliptical galaxy - (n.) A member of a class of small spheroidal galaxies, similar to standard elliptical galaxies except for their small size and low luminosity. Dwarf galaxies are probably the most common in the universe, but cannot be detected at distances beyond the Local Group of galaxies. dwarf nova - (n.) A close binary-star system containing a white dwarf; material from the companion star falls onto the other at sporadic intervals, creating brief nuclear outbursts. dynamo - (n.) A device that generates electricity through the effect of motion in the presence of a magnetic field. The solar dynamo explains sunspots and the solar activity cycle. earthshine - (n.) Sunlight illuminating the moon after having been reflected by the earth. eccentricity - (n.) A measure of the flatness of an ellipse, defined as half the distance between the foci divided by the semi-major axis. eclipse - (n.) The hiding of one celestial body by another. The hiding of a star by a body of larger angular size is usually called an occultation. ecliptic - (n.) The plane of the earth's orbit about the sun, which is approximately the plane of the solar system as a whole. The apparent path of the sun across the sky is the projection of the ecliptic onto the celestial sphere. electric field - (n.) A force field set up by an electric charge. electromagnetic force - (n.) The force created by the interaction of electric and magnetic fields. The electromagnetic force can be either attractive or repulsive, and is important in countless situations in astrophysics. electromagnetic radiation (wave, spectrum) - (n.) Energy involving electric fields and magnetic fields oscillating in phase at right angles to each other, propagated in a direction at right angles to both fields with a velocity in free space equal to c (approximately 300,000 km/sec, or 186,000 miles/sec), a universal constant. electron - (n.) A tiny (1/1830 the mass of a proton), negatively charged particle that orbits the nucleus of an atom. The charge is equal and opposite to that of a proton in the nucleus, and in a normal atom the number of electrons and protons is equal, so that the overall electrical charge is zero. It is the electrons that emit and absorb electromagnetic radiation, by making transitions between fixed energy levels. electron carrier - (n.) In a chain of chemical reactions, molecules that accept electrons from an electron donor and pass these to an electron acceptor. electron configuration - (n.) The arrangement of an atom's electrons in space. element - (n.) A fundamental substance in which all atoms have the same number of protons. elementary particle - (n.) Any of a number of sub-atomic particles. ellipse - (n.) A geometrical shape such that the sum of the distances from any point on it to two fixed points called foci is constant. In any bound system where two objects orbit a common center of mass, their orbits are ellipses, with the center of mass at one focus. elliptical galaxy - (n.) One of a class of galaxies characterized by smooth spheroidal forms, few young stars, and little interstellar matter. emission line - (n.) A wavelength at which radiation is emitted, creating a bright line in the spectrum. emission nebula - (n.) A cloud of interstellar gas that glows by the light of emission lines. The source of excitation that causes the gas to emit may be radiation from a nearby star, or heating by any of a variety of mechanisms. energy - (n.) The ability to do work. Energy can be in either kinetic form, when it is a measure of the motion of an object, or potential form, when it is stored but capable of being released into kinetic form. energy levels - (n.) The specific, quantized energy levels that an electron may have in an atom. enrichment (isotope) - (n.) The process by which the proportion of one isotope of an element is increased relative to the others. entropy - (n.) Tendency of systems to become more disordered (and thus more uniform) over time; also a measure of disorder; in thermodynamics, a measure of the amount of heat energy in a closed system that is not available to do work. equator - (n.) (a) Of the earth, a great circle on the earth, midway between the poles; ( Celestial, the projection of the earth's equator onto the celestial sphere. equatorial coordinates - (n.) The astronomical coordinate system in which positions are measured with respect to the celestial equator (in the north-south direction) and with respect to a fixed direction (in the east-west direction). The coordinates used are declination (north- south, in units of angle) and right ascension (east-west, in units of time). equinox - (n.) An intersection of the ecliptic and the celestial equator. The center of the sun is geometrically above and below the horizon for equal lengths of time on the two days of the year when the sun passes the equinoxes; if the sun were a point and atmospheric refraction were absent, then day and night would be of equal length on those days. eras (geologic) - (n.) All of Earth's history since the appearance of the first life forms is divided roughly into four eras: Precambrian, from 3.5 billion to 570 million years ago; Paleozoic, from 570 to 225 million; Mesozoic, from 225 to 65 million; and Cenozoic, from 65 million to the present. The last two eras are broken down into the following periods: the Mesozoic into Triassic) Jurassic, and Cretaceous; the Cenozoic into Tertiary and Quaternary. erg - (n.) A unit of energy in the metric system, corresponding to the work done by a force of one dyne (the force that is required to accelerate one gram by one cm/sec2) producing a displacement of one centimeter. ergosphere - (n.) A region surrounding a rotating black hole (or other system satisfying Kerr's solution) from which work can be extracted. escape velocity - (n.) The velocity required for an object to escape the gravitational field of a body such as a planet .In a more technical sense, the escape velocity is the velocity at which the kinetic energy of the object equals its gravitational potential energy; if the object moves any faster, its kinetic energy exceeds its potential energy, and the object can escape the gravitational field. ETI - (n.) Extraterrestrial intelligence; also used to signify extraterrestrial intelligent species. Euclidean space - (n.) A space with zero curvature; a space where the sum of the angles of a triangle is 180°. event horizon - (n.) The "surface" of a black hole; the boundary of the region from within which no light can escape. exobiology - (n.) The study of life as it might occur elsewhere than on earth. Also, the study of the origin of life, at any location. feed (antenna feed, line feed) - (n.) in a reflecting antenna system, the device that converts a guided (by wire, cable, or other wave guide) electromagnetic wave intc? an electromagnetic radiation field, and vice versa, when reciprocity theorem holds as it so often does. Commonly, feeds are some form of horn antenna, but they may be dipole arrays or their. filament - (n.) (a) A feature of the solar surface seen in Ha as a thin, dark wavy line. A filament is a prominence projected on the solar disk. ( In microbiology, a collective term for the cylindrical external sheath and cellular internal trichome of a filamentous prokaryote. filtergram - (n.) A photograph taken through a filter that passes only a very narrow band of wavelengths; usually applied to solar photographs. fireball - (n.) An exceptionally bright meteor. ~ See Also: bolide. first law of thermodynamics - (n.) Energy is neither created nor destroyed. fission, nuclear - (n.) The splitting of an atomic nucleus. flare - (n.) A rapid eruption of material from the surface of the sun or other star. flux - (n.) The amount of something (such as energy) passing through a surface per unit time. focal length - (n.) The distance from a lens or mirror to the point at which rays from an object at infinity are focused. force - (n.) in physics, something that can or does cause a change of momentum, measured by the rate of change of momentum with time. force field - (n.) A way of describing phenomena that result from action at a distance, that is, even though objects are not touching. Fraunhofer lines - (n.) The absorption lines of a solar or other stellar spectrum. frequency - (n.) The rate (in units of Hertz, or cycles per second) at which electromagnetic waves pass a fixed point. The frequency, usually designated ƒ, is related to the wavelength ? and the speed of light c by ƒ = c/?. fusion - (n.) The amalgamation of nuclei into heavier nuclei. gain stability - (n.) crudely defined by AG/G = g(t)', the smaller this quantity over the relevant time interval, the less the gain instability or the greater the gain stability. Gain (amplification) of analog signaling systems always varies somewhat with time; g(t) contains a variety of "noise" terms of zero mean and various secular terms; the latter dominate unless proper precautions are in force. galactic cluster - (n.) An asymmetric type of collection of stars that shared a common origin. galaxy - (n.) spelled with a lower-case g, galaxy means any of millions of stellar systems once called "island universes" or extragalactic nebulae. Depending on their form, galaxies may be called spirals, barred spirals, ellipticals, or irregulars. Spelled with a capital G, Galaxy refers to that particular stellar system which includes our Sun and all the stars visible to the naked eye. The Milky Way is our view of the Galaxy. galaxy epoch - (n.) fifth epoch in the history of the Universe, lasting on the order of 1010 yr, during which matter largely coagulated into galactic masses. Galilean satellites - (n.) The four brightest satellites of Jupiter, discovered by Galileo. gamma ray - (n.) high-energy electromagnetic particle or photon, especially as emitted by a nucleus in its transition from one energy level to another. Radiation whose wavelength is less than one Angstrom is usually considered to be gamma-ray radiation. gas - (n.) The state of matter in which the substance maintains neither shape nor volume. gegenschein - (n.) The diffuse glowing spot, seen on the ecliptic opposite the sun's direction, created by sunlight reflected off of interplanetary dust. ~ See Also: zodiacal light. geothermal energy - (n.) Energy derived from the heat of Earth's interior. giant planets - (n.) Jupiter, Saturn, Uranus, and Neptune. giant star - (n.) A type of star brighter than main sequence stars of the same spectral type. gibbous moon - (n.) The phases between half moon and full moon giga - (n.) 109 (as in gigahertz, GHz); one billion (U.S.A.). globular cluster - (n.) A large, spherical cluster of stars located in the halo of the galaxy.These clusters, containing up to several hundred thousand members, are thought to be among the oldest objects in the galaxy. gram - (n.) A unit of mass equal to the quantity' of mass contained in one cubic centimeter of water. grating - (n.) A surface ruled with closely spaced lines that, through diffraction, breaks up light into its spectrum. gravitation - (n.) One of the four fundamental forces of nature, the force by which two masses attract each other. gravitational wave - (n.) propagating field predicted by general relativity to occur as a result of any large-scale change in the distribution of matter (as in the collapse of a star). great circle - (n.) The intersection of a plane that passes through the center of a sphere with the surface of that sphere; the largest possible circle that can be drawn on the surface of a sphere. greenhouse effect - (n.) A warming of the Earth's surface and lower layers of the atmosphere caused by interaction of solar radiation with atmospheric gases (mainly carbon dioxide, methane, and water vapor) and its conversion to heat because it is transparent to incoming visible radiation but opaque to the infrared radiation that is emitted by the surface of the planet. greenhouse gas - (n.) Gases such as carbon dioxide, methane, and water vapor that produce a greenhouse effect. Gregorian calendar - (n.) The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that are divisible by 100 but not by 400. ground state - (n.) The state of an atom in which all electrons are in the lowest possible energy levels. group - (n.) A vertical column of the periodic table; a family of elements; also, A characteristic part of a molecule. H0 - (n.) The Hubble constant. H I region - (n.) An interstellar region of neutral hydrogen. H II region - (n.) An interstellar region of ionized hydrogen. Ha - (n.) The first line of the Balmer series of hydrogen, at 6563 angstroms. hadron epoch - (n.) second epoch in the history of the Universe, lasting on the order of a second; named for the heavy elementary particles (protons, neutrons, mesons) that were the most abundant form of matter at the time. half life - (n.) The period of time needed for half of the radioactive isotopes in a sample to decay to daughter atoms. halo - (n.) (a) The extended outer portions far above and below the plane of a galaxy such as the Milky Way. The halo is thought to contain a large fraction of the total mass of the galaxy, mostly in the form of dim stars and interstellar gas, ( the extensive cloud of gas surrounding the head of a comet. head - (n.) Of a comet, the nucleus and coma together. heat - (n.) A measure of a quantity of energy; of how much energy a sample contains. heat capacity (of a substance) - (n.) The amount of heat needed to change the temperature of the substance by 1 °C. heat of vaporization (of a substance) - (n.) The amount of heat involved in the evaporation or condensation of 1 g of the substance. heliocentric - (n.) Sun-centered; using the sun rather than the earth as the point to which we refer. A heliocentric measurement, for example, omits the effect of the Doppler shift caused by the earth's orbital motion. helium - (n.) atom consisting of two protons and two electrons. helium flash - (n.) A rapid burst of nuclear reactions in the degenerate core of a moderate-mass star in the hydrogen shell-burning phase. The flash occurs when the core temperature reaches a sufficiently high temperature to trigger the triple-alpha reaction. Hertz (Hz) - (n.) measure of frequency with units of sec-1, formerly called cycles per second; an oscillating system that completes a cycle a second has a frequency of 1 Hz. Hertzsprung- Russell diagram - (n.) A diagram on which stars are represented according to their absolute magnitudes (on the vertical axis) and spectral types (on the horizontal axis).Because the physical properties of stars are interrelated, stars do not fall randomly on such a diagram, but instead lie in well-defined regions according to their state of evolution. Very similar diagrams can be constructed that show luminosity instead of absolute magnitude, and temperature or color index in place of spectral type. Hertzsprung gap - (n.) A region above the main sequence in a Hertzsprung-Russell diagram through which stars evolve rapidly and thus in which few stars are found. heterocyclic compound - (n.) A cyclic compound in which one or more atoms in the ring is (are) not carbon. heterotrophic hypothesis - (n.) The concept introduced by A.I. Oparin and J.B.S. Bernal that the earliest forms of life were heterotrophs that used nonbiologically produced organic matter as their carbon source. heterotrophy - (n.) Literally, other-feeding; the condition of an organism that is not able to obtain nutrients by synthesizing non organic materials from the environment, and that therefore must consume other life forms to obtain the organic products necessary for life e.g., animals, fungi, most bacteria. high-velocity star - (n.) A star whose velocity relative to the solar system is large. As a rule, high-velocity stars are Population II objects following orbital paths that are highly inclined to the plane of the galactic disk. homeobox - (n.) A highly conserved sequence of 180 nucleotides common to many regulatory genes and coding for the DNA-binding part of the corresponding regulatory proteins. ~ See Also: Homeotic gene, Transcription factor. homeostasis - (n.) The ability of living organisms to keep constant certain of their physical or chemical properties by self-regulation. homeotic gene - (n.) A regulatory gene containing a homeobox sequence. ~ See Also: Homeobox. homogeneous - (n.) Having the quality of being uniform in properties throughout. In astronomy, this term is often applied to the universe as a whole, which is postulated to be homogeneous. homologous series - (n.) A series of compounds in which adjacent members of the series differ by a fixed unit of structure. horizontal gene transfer - (n.) The transfer of genes from one organism to another, as opposed to vertical gene transfer, from parent to offspring. horizontal branch - (n.) A sequence of stars in the H-R diagram of a globular cluster, extending horizontally across the diagram to the left from the red-giant region. These are probably stars undergoing helium burning in their cores, by the triple-alpha reaction. hormone - (n.) A chemical transmitter transported by the blood stream or some other humoral connection from the cells that secrete it to the cells on which it acts. ~ See Also: Chemical transmitter. hour angle - (n.) Of a celestial object as seen from a particular location, the difference between the local sidereal time and the right ascension (H.A. = L.S.T. - R.A.). H-R diagram - (n.) Hertzsprung-Russell diagram. Hubble constant - (n.) The numerical factor, usually denoted H, that describes the rate of expansion of the universe. It is the proportionality constant in the Hubble law v ~Hd, which relates the speed of recession of a galaxy (v) to its distance (d) . The present value of H has recently become fairly well known; estimates range between 47 and 63 km/sec Y Mpc, giving an age of the universe around 14 billion years. Hubble law - (n.) The linear relation between the velocity of recession of a distant object and its distance from us, v = H0d. human genome project - (n.) An international protect to map the entire genome of Homo sapiens. humectant - (n.) A moistening agent. hydrocarbon - (n.) Any of a diverse group of organic compounds composed of hydrogen and carbon. hydrogen - (n.) simplest atom, consisting only of one proton and one electron; the most abundant element in the Universe. hypotheses - (n.) Guesses that can be tested by experiment. Hz - (n.) see Hertz. IC - (n.) Index Catalogue, one of the supplements to Dreyer's New General Catalogue (NGC). ideal gas law - (n.) The volume of a gas is proportional to the amount of gas and its Kelvin temperature and inversely proportional to its pressure. IDP - (n.) interplanetary dust particle. image tube - (n.) An electronic device that receives incident radiation and intensifies it or converts it to a wavelength at which photographic plates are sensitive. impact crater - (n.) A crater formed on the surface of a terrestrial planet or a satellite by the impact of a meteoroid or planetesimal. inclination - (n.) Of an orbit, the angle of the plane of the orbit with respect to the ecliptic plane. index of refraction - (n.) ratio of the speed of light in a vacuum to its speed in a given medium. inertia - (n.) The tendency of an object to remain in its state of rest or uniform motion; this tendency is directly related to the mass of the object. infrared - (n.) Radiation beyond the red, from about 7000 angstroms up to 1 mm. interference - (n.) The property of radiation, explainable by the wave theory, in which waves that are in phase can add (constructive interference) and waves that are out of phase can subtract (destructive interference); for light, this gives alternate light and dark bands. interferometer - (n.) A device that uses the property of interference to measure such properties of objects as their positions or structure. interferometry - (n.) an astronomical technique in which the images from two or more telescopes are superimposed. Interferometry has many uses, and one of the most intriguing is its ability to cancel out the light from a point source such as a star by superimposing the images in such a way that the oscillations in light intensity from one image are the reverse of the oscillations from the other image. With this method, planets orbiting the star can be directly observed, and the composition of their atmospheres can be investigated using spectral analyses. Note that this has not yet been accomplished as of summer 2000. interplanetary dust particle (IDP) - (n.) Small, often microscopic rocky particles of interplanetary debris. interplanetary medium - (n.) Gas and dust between the planets. interstellar cloud - (n.) A region of relatively high density in the inter- stellar medium. Interstellar clouds have densities ranging between 1 and 10'' particles per cubic centimeter, and in aggregate, contain most of the mass in interstellar space. interstellar extinction - (n.) The obscuration of starlight by interstellar dust. Light is scattered off of dust grains, so that a distant star appears dimmer than it otherwise would. The scattering process is most effective at short (blue) wavelengths, so that stars seen through inter- stellar dust appear reddened and dimmed. inverse Compton scattering - (n.) A method of converting photons from lower to higher energy through interaction with electrons that are moving with velocities close to the speed of light. inverse-square law - (n.) Any law describing a force or other phenomenon that decreases in strength as the square of the distance from some central reference point. The term inverse-square law is often used by itself to mean the law staling that the intensity of light emitted by a source such as a star diminishes as the square of the distance from the source. ionizing radiation - (n.) Radiation that produces ions as it passes through matter. ionosphere - (n.) The zone of the earth's upper atmosphere, between 80- and 500-km altitude, where charged subatomic particles (chiefly protons and electrons) are trapped by the earth's magnetic field. ~ See Also: Van Allen belts. irregular galaxy - (n.) A type of galaxy that lacks defined shape or symmetry. isochron - (n.) A parameter used in isotopic dating of geologic materials, experimentally determined from comparison of the isotopic compositions of two or more components (usually minerals) that share a common age. isomerization - (n.) The conversion of a compound into one or more of its isomers. isomers - (n.) Compounds that have the same molecular formula but different structural formulas and properties. isotopes - (n.) different forms of an element that have the same number of protons in their nuclei, and thus the same atomic number, but that have different numbers of neutrons and thus different atomic masses. There are two kinds of isotopes, stable and unstable. Isotopes that are unstable are called radioactive and disintegrate at a constant decay rate. Examples of stable isotopes include carbon-12 and carbon-13. Carbon-14, and uranium-238 and -235, are examples of unstable isotopes. isotopic date - (n.) Age of a rock (or organic substance less than 60,000 years old) determined by measurement of the ratio of a parent isotope to one of the products of its radioactive decay. isotopic fractionation - (n.) Separation of isotopes of an element, in organisms often mediated by an enzyme. isotropic - (n.) Having the property of appearing the same in all directions. In astronomy, this term is often postulated to apply to the universe as a whole. Jansky (Jy) - (n.) convenient unit of incident spectral flux density used in radio astronomy; 1 Jy = 10-26 W/m2 Hz (named for Karl G. Jansky, initial discoverer of extraterrestrial radio radiations). Joule (J ) - (n.) unit for work in the MKS system of units: 1 J = 107 ergs = 0.239 cal. Jovian planet - (n.) Same as giant planet; in our solar system: Jupiter (Jove), Saturn, Uranus and Neptune.. Kardashev cultures - (n.) N. X. Kardashev has distinguished three types of technological societies according to the amount of power they can harness: Type I can engage the power available on a planet; Type II. the power output of a star; and Type III, the power output of a galaxy. Kelvin - (n.) A unit of temperature equal to one one-hundredth of the difference between the freezing and boiling points of water, and used in a scale whose zero point is absolute zero. A Kelvin is usually denoted by K. kHz - (n.) kilohertz, 1000Hz (see Hertz). kilocalorie - (n.) A unit of energy, the amount of heat required to raise the temperature of 1 kg of water 1° C, 1000 calories. kilogram - (n.) The SI unit of mass, a quantity about equal to 2.2 pounds. kiloparsec (kpc) - (n.) A unit of distance equal to 1,000 parsecs. kinetic energy - (n.) The energy of motion. The kinetic energy of a moving object is equal to one-half times its mass times the square of its velocity. kinetic isotopic fractionation - (n.) Separation of isotopes of an element as a result of their speeds of movement, in organisms mediated by an enzyme that interacts more readily with one of two or more isotopes of an element. kinetic-molecular theory - (n.) A model that uses the motion of molecules to explain the behavior of the three states of matter. Kirkwood's gaps - (n.) Narrow gaps in the asteroid belt created by orbital resonance with Jupiter launch window - (n.) The time opportunity for launching a spacecraft to another body in the Solar System involving minimal energy or the shortest flight time. law of conservation of energy - (n.) The amount of energy within the universe is constant; energy cannot be created or destroyed, only transformed. lepton epoch - (n.) third epoch in the history of the Universe, lasting about 100 sec, in which the lighter elementary particles such as electrons, neutrinos, and muons were the dominant form of matter. life era - (n.) era in the history of the Universe when life emerges as the dominant element. light-gathering power - (n.) The ability of a telescope to collect light from an astronomical source. The light-gathering power is directly related to the area of the primary mirror or lens. light-harvesting pigment - (n.) Organic compounds, such as chlorophyll and bacteriochlorophyll, that absorb light energy in photosynthesis. light year - (n.) distance traveled by light in a vacuum in one year: 1 light year = 9.46>(10' 55 m. limb darkening - (n.) The dark region around the edge of the visible disk of the sun or a planet, caused by a decrease in temperature with height in the atmosphere. limestone - (n.) A kind of sedimentary rock consisting mainly of calcium carbonate minerals. line spectrum - (n.) The pattern of colored lines emitted by an element. liquid - (n.) A state of matter in which the substance assumes the shape of its container, flows readily, and maintains a fairly constant volume. liquid metallic hydrogen - (n.) Hydrogen in a state of semi rigidity that can exist only under conditions of extremely high pressure, as in the interiors of Jupiter and Saturn. liter - (n.) Unit of volume equal to a cube with sides 10 cm. lithosphere - (n.) The layer in the earth, moon, and terrestrial planets that includes the crust and the outer part of the mantle. local group - (n.) The cluster of about thirty galaxies to which the Milky Way belongs. luminosity - (n.) the total energy emitted by an object per second; that is, the power of the object. for stars the luminosity is usually measured in units of ergs per second. luminosity class - (n.) One of several classes to which a star can be assigned on the basis of certain luminosity indicators in its spectrum. The classes range from I for supergiants to V for main-sequence stars (also known as dwarfs). lunar month - (n.) The synodic period of the moon, equal to 27 days 7 hours 43 minutes 11.5 seconds. L wave - (n.) A type of seismic wave that travels only over the surface of the earth. Ma - (n.) Mega anna, one million (1,000,000 or 1 x 106) years. Magellanic clouds - (n.) The two irregular galaxies that are the nearest neighbors of the Milky Way; they are visible to the unaided eye in the southern hemisphere. magnetic braking - (n.) The slowing of the spin of a young star (such as the early sun) by magnetic forces exerted on the surrounding ionized gas. magnetic dynamo - (n.) A rotating internal zone inside the sun or a planet, thought to carry the electrical currents that create the solar or planetary magnetic field. magnetosphere - (n.) A region, surrounding a star or planet, that is permeated by the magnetic field of that body. magnitude - (n.) A measure of the brightness of a star. It is based on a system established by Hipparchus, in which stars were ranked according to how bright they appeared to the unaided eye. In the modern system, a difference of five magnitudes corresponds exactly to a brightness ratio of 100, so that a star of a given magnitude has a brightness that is 1001/5 = 2.51 times that of a star one magnitude fainter. main group elements - (n.) The elements in the A groups of the periodic table that is customary in the United States and in Groups 1, 2, and 13 to 18 in the periodic table recommended by IUPAC. main sequence - (n.) principal sequence of stars on the graph of luminosity versus effective temperature (H-R diagram), encompassing more than 90% of observable stars. These stars are converting hydrogen to helium by nuclear reactions in their cores The lower mass limit for the Main Sequence is 0.085 Mo and the upper limit is about 60 Mo. main-sequence fitting - (n.) A distance-determination technique in which an H-R diagram for a cluster of stars is compared with a standard H-R diagram to establish the absolute magnitude scale for the cluster H-R diagram. main-sequence turn-off - (n.) In an H-R diagram for a cluster of stars, the point where the main sequence turns off toward the upper right. The main-sequence turn-off, showing which stars in the cluster have evolved to become red giants, is an indicator of the age of the cluster. major axis - (n.) The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line. mare - (n.) One of the smooth areas on the moon or on some of the other planets. mascon - (n.) A concentration of mass under the surface of the moon, discovered from its gravitational effect on spacecraft orbiting the moon. maser - (n.) An acronym for "microwave amplification by stimulated emission of radiation," a device by which certain energy levels are more populated than normal, resulting in an especially dense emission of radio radiation at a certain frequency when the system drops to a lower energy level. A laser operating in the microwave region of the electromagnetic spectrum. mass - (n.) A measure of the inherent amount of matter in a body. That property which resists change of position by applied forces. mass-energy equation - (n.) Einstein's equation E = mc2, in which E is energy, m is mass, and c is the speed of light. mass-luminosity relation - (n.) A well-defined relation between the mass and luminosity for main sequence stars. mass number A - (n.) Nucleon number, the sum of the numbers of protons and of neutrons in the nucleus of an atom. mass spectrometry - (n.) Instrumental method of identifying the chemical constituents of a substance by means of the separation of gaseous ions according to their differing mass and charge. matter - (n.) Stuff of which all materials are made; anything that has mass. As opposed to pure energy. matter era - (n.) collective name for the most recent three epochs in the history of the Universe (atom, galaxy, stellar), covering all of time after the Radiation Era. Maunder minimum - (n.) virtual disappearance of sunspots in the period 1645 to 1715. MCSA - (n.) short for MCSA/SD, a digital, energy-efficient, real-time, multichannel spectrum analyzer and signal detector; hardware device that continuously accepts a significant portion of the electromagnetic spectrum and divides the spectral energy into a set of many contiguous frequency bins or output channels. Operational functions included are frequency analysis and signal detection, averaging, threshold testing, etc.. mega - (n.) –106 (as in megahertz, MHz); one million. megaparsec (Mpc) - (n.) A million parsecs. melting point - (n.) The temperature at which a substance changes from the solid to the liquid state. metal - (n.) (a) For stellar abundances, any element higher in atomic number than 2, that is, heavier than helium, ( For a planet or solid, matter that is a good conductor of heat and electricity. meteor - (n.) Solar system matter observable when it falls through Earth's atmosphere and is heated by friction to temporary incandescence; a "shooting star." meteorite - (n.) An interplanetary chunk of rock after it impacts on a planet or moon, especially on the earth. meter - (n.) The SI unit of length, slightly longer than 39 inches. methane - (n.) The colorless gaseous hydrocarbon CH4. MHz - (n.) one million Hertz; megahertz ; a million cycles per second. micrometeorite - (n.) a particle from space that is small enough to be slowed down when it reaches the Earth's atmosphere without being burnt up. Approximately 50 micrometeorites per square meter hit the top of Earth’s atmosphere each day. micrometer (?) - (n.) A unit of length, one-millionth (10-6 )of a meter. microwave - (n.) electromagnetic wave roughly in the range 0.01-1 m in wavelength (ordinary broadcasting utilizes waves in the 200-600 m range; the "short waves" used in long-distance communications are rarely shorter than 10 m). Milky Way - (n.) The band of light across the sky from the stars and gas in the plane of the Milky Way Galaxy. MIL - (N.) Mother in law a super massive blackhole minor axis - (n.) The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is perpendicular to the major axis. Also, the length of that line. Mo - (n.) solar mass. molar mass - (n.) The formula weight expressed in grams. molar volume - (n.) The volume occupied by 1 mole of a substance under specified conditions. mole - (n.) The formula weight in grams of an element or compound; or a quantity of chemical substance that contains 6.02 x 1023 formula units of the substance. molecule - (n.) A collection of atoms bound together that is the smallest collection that exhibits a certain set of chemical properties, mollusk : Any of an animal phylum (Mollusca) characterized by a large muscular foot and a mantle that secretes spicules or shells, such as a snail, clam, or squid. momentum - (n.) A measure of the tendency that a moving body has to keep moving. The momentum in a given direction (the "linear momentum") is equal to the mass of the body times its component of velocity in that direction. ~ See Also: angular momentum. nanometer (nm) - (n.) A unit of length, one billionth (10-9) of a meter. NASA - (n.) National Aeronautics and Space Administration of the United States of America. natural philosophy - (n.) Philosophical speculation about nature. nebula - (n.) rarefied cloud of gas or dust observed in interstellar space. nebular hypothesis - (n.) a general theory that describes how stars and their associated solar systems are formed from the condensation of clouds of dust and gas in space neutrino - (n.) Spinning, neutral elementary particle. A neutrino has no rest mass and always travels at the speed of light. neutron - (n.) A massive, neutral elementary particle, one of the fundamental constituents of an atom. Neutrons and protons are similar in mass, about 1830 times more massive than electrons. neutron star - (n.) A star that has collapsed to the point where it is supported against gravity by neutron degeneracy. new moon - (n.) The phase of the moon when the side of the moon facing the earth is the side that is not illuminated by sunlight. Newtonian telescope - (n.) A type of reflecting telescope where the beam reflected by the primary mirror is reflected by a flat secondary mirror so that the focus falls to the side of the telescope tube. New General Catalogue (NGC) - (n.) The common name for "A New General Catalogue of Nebulae and Clusters of Stars," put together by J. L. E. Dreyer in 1888. noble gas - (n.) one of a group of rare but extremely stable gases with low reaction rates (helium, neon, argon, krypton, xenon, radon). node - (n.) A point of intersection between two great circles. Eclipses of the Sun and Moon occur when these bodies are simultaneously near the nodes of their paths in the sky. nonmetals - (n.) The group of elements to the right of the heavy, stepped, diagonal line in the periodic table. nonthermal radiation - (n.) Radiation that cannot be characterized by a single number (the temperature). Normally, we derive this number from Planck's law, so that radiation that does not follow Planck's law is called nonthermal. nova - (n.) A star that suddenly increases in brightness. Most novae are thought to result in binary systems when matter from the giant component falls on the white dwarf component. nuclear fission - (n.) The splitting of an atomic nucleus into two large fragments. nuclear fusion - (n.) Combination of two small atomic nuclei to produce one larger nucleus. nuclear reactor - (n.) A plant that produces energy by nuclear fission. nuclear winter - (n.) A period of dark, cold weather that may be caused by the dust and smoke entering the atmosphere from the explosion of nuclear bombs. nucleon number - (n.) The total number of protons and neutrons in an atom; the mass number A. nucleons - (n.) Protons and neutrons in an atomic nucleus. nucleosynthesis - (n.) The formation of the elements. nucleus - (n.) (a.) Of an atom, the core of an atom, which has a positive charge, contains most of the mass, and takes up only a small part of the volume; ( of a comet, the chunks of matter, taking up a volume no more than a few kilometers across, at the center of the head of a comet; © of a galaxy, the innermost regions of a spiral galaxy; it does not show spiral structure and is visible from the sky as a bulge in the otherwise flat disk of the galaxy. O and B association - (n.) A group of O and B giant Young stars close together in space. The members of an O and B association were formed at roughly the same time. objective - (n.) The principal lens or mirror of an optical system. oblate - (n.) Having an equatorial diameter greater than the polar diameter. obliquity - (n.) The angle by which the spin axis of a planet to the plane of its ecliptic differs from 90°. occultation - (n.) The hiding of one astronomical body by another, such as the occultation of a star by the moon. Olbers's paradox - (n.) Tine observation that the sky is dark at night contrasted to a simple argument that shows that the sky should be uniformly bright, due to there being a star in every line of sight in an infinite and uniform universe. The paradox is resolved by the redshift of the expanding universe. Oort cloud - (n.) a vast collection of bodies made up of rock and ice that orbit the Sun at a distance starting in the region beyond the orbit of Pluto and extending out to nearly 1.5 light-years or 50,000 A. U. Unlike the bodies of the Solar System, which orbit the Sun in roughly the same plane, the Oort cloud objects form a vast sphere around the Sun. It is estimated that billions of objects exist in this region, and there is evidence that this is the place where most comets originate. opacity - (n.) The lack of complete transparency of a gas. open cluster - (n.) A galactic cluster, an asymmetric type of star cluster. open universe - (n.) The version of big bang cosmology in which the universe will expand forever. An open universe has infinite volume. opposition - (n.) The passage of a planet through the point most directly opposite the sun on the other side of the earth. optical activity - (n.) the property of some crystals, gases, liquids, and solutions to rotate plane-polarized light to the left or right. It occurs because the molecules that make up the substance through which the light beam is shone are asymmetric, i.e., they have no plane of symmetry. Asymmetric molecules are mirror images of each other that cannot be superimposed. This asymmetric property is also referred to as handedness. Examples are the L- and D-forms of amino acids. optical brightener - (n.) A compound that absorbs the invisible ultraviolet component of sunlight and re-emits it as visible light at the blue end of the spectrum. optical depth - (n.) The number of factors of the transcendental number (2.71828. . . ) that radiation is dimmed in passage through a gas. optical double - (n.) A pair of stars that appear extremely close together in the sky even though they are at different distances from us and are not physically linked. orbital - (n.) A region of space in an atom occupied by one or two electrons. ozone - (n.) O3 a gaseous molecule, made of three oxygen atoms, which forms a layer in the upper atmosphere that shields the Earth against excessive ultraviolet radiation. ozone layer - (n.) layer of Earth's atmosphere at about 20 to 30 miles, marked by a high ozone (O3) content. polycyclic aromatic hydrocarbon (PAH) - (n.) Any of various organic compounds composed of a few to many six-membered rings of carbon atoms linked by an alternating sequence of single and double bonds and to which hydrogen atoms are attached. PAHs - (n.) Polycyclic aromatic hydrocarbons, i.e., hydrocarbons in which the carbon atoms are arranged in rings which are themselves linked together. paradigm - (n.) A well-established example or model, an archetype. parallax - (n.) (a) When used by itself, the word "parallax" refers to trigonometric parallax, half the angle through which a star appears to be displaced when the earth moves from one side of the sun to the other, that is, through 2 A.U. The parallax of a star is inversely proportional to the distance to the star from the sun. ( Some of the other ways of measuring distances, usually in those cases referred to with an adjective, as in spectroscopic parallax. parsec (pc) - (n.) parallax second, the distance at which I AU subtends an angle of 1second of arc: 1 parsec = 1.9x1013 mi. = 3.26 light years. peculiar velocity - (n.) The velocity of a star with respect to the local standard of rest. penumbra - (n.) (a.) For an eclipse, the part of the shadow from which the sun or other radiating body is only partially occulted; ( of a sunspot, the outer region of the sunspot, not as dark as the central umbra. perfect cosmological principle - (n.) The assumption that on a large scale the universe is homogeneous and isotropic in space and unchanging in time. periastron - (n.) The near point of the orbit of a body to the star around which it is orbiting. perihelion - (n.) The near point to the sun of the orbit of a body orbiting the sun. periodic table - (n.) A systematic arrangement of the elements in columns and rows; elements in a given column have similar properties. photomultiplier - (n.) An electronic device that through a series of internal stages multiplies the small current that is given off when light is incident on the device so that a relatively large current results. photon - (n.) A packet of energy that can be thought of as a particle of light travelling at the speed of light. A photon of energy E is equivalent to an electromagnetic wave of wavelength ?=hc/E, where h is Planck's constant and c is the speed of light in a vacuum. photosphere - (n.) The visible surface layer of the sun and stars; the layer from which continuous radiation escapes and where absorption lines form. photovoltaic cell - (n.) A solar cell; a cell that converts sunlight directly to electrical energy. physical change - (n.) A change in physical state or form. physical properties - (n.) The qualities of a substance that can be demonstrated without changing the composition of the substance. plage - (n.) The part of a solar active region that appears bright when viewed in H0. Planck constant - (n.) the numerical factor h relating the frequency v of a photon to its energy e in the expression e = hn. the Planck constant has the value h = 6.62620 x 10-27 erg sec. Planck function (also known as the Planck law) - (n.) The mathematical expression describing the continuous thermal spectrum of a glowing object. For a given temperature, the Planck function specifies the intensity of radiation as a function of either frequency or wavelength. planet - (n.) A celestial body of substantial size (more than about 1000 km across), basically non-radiating and of insufficient mass for nuclear reactions ever to begin, ordinarily in orbit around a star. planetary nebula - (n.) A cloud of glowing, ionized gas, usually taking the form of a hollow sphere or shell, ejected by a star in the late stages of its evolution. planetesimal - (n.) A small (diameter up to several hundred kilometers) solar-system body of the type that first condensed from the solar nebula. Planetesimals are thought to have been the principal bodies that combined to form the planets. plasma - (n.) A state of matter similar to a gas but composed of isolated electrons and nuclei rather than discrete whole atoms or molecules. plate tectonics - (n.) the theory that the Earth's continental and oceanic crust and outermost portion of the mantle is fractured into large plates that move relative to each other. Convective currents in the mantle provide the driving force for this motion. The plate motion is responsible for global mountain building, earthquake activity, and volcanism, all of which are most pronounced along plate boundaries ~ See Also: continental drift. polar axis - (n.) The axis of an equatorial telescope mounting that is parallel to the earth's axis of rotation. polarization - (n.) The arrangement of electromagnetic waves so that all the planes in which the waves are oscillating are parallel to each other. population I - (n.) The class of stars with relatively high abundances of heavy elements. These stars are generally found in the disk and spiral arms of spiral galaxies, and are relatively young. The term Population I is also commonly applied to other components of galaxies associated with the star formation, such as the interstellar material. population II - (n.) The class of stars with relatively low abundances of heavy elements. These stars are generally found in a spheroidal distribution about the galactic center and throughout the halo, and are relatively old. Population III - (n.) Hypothetical class of 1st generation stars composed solely of H and He, with mass 100-1000 x solar and luminosity 1-30 million times greater. As usual in astronomy, the numbering is backward: these are the first stars. positron - (n.) A subatomic particle with the same mass as the electron, but with a positive electrical charge; the antiparticle of the electron. potential energy - (n.) Energy that is stored, and which may be converted into kinetic energy under certain circumstances. In astronomy, the most common form of potential energy is gravitational potential energy. prebiotic - (n.) relating to the chemical or environmental precursors of the origin of life. precession - (n.) The slowly changing position of stars in the sky with respect to earth-based coordinates, resulting from the slowly varying orientation (the precession) of the earth's axis. The apparent precession of the stars is the result of the actual precession of the earth's axis. primary cosmic rays - (n.) The cosmic rays arriving at the top of the earth's atmosphere. primary mirror - (n.) The principal light-gathering mirror in a reflecting telescope. prime focus - (n.) The location at which the main lens or mirror of a telescope focuses an image without being reflected or refocused by another mirror or other optical element. primordial background radiation - (n.) Radiation detected in millimeter and submillimeter wavelength regions that is coming from all directions in space and interpreted to be the remnant of the big bang. Also known as 3° background radiation, background radiation, or the remnant of the primeval fireball. prograde motion - (n.) Orbital or spin motion in the forward or "normal" direction; in the solar system, this is counterclockwise as viewed looking down from above the north pole. Project Cyclops - (n.) a 10-week design study sponsored by NASA, Stanford University, and the American Institute for Engineering Education, of possible means for detecting extraterrestrial civilizations. prolate - (n.) Having the diameter along the axis of rotation longer than the equatorial diameter. prominence - (n.) Solar gas protruding over the limb, visible to the naked eye only at eclipses but also observed outside of eclipses by its emission line spectrum. Prominences are at approximately the same temperature as the chromosphere. proper motion - (n.) Motion across the sky with respect to a framework of galaxies or fixed stars, usually measured in seconds of arc per century. proto- - (n.) A prefix from the Greek for "before." When used in conjunction with the name of a celestial body, means the state of the body just before it is considered to have formed. proton - (n.) A positively charged elementary particle of mass equal to that of a neutron and almost two thousand times that of an electron; a component, with neutrons, of all atomic nuclei : The nucleus of the hydrogen atom : A hydrogen ion, H*, arising either through the removal of the peripheral electron from a hydrogen atom or, together with a hydroxyl ion, from the dissociation of a water molecule. ~ See Also: Electron, Hydroxyl ion, Isotopes, Neutron, Nucleus. protostar - (n.) A star in the process of formation, specifically one that has entered the slow gravitational contraction phase. pulsar - (n.) A rapidly rotating object, now known to be a neutron star, an extremely dense collapsed star where the electrons have been forced into the protons. The object is thus made up mainly of neutrons and a few kilometers in diameter. Pulsars emit magnetic field constrained beams of radio, visible, x-ray and gamma radiation, which we perceive as pulses as these objects whirl, sometimes at hundreds of revolutions a second. q0 - (n.) The deceleration parameter, a cosmological parameter that describes the rate at which the expansion of the universe is slowing up. QSO - (n.) Quasi-stellar object. ~ See Also: quasar. quadrature - (n.) The configuration where a superior planet or the moon is 90 degrees away from the sun, as seen from the earth. quantum - (n.) The amount of energy associated with a photon, equal to hv, where h is the Planck constant, and v is the frequency. The quantum is the smallest amount of energy that can exist at a given frequency. quantum mechanics - (n.) The branch of 20th century physics that describes atoms and radiation; the theory involves bundles of energy known as quanta. quark - (n.) One of the subatomic particles from which many modern theoreticians believe such elementary particles as protons and neutrons are composed. The various kinds of quarks have positive or negative charges of 1/3 or 2/3. quasi-stellar object - (n.) Any of a class of extragalactic objects also known as quasars, characterized by emission lines with very large redshifts. The quasi-stellar objects are thought to lie at great distances, in which case they existed only at earlier times in the history of the universe; they may be cores of young galaxies. quiet sun - (n.) The collection of solar phenomena that do not vary with the solar activity cycle. radar - (n.) The acronym for radio detection and ranging; an active rather than passive radio technique in which radio signals are transmitted and their reflections received and studied. radial velocity - (n.) The velocity of an object along a line (the radius) joining the object and the observer; the component of velocity toward or away from the observer. radian - (n.) The unit of angular measure, defined as the ratio of a length of arc intercepted by two radii to the length of the radius. P radians = 180°. radiant - (n.) The point in the sky from which all the meteors in a meteor shower appear to be coming. radiation - (n.) Electromagnetic radiation. radiation era - (n.) collective name for the first three epochs of the history of the Universe (chaos, hadron epoch, lepton epoch), lasting overall about 100 sec and dominated by radiation rather than matter. radiation pressure - (n.) Pressure created by light hitting a surface. radiative transport - (n.) The transport of energy, inside of a star or in other situations, by radiation. radioactive - (n.) Having the property of spontaneously changing into another isotope or element. radioactive dating - (n.) A technique for estimating the age of material, such as rock, based on the known initial isotopic composition and the known rate of radioactive decay for unstable isotopes originally present. radioactive decay - (n.) Disintegration of an unstable atomic nucleus by spontaneous emission of radiation. radioactivity - (n.) Spontaneous emission of alpha, beta, or gamma rays by the disintegration of the nuclei of atoms. radio galaxy - (n.) Any of a class of galaxies whose luminosity is greatest in radio wavelengths. Radio galaxies are usually large elliptical galaxies, with synchrotron radiation emitted from one or more pairs of lobes located on opposite sides of the visible galaxy. radionuclide - (n.) radioactive nuclear species or nuclide. radio telescope - (n.) An antenna or set of antennas, often together with a focusing reflecting dish, that is used to detect radio radiation from space. radio waves - (n.) Electromagnetic radiation with wavelengths longer than about one millimeter. rare earth elements - (n.) series of elements usually taken to include elements with atomic numbers 58 to 71 (lanthanum) and sometimes yttrium and scandium. ray - (n.) (a) A light ray, a wave of electromagnetic radiation; ( on the surface of a moon or planet, a streak of material that is relatively light in shade, presumable representing material ejected when a crater was formed. recombination - (n.) The addition of an electron to an ion, usually resulting in radiation (recombination lines) when the electron subsequently jumps down to lower energy states. recurrent nova - (n.) A star known to flare up in nova outbursts more than once. A recurrent nova appears to be a binary system containing a white dwarf and a mass-losing star, in which the white dwarf sporadically flares up when material falls onto it from the companion. reddening - (n.) The phenomenon by which the extinction of blue light by interstellar matter is greater than the extinction of red light so that the redder part of the continuous spectrum is enhanced. red giant - (n.) A post-main-sequence stage of the lifetime of a star; the star becomes relatively bright and relatively cool. redshift - (n.) The shift of a spectrum, usually of spectral lines in particular, to longer wavelengths. reducing agent - (n.) A substance that causes reduction and is itself oxidized. reducing atmosphere - (n.) atmosphere comprised of substances that readily provide electrons. reduction - (n.) The gain of one or more electrons or hydrogen atoms (electrons + protons) by an atom or molecule. ~ See Also: Electron, Oxidation, Proton : In cell biology, chromosome reduction refers to the halving of the chromosome number that occurs at meiosis. ~ See Also: Diploid, Haploid, Meiosis. reflecting telescope - (n.) A type of telescope that uses a mirror or mirrors to form the primary image. reflection nebula - (n.) An interstellar cloud containing dust that shines by light reflected from a nearby star. refracting telescope - (n.) A telescope that uses lenses to bring light to a focus. refraction - (n.) The bending of electromagnetic radiation as it passes from one medium to another or between parts of a medium that has varying properties. The index of refraction of a substance is the ratio of the speed of light in a vacuum to that in the substance. refractory - (n.) The property of being able to exist in solid form under conditions of very high temperature. Refractory elements are characterized by a high temperature of vaporization; they are the first to condense into solid form when a gas cools, as in the solar nebula. regolith - (n.) The mantle of unconsolidated fragmental material that covers a land surface; i.e., soil and fractured rock. relativistic - (n.) Having a velocity that is such a large fraction of the speed of light that the special theory of relativity must be applied. relativity - (n.) Either of the theories of relativity worked out by Albert Einstein. The special theory of relativity (1905) is a theory of relative motion. The general theory of relativity (1916) is a theory of gravitation. research - (n.) A study or investigation to find new information. resolution - (n.) The ability of an optical system to distinguish fine detail. rest wavelength - (n.) The wavelength of a spectral line as measured in a laboratory, when there is no relative motion between source and observer. retrograde motion - (n.) Orbital or spin motion in the opposite direction from prograde motion; in the solar system, retrograde motions are clockwise as seen from above what we call the North Pole of the Earth. revolution - (n.) The orbiting of one body around another. right ascension (RA) - (n.) The east-west coordinate in the equatorial coordinate system. The right ascension is measured in units of hours, minutes, and seconds to the east from a fixed direction in the sky, which itself is defined as the line of intersection of the ecliptic and the celestial equator. rille - (n.) A type of winding, sinuous valley commonly found on the moon. Roche limit - (n.) The point near a massive body such as a planet or star, inside of which the tidal forces acting on an orbiting body exceed the gravitational force holding that body together. The location of the Roche limit depends on the size of the orbiting body. Roentgen - (n.) unit of X or gamma radiation dosage: The amount of such radiation sufficient to produce ions carrying 1 electrostatic unit of charge in I ~11733 of air. Rosetta stone - (n.) A slab of black basalt stone found in 1799 that bears an inscription in hieroglyphics, demotic characters, and Greek, celebrated for having given the first clue to the decipherment of Egyptian hieroglyphics; from this, any breakthrough discovery of great magnitude. rotation - (n.) Spin on an axis. RR Lyrae variable - (n.) A member of a class of pulsating variable stars named after the prototype star, RR Lyrae. These stars are blue-white giants with pulsation periods of less than one day, and are Population II objects found primarily in globular clusters. Russell-Vogt theorem - (n.) The theorem that the evolution of a star is completely determined by its mass and chemical composition. saros cycle - (n.) An 18-year, 11-day repeating pattern of solar and lunar eclipses caused by a combination of the tilt of the lunar orbit with respect to the ecliptic and the precession of the plane of the moon's orbit. scattering - (n.) The random reflection of photons by particles such as atoms or ions in a gas, or dust particles in interstellar space. Schmidt camera - (n.) A type of telescope that uses a spherical mirror and a thin lens to provide photographs of a wide field. Schwarzschild radius - (n.) The radius that, according to Schwarzschild's solutions to Einstein's equations of the general theory of relativity, corresponds to the event horizon of a black hole. science : A branch of knowledge based on the laws of nature. science - (n.) A branch of knowledge based on the laws of nature. scientific law - (n.) A summary of experimental data; often expressed in the form of a mathematical equation. scientific model - (n.) A representation that serves to explain a scientific phenomenon. secondary cosmic rays - (n.) High energy particles that are generated in the earth's atmosphere by primary cosmic rays. secondary mirror - (n.) The second mirror in a reflecting telescope (after the primary mirror), usually either convex, to reflect the image out of a hole in the bottom of the telescope to the Cassegrain focus or along the telescope mount axis to the coudé focus; or flat, to reflect the image out of the side of the telescope to the Newtonian focus. second law of thermodynamics - (n.) The degree of randomness in the universe increases in any spontaneous process. seeing - (n.) The steadiness of the earth's atmosphere as it affects the resolution that can be obtained in astronomical observations. Good seeing corresponds to a steady atmosphere, and bad seeing corresponds to an unsteady atmosphere. seismic wave - (n.) A wave created in a planetary or satellite interior, usually caused by an earthquake. seismology - (n.) The study of waves propagating through a body and the resulting deduction of the internal properties of the body. The prefix "seismo-" comes from the Greek word for earthquake. selection effect - (n.) The tendency for a conclusion based on observations to be influenced by the method used to select the objects for observation. An example was the early belief that all quasars are radio sources, when the principal method used to discover quasars was to look for radio sources and then to find out whether they had other properties associated with quasars. semimajor axis - (n.) Half the major axis, that is, for an ellipse, half the longest diameter. SETI - (n.) Search for Extra-Terrestrial Intelligence. Seyfert galaxy - (n.) A type of spiral galaxy that has a bright nucleus and whose spectrum shows emission lines. Historically, N galaxies and Seyfert galaxies were defined by different astronomers on the basis of different information, and the difference between them is not always clear cut. shear wave - (n.) A wave that consists of transverse motions; that is, motions perpendicular to the direction of wave travel. shock wave - (n.) discontinuity in the flow of a fluid (including a gas or plasma) marked by an abrupt increase in pressure, temperature, and flow velocity at the shock front. sidereal - (n.) time measured in relation to the fixed stars: the length of a sidereal day is 23 hr, 56 min, 4.09 sec of mean solar time. siderophile element - (n.) element with a weak affinity for oxygen and sulfur and readily soluble in molten iron (including iron, nickel, cobalt, platinum, gold, tin, and tantalum). Those measured digits that are known with certainty plus one uncertain digit. signal-to-noise ratio - (n.) Those measured digits that are known with certainty plus one uncertain digit. significant figures - (n.) Those measured digits that are known with certainty plus one uncertain digit. singularity - (n.) A point in space where quantities become exactly zero or become infinitely large; a singularity is present in a black hole. Sol - (n.) A Martian day, 24 hours and 37 minutes. solar activity cycle - (n.) The 11- or 22-year cycle with which such solar activity as sunspots, flares, and prominences varies solar cell - (n.) A device used for converting sunlight into electricity; a photoelectric cell. solar constant - (n.) rate at which radiant solar energy is received normally per unit area at the outer layer of Earth's atmosphere; its value is about 1.94 gram calories/cm2 /min. solar day - (n.) The synodic rotation period of the earth with respect to the sun; that is, the length of time from one local noon, when the sun is on the meridian, to the next local noon. solar flare - (n.) An explosive outburst of ionized gas from the sun, us ally accompanied by X-ray emission and the injection of large quantities of charged particles into the solar wind. solar mass (Mg) - (n.) mass of the Sun, 2x1030 kg, used commonly as a unit to measure the masses of stars. solar motion - (n.) The deviation of the sun's velocity from perfect circular motion about the center of the galaxy; that is, the sun's peculiar velocity. solar nebula - (n.) cloud of gas and dust out of which a star condenses. The primordial gas and dust cloud from which the sun and planets condensed. solar (stellar) wind - (n.) radial outflow of hot plasma from a star's corona, carrying both mass, angular momentum, and energy away from the star solar time - (n.) A system of time-keeping with respect to the sun such that the sun is overhead of a given location at noon. solar wind - (n.) The stream of charged subatomic particles flowing steadily outward from the sun. solid angle - (n.) A three-dimensional angle. solstice - (n.) The occasion when the sun, as viewed from the earth, reaches its farthest northern point (the summer solstice) or its farthest southern point (the winter solstice). space velocity - (n.) The velocity of a star with respect to the sun. specific heat - (n.) The amount of heat required to raise the temperature of 1 g of the substance by 1 °C. speckle interferometry - (n.) method of using short-exposure photographs to recover information down to the diffraction limit of large optical telescopes. spectral analysis - (n.) study of the distribution by wavelength or frequency of the radiation emitted by an object of interest. spectral binary - (n.) : A binary system recognized as a binary because its spectrum contains lines of two stars of different spectral types. spectral type/class - (n.) classification used to sort stars by photospheric temperature and intrinsic brightness. The seven spectral classes O-B-A-F-G-K-M, listed in order of decreasing temperature, include 99% of all known stars. Each spectral type is divided into a variable number of subtypes. spectrogram - (n.) A photograph of a spectrum. spectrograph - (n.) An instrument for recording the spectra of astronomical bodies or other sources of light. spectroscope - (n.) An instrument allowing an observer to view the spectrum of a source of light. spectroscopic binary - (n.) binary star that can be distinguished from a single star only through analysis of the Doppler shift of the spectral lines of one or both stars as they revolve about their common center of mass. spectroscopic parallax - (n.) The distance to a star derived from comparison of its apparent magnitude with its absolute magnitude deduced from study of its position on an H-R diagram determined by observation of its spectrum (spectral type and luminosity class). spectroscopy - (n.) The science of analyzing the spectra of stars or other sources of light. spectrum - (n.) An arrangement of electromagnetic radiation according to wavelength. spin-orbit coupling - (n.) A simple relationship between the orbital and spin periods of a satellite or planet, caused by tidal forces that have slowed the rate of rotation of the orbiting body. Synchronous rotation is the simplest and most common form of spin-orbit coupling. spiral density wave - (n.) A spiral wave pattern in a rotating, thin disk, such as the rings of Saturn or the plane of a spiral galaxy like the Milky Way. spiral galaxy - (n.) Any of a large class of galaxies exhibiting a disk with spiral arms. sporadic meteor - (n.) A meteor that is not associated with a shower. standard candle - (n.) A general term for any astronomical object whose absolute magnitude can be inferred from its other observed characteristics, and which is therefore useful as a distance indicator. star - (n.) : A self-luminous ball of gas that shines or has shone because of nuclear reactions in its interior. steady-state theory - (n.) The theory of cosmology' in which the universe is thought to have had no beginning and is postulated not to change with time. Stefan-Boltzmann Law - (n.) The radiation law that states that the energy emitted by a black body varies with the fourth power of the temperature. Stellar epoch - (n.) sixth epoch in the history of the Universe, lasting perhaps 1010 yr from the galactic era to the present, dominated by the formation of stars. Stellar parallax - (n.) The apparent annual shifting of position of a nearby star with respect to more distant background stars. The term stellar parallax is often assumed to mean the parallax angle, which is one- half of the total angular motion a star undergoes. ~ See also parallax, parsec. Stellar wind - (n.) Any stream of gas flowing outward from a star, including the very rapid winds from hot, luminous stars; the intermediate- velocity, rarefied winds from stars like the sun; and the slow, dense winds from cool supergiant stars Stones - (n.) A stony type of meteorite, including the chondrites. STP - (n.) Standard temperature (0 °C) and pressure (1 atm). Stradian - (n.) The unit of solid angular measure, defined as the ratio of the surface area of that section of a sphere intercepted by a solid angle to the square of the radius. A full sphere subtends 4 P steradians from its center. stratosphere - (n.) One of the upper layers of the atmosphere of a planet, above the weather. The earth's stratosphere ranges from about 20 to 50 km in altitude. strong force - (n.) The nuclear force, the strongest of the four fundamental forces of nature. sublimation - (n.) The process of passing from gas to solid state (or vice versa) without becoming a liquid. subtend - (n.) The angle that an object appears to take up in your field of view; actually, the angle between lines drawn from opposite sides of the object to your eye. For example, the full moon subtends 1/2°. subtypes - (n.) designated by Arabic numerals. Further, stars are sorted by intrinsic brightness into luminosity classes designated by the first five Roman numerals. In turn, these are subdivided into a small number of subclasses designated by the first few letters of the lower case English alphabet; for example, the Sun is a G2 V star (also sometimes denoted as a dwarf G2 star) and Betelgeuse (a Orionis) is classified as M2 lab (i.e., intermediate between la and lb). sunspot - (n.) A relatively dark area of the solar surface. Sunspots appear dark because they are relatively cool; they represent regions of extremely high magnetic field. sunspot cycle - (n.) The roughly 11-year cycle of variation of the number of sunspots visible on the sun. supercluster - (n.) A cluster of clusters of galaxies. supergiant - (n.) A post-main-sequence phase of evolution of stars of more than about 4 solar masses. Supergiants fall in the extreme upper right of the H-R diagram. supergranulation - (n.) The pattern of large cells seen in the sun's chromosphere, when viewed in the light of the strong emission line of ionized hydrogen. superior planet - (n.) Any planet whose orbit lies beyond the earth's orbit around the sun. supernova - (n.) An explosion of a massive star (with mass greater than 8 to 10 times the mass of the Sun) which ejects, at speeds about a tenth the speed of light, most of the original mass into space. The shell of material left behind may form a white dwarf, neutron star, or black hole, a region where gravity is so strong that no radiation or matter can escape. There is a short billionfold increase in luminosity. S wave - (n.) A type of seismic wave that is a transverse, or shear, wave, and which can travel only through rigid materials. synchotron self-absorption - (n.) re-absorption of radiation from accelerated electrons by other nearby electrons; this is a possible source of low frequency turnovers observed in the radio spectra of compact sources. synchotron emission - (n.) : radiation from electrons constantly accelerated in a magnetic field at a rate great enough for relativistic effects to be important. Predicted long ago, this radiation was first encountered in the particle accelerator called the synchrotron. Much of the radiation observed by radio astronomers originates in this fashion. synchotron rotation - (n.) A situation in which the rotational and orbital periods of an orbiting body are equal, so that the same side is always facing the companion object. synergistic effect - (n.) An effect much greater than the sum of the expected effects. synodic - (n.) Measured with respect to an alignment of astronomical bodies other than or in addition to the sun or the stars (usually the moon or a planet). For example, a synodic month depends on the positions of the sun, earth, and moon. synodic period - (n.) The orbital or rotational period of an object as seen by an observer on the earth. For the moon or a planet, the synodic period is the interval between repetitions of the same phase or configuration. The synodic period of Mars depends on the relative positions of the earth and Mars as they orbit the sun. systems analysis - (n.) analysis of the response to inputs of a set of interconnected units whose individual characteristics are known. syzygy - (n.) An alignment of three celestial bodies. Sometimes applied more specifically to an alignment of the sun, earth, and moon. T association - (n.) A grouping of several T Tauri stars, presumably formed out of the same cloud of interstellar dust and gas. technology - (n.) The sum total of processes by which humans modify the materials of nature to better satisfy their needs and wants. tektites - (n.) Small glassy objects found scattered around the southern part of the southern hemisphere of the earth. temperature - (n.) A measure of heat intensity, or how energetic the particles of a sample are. terminator - (n.) The line between nighttime and daytime on a moon or planet; the edge of the part of a moon or planet that is lighted by the sun. terrestrial planets - (n.) Mercury, Venus, Earth, and Mars. theories - (n.) Detailed explanations of the behavior of matter based on experiments; may be revised if new data warrant. thermonuclear reactions - (n.) Radiation whose distribution of intensity over wavelength can be characterized by a single number (the temperature). Black-body radiation, which follows Planck's law, is an example of thermal radiation. total eclipse - (n.) Any eclipse in which the eclipsed body is totally blocked from view or totally immersed in shadow. transit - (n.) The passage of one celestial body in front of another celestial body. When a planet is in transit, we understand that it is passing in front of the sun. transition elements - (n.) Metallic elements situated in the center portion of the periodic table in the B groups. transit telescope - (n.) : A telescope designed to point straight overhead and accurately measure the times at which stars cross the meridian triple-alpha process - (n.) : A chain of fusion processes by which three helium nuclei (alpha particles) combine to form a carbon nucleus. tritium - (n.) : A rare radioactive isotope of hydrogen with two neutrons and one proton in the nucleus (a mass of 3 atomic mass units). Trojan asteroids - (n.) A group of asteroids that precede or follow Jupiter in its orbit by 60°. tropical year - (n.) The length of time between two successive vernal equinoxes. T-Tauri stars - (n.) luminous variable stars associated with interstellar clouds and found in very young clusters; they are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the Main Sequence and begun to burn hydrogen. UBV system - (n.) A system of photometry that uses three standard filters to define wavelength regions in the ultraviolet, blue, and green-yellow (visual) regions of the spectrum. ULE - (n.) Ultra-Low Expansion, Corning Glass's successor to Pyrex as a material out of which telescope mirrors are made. U.L.E. expands or contracts very little when the temperature changes, and thus a mirror made out of it holds its shape as the temperature varies without distorting the image. ultraviolet - (n.) The region of the spectrum between about 100 and 4000 angstroms; also used in the restricted sense of ultraviolet radiation that reaches the ground, namely, that between about 3000 and 4000 angstroms. umbra - (n.) : (a) The dark inner portion of a shadow, such as the part of the earth's shadow which the moon is in total eclipse during a lunar eclipse; ( the dark central portion of a sunspot. universe - (n.) everything that came into existence at the moment of the Big Bang, and everything that evolved from that initial mass of energy, or everything we can in principle, observe. UVBY - (n.) A system of photometry that uses four standard filters to define wavelength regions in the ultraviolet, blue, and yellow regions of the spectrum. UV light - (n.) ultraviolet (UV) light. van Allen belts - (n.) Zones in the earth's magnetosphere where charged particles are confined by the earth's magnetic field. There are two main belts, one centered at an altitude of roughly 1.5 times the earth' radius, and the other between 4.5 and 6.0 times the earth's radius. vaporization - (n.) The process in which a substance changes from the liquid to the gaseous (vapor) state. variable star - (n.) A star whose brightness changes over time. velocity - (n.) The measure of the speed and direction of an object; the distance traveled by an object per unit of time. velocity curve - (n.) A plot showing the orbital velocity of stars in a spiral galaxy versus distance from the galactic center. velocity dispersion - (n.) A measure of the average velocity of stars in a group or cluster with random internal motions. In globular clusters and elliptical galaxies, the velocity dispersion can be used to infer the central mass. vernal equinox - (n.) The intersection of the ecliptic and the celestial equator that the sun passes each year when moving from southern to northern inclinations. visible light - (n.) That part of the electromagnetic spectrum visible to humans and lying between the infrared and ultraviolet parts of the spectrum. Visual Binary - (n.) binary star system whose components can be identified with an optical telescope. VLA - (n.) The Very Large Array, a set of radio telescopes being built by the National Radio Astronomy Observatory in New Mexico to make aperture synthesis measurements. VLBI - (n.) Very-long-baseline interferometry, the technique of using simultaneous measurements made with radio telescopes at widely separated locations to obtain extremely high resolution. volatile - (n.) The property of being easily vaporized. Volatile elements stay in gaseous form except at very low temperatures; they did not condense into solid form during the formation of the solar system. volatile organic compounds - (n.) class of organics that is easily vaporizable at low temperatures and pressures. wave guide - (n.) special transmission medium resembling a pipe and often having a rectangular cross section, inside of which radio waves may be propagated. wavelength - (n.) The distance between wave crests in any type of wave. weak force - (n.) One of the four fundamental forces of nature, weaker than the strong force and the electromagnetic force. It is important only in the decay of certain elementary particles. weight - (n.) The force determined by the gravitational pull on a mass. whistler - (n.) electromagnetic ultra-low frequency radiation observed in planetary magnetospheres; energized by lightning and other discharges. white dwarf - (n.) The final stage of the evolution of a star of between 0.07 and 1.4 solar masses; a star supported by electron degeneracy. white dwarfs are found to the lower left of the main sequence of the H-R diagram. Wien’s law - (n.) An experimentally discovered law applicable to thermal continuum radiation, which states that the wavelength of maximum emission intensity is inversely proportional to the absolute temperature. Wm-2 (or W/m2 ) - (n.) watts per square meter of incident signal flux per whatever resolution bandwidth is in use. It is the total signal flux if the receiving bandwidth equals or is greater than the bandwidth of the signal. Wm-2 Hz-1 (or W/m2 Hz) - (n.) spectral flux density (see Jansky). Wolf-Rayet star - (n.) A type of O star whose spectrum shows very broad emission lines. W Virginis star - (n.) A type of II Cepheid, one of the fainter class of Cepheid variable stars characteristic of Cepheids in globular clusters. x radiation - (n.) electromagnetic radiation in the range of approximately 0.05-100 A. X-ray - (n.) A photon of electromagnetic radiation in the wavelength interval between about 1 Angstrom and 100 Angstroms. year - (n.) The period of revolution of a planet around its central star; more particularly, the earth's period of revolution around the sun. Zeeman effect - (n.) The broadening or splitting of spectral lines caused by the presence of a (strong) magnetic field in the gas where the lines are formed. zero-age main sequence (ZAMS) - (n.) The main sequence in the H-R diagram formed by stars that have just begun their hydrogen-burning life- times, and have not yet converted any significant fraction of their core mass into helium. The zero-age main sequence forms the lower left boundary of the broader band representing the general main sequence. zircon - (n.) a silicate mineral, ZrSiO4, diamondlike in appearance, that contains silica silicon dioxide, and the element zirconium. Some zircons in meteorites contain material from the period before the Solar System formed. zodiac - (n.) A band circling the celestial sphere along the ecliptic, broad enough to encompass the paths of all the planets visible to the naked eye. .In some usages, the sequence of constellations lying along the ecliptic. zodiacal light - (n.) A diffuse band of light visible along the ecliptic near sunrise and sunset, created by sunlight scattered off of interplanetary dust.
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