Qualia

Introduction

If someone were to ask what a galaxy is, the simplest answer would be to say a group of thousands of millions of stars, lots of planets, dust, gas and dark matter rotating within the emptiness of space.

If they wanted a little more detail, you could point out that some galaxies are so small they contain no more than 10 million stars or so, while others are so big they could have over a billion stars, that is, a million times a million. 1 followed by 12 zeros. You could say that there are an estimated 200,000 million galaxies in the visible universe and the average distance between them is 15 to 20 million light years.  

Unable to imagine such vastness, it comes as no surprise that even Andromeda, the closest galaxy to us at about 2 million light years, appears on the darkest, clearest nights little more than a subtle misty blur on the edge of visibility.

Knowing that this will be the oldest light you will ever see without the aid of a telescope, cranking open the aperture will still be a night of listening to faint whispers and delicate murmurs as you hunt out galaxies. The world of deep space is a world of almost silence, of muted sounds where nothing breathes and everything is transfixed.

The Milky Way Galaxy

Before venturing out and exploring, it makes sense to appreciate our own surroundings. That way, we have something to compare our own world to. If home is a place on the planet Earth and the Solar system is our neighbourhood, then on this cosmic scale the Milky Way Galaxy can be considered our country, a single island in a world of hundreds of thousands of millions more. 

Our cosmic island is around 13,500 million years old and may contain over 400,000 million stars whose average distance is about five light years apart. These stars are spread across a disc about 100,000 light years across and 10,000 light years deep increasing to around 30,000 light years at the Galaxy’s nucleus. An edge on view of the Milky Way would look like two fried eggs stacked back to back.

At the centre of the Milky Way is a nucleus containing a bulge of stars while surrounding the entire disc is a spherical halo of older stars and globular clusters extending to a diameter of about 130,000 light-years and containing some of the most ancient stars in the Galaxy. There is also a vast outer spherical region made up of hot, ionized gas which could be as much as 600,000 light years in diameter.

Even though there are thousands of millions of stars and each individual star may have a handful of planets, several dozen moons and tens of thousands of asteroids, all this would still only make up a tiny fraction of the Galaxy’s total mass. The Milky Way, along with all the other galaxies, is also in the grip of something mysterious called dark matter. No one knows much about dark matter other than it does not seem to be made of the atoms that make up you and me, or the stars and planets but it outweighs the mass of everything we can detect many times over.

There are over 150 globular clusters in the Milky Way. These objects belong to the Galactic Halo and are compact groups of about a million stars moving together around the Galaxy. Globulars are among the oldest objects in the universe and no doubt there are many more to be found but we cannot see them because of the bright band of light reaching across the sky between us and them which rather confusingly is also called the Milky Way.

The Sun is about two-thirds of the way out from the centre of the Milky Way, around 27,000 light years toward the edge of the visible disc and perhaps about 22 light years above the centre of the Galaxy’s plane.

Like all other stars in the Galaxy, the Sun orbits the galactic centre at about 900,000 km/h and takes about 250 million years to complete one circuit. On that reckoning the Sun and the entire Solar system it carries about with it is about 18 galactic years old while the Sun has barely covered a thousandth of its present circuit in the entire history of Homo sapiens.

If we were to fly over the Milky Way, we would see a central bar crossing over the bulge composed mainly of stars and about 30 light years long. From here four tightly wound arms are spiraling from the centre. It is for this reason that the Milky Way is known as a Barred Spiral galaxy.

The arms contain mainly hot, young stars which are typically bright and big. Because the more massive a star is, the more intensely it must burn its fuel to hold itself up against the pull of gravity, the more quickly it runs through its fuel and the shorter lived is the star. Such stars may live as little as 10 million years and as such, spiral arms are the sites of rich star formation and death. The Sun is a smaller, mellower star which although forming in one of the spiral arms does not burn nearly so brightly and will outlive many of those hotter stars by a thousand million years.

At present the Sun belongs to a large concentration of stars located in the Orion or Local Arm, which is a kind of star bridge between the two major arms of Sagittarius and Perseus. The Orion Arm contains about 25 Messier Objects the majority being open clusters with a small handful of planetary nebulae such as M 27, M 57 and M 97.  

Stars such as the Sun which form in the arms and disc of the Milky Way are called Population I stars. These contain the recycled material from previous generations of stars which died long ago and so have a greater abundance of elements heavier than helium, including the elements which make up planets and all known life.

Older stars are found further out either in the galactic halo or the bulge at the centre of the Milky Way. These are known as Population II stars and their ages range from 10,000 to 13,000 million years old. They tend to be redder than Popuation I stars and less rich in elements heavier than helium. Old star clusters are called globular clusters and because they swarm around the Galactic centre like bees around a hive, they have been used to locate the centre of the Milky Way.

If all the stars are circling the galactic centre at colosal speeds and those stars nearer the centre are rotating more rapidly than the stars in the outer regions and thus ceaselessly changing their positions relative to one another, how does the spiral pattern remain?

There might seem an easy solution. Stars orbit the Galaxy and the faster stars nearer the galactic centre twist around the nucleus faster than the more distant stars which overtime produces the Milky Way’s spiral shape.

The problem is we know that if the idea were correct the spiral would become tightly wound and smear out within 1,000 million years or so.

Spirals persist because they appear to be riding a kind of shock wave or sonic boom called a density wave. These waves move through the galactic disc, just as sound waves might move through the air or ocean waves pass through water, compressing and slowing down different parts of the disc at different times.

The spiral arms we observe are defined by the denser clouds of interstellar gas and stars the density waves have created and so in a way, spirals are not great masses of stars being transported from place to place, but patterns moving through the Galaxy.  

The younger stars found in the spirals are the visible feature of the shock waves travelling around the Galaxy. Clouds of gas and dust are also being squeezed as the density waves go on their way. Some of these clouds get compressed sufficiently to trigger star formation.

This process begins in large clouds of dust and gas thousands of light years across and containing the material to build millions of stars like the Sun. Within these gigantic cloud complexes are significantly smaller, tighter knots of clouds whose collapse was probably caused by the impact of density waves and the turbulence from the explosions of super massive stars known as supernovae.

With the pull of gravity, the gradual knotting of these denser clouds leads to the formation of a star bearing core which gains mass as its gravity pulls in material from the surrounding area. The mass of the star is dependent on the quantity of material nearby and once the star begins to shine, the radiation from it blows away the rest of the cloud.

By the aid of density waves and supernovae, star formation and star death becomes a self-sustaining process within the vicinity of the spiral arms. The balance between interstellar material being converted into new stars and the amount of material thrown back into space by dying stars maintains the Galaxy’s spiral shape and enables the process of star birth, life and death to continue for billions of years.

Most newly born stars are part of binary or triple solar systems; many will form open clusters while solitary stars like the Sun are rarer. Whether the Sun was part of a more complex star system is open to debate which perhaps is ultimately premised on the anthropic principle. If our Sun were part of a more complex star system or cluster, we would probably not be here to observe it, for either life would never have developed or evolution would have taken a vastly different course.

Astronomers estimate that there are over 100 million black holes in the Milky Way Galaxy, the stellar remnants of stars whose mass was at least twenty times that of the Sun.

In the life of a star there is a cosmic game of tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the star produce sufficient pressure to balance the force of gravity and depending on its initial mass the star will remain stable for millions, if not thousands of millions of years.

When a star nears the end of its life and starts to run out of fuel, gravity takes over and the material in the core is compressed even further. The more massive the star’s core, greater is the force of gravity that compresses it.

For smaller stars, the repulsive forces of electrons within the star eventually counter further gravitational collapse. The star cools; it releases its atmosphere in the form of a planetary nebula and dies peacefully. What remains is a star cinder we call a white dwarf.

More massive stars explode as supernova; the star’s atmosphere is violently expelled into space while the core unable to hold itself up under its own weight, continues to collapse, shrinking to a point of zero volume called a singularity. All the particles, the atoms, neutrons and electrons which made up the star’s core are crushed out of existence and the gravitational attraction of the object becomes so powerful that nothing can escape, not even light.

Needless to say, black holes are invisible but we know they exist because of the gravitational influence they have on their surroundings. Some black holes are in orbit around binary systems and astronomers can detect the effect of the black hole on its companion’s orbit. Black holes can also rip matter from the companion star, which tumbling into its hungry mouth gets hot enough to emit x-rays as the particles speed up and collide.

In principle any object can become a black hole if it is sufficietly crushed. There is a critical radius for which this will occur for every object. The Sun would have to be compressed to about 3km in radius, the Earth about 1cm, and you and I to something smaller than a neutrino. Black holes are simply matter squeezed to extremely high densities but the black hole at the centre of the Milky Way is something a little different.

About 27,000 light years from Earth, at the core of the Milky Way is a huge black hole. Our Galaxy’s nucleus lies in the direction of the constellation Sagittarius. Unfortunately, there is a significant amount of gas and dust in the plane of the Milky Way, so all visible light from this direction is blocked from view.

However, with longer wavelengths like radiowaves and infrared we can penetrate the dust to garner a better understanding of the galactic centre. The centre of our Galaxy is known as Sagittarius A and from radio telescope observations we know that it is made up of three components.

The first is Sgr A East, an expanding bubble of gas associated with a supernova remnant. The second is Sgr A West, a region of hot ionized hydrogen gas and finally there is Sgr A*. This is an area revealing the traces of shock waves from recent star explosions. There are also x-rays and gamma rays pouring out from the region and there is indication that a dense cluster made up of over 20 million stars are packed into a volume of about 3 light years across while at its centre is a supermassive black hole.

Stars close to this black oblivion orbit it at some 32 million km/hr. They are moving so fast that even though they are very far away, their positions taken in infrared are seen to change within a matter of months. This orbit also tells us that the stars are in the grip of a monstrous object with about 4 to 5 million times the mass of the Sun yet contained in a volume of space a little less than the distance between Mercury and the Sun.

Moving far from the raging black hole and out towards the halo, we come across vanishing stellar streams. These appear to be long filaments of stars all with a similar composition but moving at an angle relative to most of the other stars. More than a dozen such streams have been identified ranging in length from some 20,000 light years to as much as a million.

Stellar streams are created when galaxies come too close to our own and are gradually torn apart by gravitational forces. The stars are pulled from the smaller galaxies by tidal waves until eventually all that remains of the galaxy are these vanishing stellar streams merging with the Milky Way.

One of the most spectacular is the Sagittarius stream which extends over a million light years and bridges the Milky Way to the Sagittarius dwarf elliptical.  One of the most studied is the Arcturus stream, located about 37 light years away, containing the star Arcturus. Arcturus and its stream is a remnant of a dwarf galaxy devoured long ago by the Milky Way.

Just like many other creations of nature the Milky Way has consumed its way to its present size, swallowing up lesser objects through intergalactic cannibalism. The interaction of galaxies is not confined to greater galaxies devouring lesser ones. The light from M 31, the Andromeda galaxy, shows a blueshift corresponding to an approaching velocity of some 360,000km/h.

This means that in about 4,000 million years time, just as the Sun is ending its own life and Earth has died long before, Andromeda and the Milky Way will collide. The stars in each galaxy will probably experience few collisions due to the colossal distances between them but the structure of both galaxies will be destroyed, perhaps forming a giant elliptical galaxy. If there is anyone about to call this galaxy home, a new story on the evolution of the Milky Way will begin.

A Brief History

If you’ve arrived this far, then a little history is in order.

It’s amazing to think that our understanding of the universe, a huge amount of expanding space generously littered with thousand of millions of other galaxies is a rather recent story, no more than a hundred years old.

At the beginning of the last century, astronomers were still debating the nature of nebulae, those fuzzy, spiral shaped clouds found all over the night sky. Were they part of the Milky Way or were they galaxies like our own but very far away?

The story of galaxies probably began back in 1750 when Thomas Wright reasoned that the Milky Way formed a disc made up of thousands of stars and that the Sun was not at the centre of the disc but out to one side. He went on to suggest that the cloudy blobs of light seen in the night sky probably resided outside the Milky Way. Kant called them island universes.

By the late 18^th century astronomers were eager to find comets which were easily mistaken for the fuzzy nebulae. Careful cateloguing became essential and two observing masters, Messier and Herschel, plotted many such nebulae. Indeed, Herschel positioned some 2,500 most of which are now known to be galaxies.

Over the following decades, many nebulae were resolved into clusters, others were identified as glowing clouds of gas within the Milky Way. But the spiral structured nebulae remained a mystery. Many astronomers at the beginning of the twentieth century would still agree that they were interstellar clouds of dust and gas.

In the 1920s, Edwin Hubble, using a 100" telescope, detected variable stars in several nebulae. His discovery was revolutionary because now distances could be measured.

Variable stars have a characteristic pattern resembling other stars called Cepheid variables. Henrietta Levitt, had shown there was a correlation between the period of a Cepheid variable star and its luminosity, its intrinsic brightness.

By knowing the luminosity of a star it is possible to measure the distance to that star by measuring how bright it appears to us. The dimmer it appears the farther away it is. Thus, by measuring the luminosity of these stars, Hubble was able to show that the nebulae were not clouds within our own Galaxy, but were external galaxies far beyond our own.

Hubble's second revolutionary discovery was based on comparing the measurements of these galaxy distances to the recession velocities, or redshift of them. He showed that more distant galaxies were moving away from us more rapidly.

When these two discoveries were put together, it dawned on astronomers that the universe must be expanding. Hubble’s work marked the beginning of modern cosmology.

Looking Out From Home

There’s a useful principle called the Copernican Principle.

It accepts that Earth is not at the centre of the universe but moves around the Sun which in turn is not at the centre of the universe but is quite an ordinary star, occupying no privileged position in the Milky Way, let alone in the universe itself.

The principle is useful for it allows us to infer the likelihood of the nature of the universe. If the Earth and the Sun are occupying no special place and the Sun appears to be a pretty ordinary star as far as stars go, then in like manner so too with the Milky Way. It must also be pretty similar to other spiral galaxies.

It was one of the justifications of the Hubble Telescope to put this inference to the test by calibrating Hubble’s Constant – the redshift or recession velocity of galaxies known to describe the expansion of the universe – and then measuring the angular diameter of a good number of galaxies resembling the Milky Way to determine average size.

Needless to say, in agreement with our principle, the Milky Way was found to be a pretty ordinary spiral just a little smaller than average.

Another key insight of Hubble was to note that there were different kinds of galaxies of which spirals were just one. Following this discovery, all galaxies are defined according to shape.

Hubble is credited with creating a classification scheme for galaxies, which is usually referred to as the Tuning Fork diagram.

Spirals

Spiral galaxies usually have two main parts. The first is a flat disc containing a lot of gas and dust between the stars which means quite a bit of star formation is taking place within the disc, particularly in the spiral arms where we find a lot of hot, young stars and their clusters and a large bulge component consisting of old Population II stars related to the globular clusters.

Spirals are further sub-divided into regular spirals and barred spirals and then further sub-divided depending on the size of the central bulge and how tightly the arms are wound around the centre.

The ‘a’ group has large bulges and tightly wound spiral arms and the ‘c’ group have almost no bulge and very loose arms. The Milky Way is somewhere in between and because we cannot really see the Milky Way, it is loosely classified as a ‘b’ and ‘c’ type spiral galaxy.

The ‘SB’ group has a large, well defined bar that passes through the bulge with subclasses of a, b, and c.  The Milky Way is believed to also have a bar, so is finally classified as a SBb or perhaps SBc type spiral galaxy.

Lenticulars

Some galaxies have no spiral arms and these are called ‘S0’ or Lenticular galaxies. Lenticulars are spiral galaxies without spirals. That is, they are disc shaped galaxies but where stellar formations have stopped and so consist of mainly old population II stars. For all intents and purposes, they can hardly be distinguished from ellipticals.

Ellipticals

Elliptical galaxies are the cosmic rugby balls of the universe. They are typically smooth looking and ellipsoidal in shape and unlike spirals, do not seem to rotate as a whole. Elliptical galaxies appear to have very little interstellar matter between the stars and so consist mainly of old population II stars.

Most Elliptical galaxies are small and dim and are sub-divided according to how flat they appear. The letter ‘E’ is followed by a number, bigger the number flatter is the galaxy.

Irregular

Irregular galaxies are where all the other types go for they have no definite stucture. Often distorted by the gravitation of their intergalactic neighbours, these galaxies typically exhibit peculiar shapes. Some irregulars have a lot of dust and gas so star formation is possible. Others have very little star formation going on.

Messier's Galaxies

When it comes to observing galaxies, Messier’s list is a good place to begin. These can be grouped into two main categories: Spring Galaxies and Autumn Galaxies.

Spring

Galaxies in Virgo: M 49, 58, 59, 60, 61 84, 85, 86, 87, 88, 89, 90, 91, 98, 99, 100. 

Galaxies in Leo: M 65, 66, 95, 96, 105.

Galaxies in and around Ursa Major: M 51, 63, 64, 81, 82, 94, 101, 102, 106, 108, 109.

Autumn

Andromeda - The Local Group: M 31, 32, 33, 110.

Pisces: M 74.

Cetus: M 77.

The Messier galaxies can also be placed into their respective types:

Spiral Galaxies

M 31, 51, 58, 61, 63, 64, 65, 66, 74, 77, 81, 83, 88, 90, 91, 94, 95, 96, 98, 99, 100, 101, 104, 106, 108, 109.

Lenticular Galaxies

M 84, 85, 86, 102.

Elliptical Galaxies

M 32, 49, 59, 60, 87, 89, 105, 110.

Irregular Galaxies

M 82, 51B

NGC Galaxies

It is also of interest to work through some non-Messier galaxies. Here I've included some of the brightest galaxies in the night sky and should have a magnitude brighter than 10.

Autumn

Spirals: NGC 253, 891, 1055, 7331, 7479.

Irregular: NGC 6822

Winter

Spiral: NGC 247, 253, 613, 1023, 1232, 1398.

Eliptical: NGC 185, 1395, 1407.

Spring

Spiral: NGC 2403, 2683, 2841, 2903, 3184, 3344, 3521, 3628, 3953, 4490, 4526, 4535, 4565, 4559, 4571, 4631, 4656, 4699, 4725, 4753, 5005, 5068, 5247, 5907, 6946.

Elliptical: NGC 3115, 3384, 3585, 3607, 4125, 4494, 4636, 4697.

Lenticular: NGC 3115.

Irregular:  NGC 2976, 3077, 4214, 4449, 5195.

Summer

Spiral: NGC 5866, 6946 and of course, the Milky Way band of stars.

What to Expect

There is a kind of deep space disappointment for many beginners when they set out to observe galaxies. Perhaps they rightly expected to find more than a smudge of light at the eyepiece, perhaps they expected Hubble like images.

But just as music is interpreted on the silence built around it, so to with observing galaxies. As sometimes happens, those moments in deep space when you have eventually caught a galaxy and you can say to yourself, “I’ve seen it! I’ve finally made visual contact with an object millions and millions of light years away”, remains much more than just a moment.

If there is something as grand as an art to galaxy observing, it may consist in nothing more than being sensitive to each moment, wholly receptive and regarding that moment as utterly new and unique.

Here are just a few sketches of galaxies taken with a 10" to give some idea of what to expect.

Sketches with a 10"

NGC 3953

NGC 3184

NGC 2903

NGC 3226/7

NGC 2683

NGC 2859

NGC 2964/68

M 109

Observing Tips

As with many things in life, observing galaxies is dependent on many variables, not least of which is experience. Within reason, the more you practice, the more you will see. Assuming you’ve already got your telescope and eyepieces, some general advice can be offered. Needless to say, there are many threads on SGL which cover these points in more detail.

Stellarium - useful for planning sessions, seeing what is about, learning the positions of constellations and much more.

Star Atlas - quite literally, you’ll be lost without one.

RACI Viewfinder - helps when it comes to hunting out deep sky objects.

Telrad or Rigel - helps to aim your scope in a given part of the night sky.

Low Magnification Eyepiece - your star-hopping workhorse.

Observing Chair - to be patience you need to be comfortable and so you need to be seated.

Red Light - helps retain night vision when you need to see something like your map etc.

Eye Patch - keeps both eyes open while observing, since squinting strains the working eye.

Light Pollution - the single worst enemy for stargazing is light pollution. A small telescope in the countryside will show faint objects better than a larger scope in the city. In effect, darker the night skies, brighter the deep sky objects.

Dark Adaptation - the human eye takes time to adjust to the dark. If you have planned an evening with deep space objects, try not to observe any bright celestial object such as the Moon or planets with your working eye. It’s best to let your eyes adjust to the dark for 30 min or so before starting a galaxy observing session.  This will give the rods in your eye time to adapt, for very faint objects like galaxies cannot be seen easily with the cones.

Clothing - you’re observing at night when it is the coldest, when the body and mind is most sensitive. Effective protection requires three-layer clothing. The inner layer should be made of cotton or synthetic microfiber. This will help absorb sweat. The middle layer is made of heat insulation material to keep body-heat cocooned. The outer layer transmits water vapour to the environment and keeps out the wind. Also, pay particular attention to your hands, feet, head and neck.

Averted Vision - looking directly at deep space objects might not be the best method for observing them. Practice by centering a dim star or DSO in the centre of the eyepiece’s field of view and concentrate your attention on an area just a little off to one side or above. Alternatively, place the object a little to the side or below the centre of vision. Either method should work, but finding your sweet spot will be a matter of trial and error.

Don’t Squint, Jiggle and Breath - try not to squint when observing celestial objects, for by doing so you are not only straining your working eye which can lead to fatigue but also limiting your powers to detect faint objects. There may also be occasions where you are certain you have the specific area where you think the deep space object resides but it appears to be lurking under the limit of visibility. If this is the case, tap the telescope or eyepiece just a little to make the field of view jiggle. It’s not guaranteed but you might find the object revealing itself. Again, when you are concentrating you might find yourself holding your breath without realizing it. Limiting the oxygen to your brain, even for just a few seconds, compromises night vision. So while observing its good practice to breathe steadily and deeply but in a calm and relaxed fashion.

Aperture and High Power - when it comes to viewing deep sky objects, aperture rules.

Sketching and Log Book - better to be a visitor, rather than a tourist. There are two essential features to visual astronomy. The first is to find the object and the second is to observe it. The former process involves star-hopping and reading star maps, the latter requires you to slow down and to engage yourself with the complexity and beauty of what is being observed. It's been said many times before but anything glanced at will always look like a featureless something or other but the trick is to go beyond this style of looking and practice picking out features and textures. It is important to slow down from time to time and sketch or write about what you are seeing.

Planning - a session is a good idea and part of this is to check out sketches by other observers. Drawings should give you an idea of what the object will more or less look like if you were to use similar aperture and magnification.

Patience - master patience you'll be master of yourself and the night sky is a good teacher. If you don't succeed one night, or you can’t go out for weeks on end, don't be down hearted. In most cases, during that time you've probably discovered something new about yourself and those stars and DSOs will be back to give you another chance, another night.

Cloudy nights - stargazing is a hobby that can be a tiresome road and one can suffer for it and be grieved, but the worst you can do is add to this frustration and curse those things beyond your control. Cloudy or uneventful evenings are just that, nothing more and when older they will appear as a singular, non-descript events, yet shining from them like a host of gleaming stars will be those evenings where everything just seemed perfect and the universe, at last, could murmur to you its secrets.