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# JulianO

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653

1. ## Luminosity of a star in solar units

I'm not sure what you are trying to calculate, if you know the star is 0.01 Lsun, then you already have the answer! What question are you trying to answer, and what data do you have?
2. ## Luminosity of a star in solar units

These are all ratios, so it will give an answer in terms of solar luminosities. I.e. and answer of 1 would be the output of the sun. R should be in meters and divided by the solar radius in meters, or both in kilometers, or both in solar radii, as long as both R's are in the same units you're fine. Same with T, but as solar temperature isn't often used as a base unit, I'd suggest K in that case. So for a star twice as big in radius as the sun, and at 12000K it would be approx L = (2/1)2 * (12000/6000)4 = 4 * 16 = 64 solar luminosities.
3. ## Proverbial Candle (Type 1A)

I think the stealing of material is a relatively slow process - remember it has to gather a significant mass to go supernova, maybe a tenth of a Sun, so it's probably not a quick process. I don't know in detail why they produce different elements, but its probably because of the way they go off. In a type 1a, they are tipped over the limit and suddenly the oxygen and helium which is in a degenerate state starts to burn. As degenerate matter is not much affected by heat, it doesn't explode immediately, but burns as a wave across the dwarf and then explodes. Strong silicon lines show up in a SN1a explosion, which is how they are identified (and no H lines). After a few weeks, strong Iron lines show up, so we know they make a lot of iron. So - its basically a wave of burning zooming across a very hot cinder that causes a type 1a A type 2 is caused by the star collapsing in on itself, imploding. A lot of the elements it has made are destroyed in this process by photodisintegration, so it almost starts from scratch. It crushes down to a neutron star and then bounces somehow (not well understood) and explodes outwards. Most of the energy is lost as neutrinos, but the remainder drives the nuclear reactions that make heavy elements. As the collapse sort of resets the state somewhat, I guess it makes more of the light elements than the heavy ones. Should say I'm not an expert on this stuff!
4. ## Proverbial Candle (Type 1A)

No, because the SN1a is giving out light, so you see emission bands, and the light is enough to outshine the galaxy. Some of it may get absorbed in gas and dust on it's way out of the galaxy, but that would come out as absorption. Just out of interest as type 1a's are a result of a companion white dwarf munching on material from a healthy neighbour and not core collapse is it thought that they still produce the heavy elements at supernova and do they still result in the dense neutron star remnants? Yes lots of heavy elements are produced from SN1a, but a different set from the core collapse type. SN2 tend to produce more of some elements called alpha elements, such as magnesium, whereas SN1a tend to produce more iron. This is a useful way to let you date galaxy formation events. As to whats left over, I suspect it would be in the white dwarf category. The star wasn't big enough to form a neutron star in the first place, so formed the white dwarf that went SN> As it looses a lot of mass and bts in the explosion, whatever is left must be smaller than it started, so can't really be a neutron star. This is just my thinking though without any evidence, I'll see if I can find out!
5. ## Proverbial Candle (Type 1A)

Not all 1a's are quite the same, so there is still some tweaking to do to get a good luminosity reading. However all 1a's have a characteristic spectroscopic signature, so by looking at the spectra you can determine what sort it is. This is why there is a 1a,1b,1c etc - they looked sort of similar, spectroscopically but have different lines in them. It turns out 1b & 1c are produced by a completely different mechanism (core collapse), but that's where hindsight is so useful
6. ## Galaxy rotation velocity question

We have not directly detected dark matter, but we have good evidence that it exists. Not 100%, but in science we never really get to 100%. The neutrino was proposed in theory in 1930, it was 1954 before one was detected directly, and that was with something you could make in the lab pretty easily. Even now they are very hard to detect. Dark Matter may not even be detectable directly, it may have a very very tiny cross section - or indeed none at all. The universe doesn't have to play by a set of rules meaning we can detect everything directly. The fact is we have about 5 or 6 lines of independent evidence that all point to something very much like a missing particle. A missing particle fits with all the data. Are there other possible explanations? Yes of course, but much like the Ptolemeic system, you have to add several layers of complexity in to get them to work out. Missing mass sort of just works pretty much, tinkering with gravity less well, other systems struggle even harder. It just makes the most sense with the data we have. We have very good data on a lot of stars in the milky way, we will have even better data soon with Gaia. If the stars were flying apart, and exiting the galaxy, we'd pretty much know by now.
7. ## Galaxy rotation velocity question

We can work out the approximate age of Andromeda, and even some of the stars in the outer reaches, and its clear they are billions of years old, and the flinging out time scale would be on the order of a few million years, so they really should have gone by now several times over. Additionally, from rotational stability dynamics, a spinning mass of particles such as the stars in Andromeda would become unstable and the disk would start to break away from the smooth disk, forming clumps and oscillations (like a badly balanced wheel) even if the velocities are right for the mass - there is a certain ratio where disks can be kept stable, and the MW and Andromeda exceed that without adding DM, so that's another good reason we think there is DM present. (There are several more).
8. ## Galaxy rotation velocity question

The stars further out are moving too fast if you ignore dark matter. The galactic disk would be unstable, and stars would be flung out. Dark matter is added so there is more mass, and so more gravity to hold them in their orbits. The stars can go around faster if there is a stronger restoring force. You can spin a rock on a rope faster around your head with a stronger string sort of thing. Basically to keep in orbit too forces have to balance, that of gravity pulling it in, and centripetal pushing it out. So you have F = Gm1 m2 / r2 being the force of gravity - m1 is the mass inside the orbit of the star m2 at distance r. F = m2v2/r being the centripetal force a star moving at velocity v experiences. These must be set equal to keep the star where it is Therefore: Gm1 m2 / r2 = m2v2/r which simplifies to Gm1 / r = v2 For a star in a fixed orbit r is fixed, G is a constant so can't change, which means the thing controlling the velocity v of the star is the amount of mass inside its orbit. We can estimate this from the number of stars, and there isnt enough to satisfy the observed velocity. Therefore there must be more "dark" (unseen) matter to compensate - or alternatively the equations that work so well on earth and the solar system do not apply on galactic scales.
9. ## Galaxy rotation velocity question

They are balanced by the gravity of the dark matter surrounding them. So - yes they are moving at the right speed, but partly because we throw in enough dark matter to make it so.
10. ## Slowing down light

As far as we know, it stayed the same speed, its just space was stretched out "under its feet" very quickly. Inflation is partly brought about as a theory to stop us having to increase the speed of light, and still end up with the universe as it is.
11. ## Galaxy rotation velocity question

Depends how much dark matter you add, but if you add the right amount, then they are just fine too. Typically DM dwarfs the stuff you can see. E.g.,
12. ## Galaxy rotation velocity question

The ones around the centre are ok, its the ones further out that are going too fast.
13. ## Catching a rogue planet

I think most eject planets will be very small. We've only seen the big ones, because they are the only ones we stand a chance of detecting. Simulations of planetary formation show a lot of stuff getting ejected or thrown into the sun. Similarly Jupiter has cleared out large numbers of asteroids, sending them into the sun or outer space. I think I'm right that you need a 3 body interaction to eject something typically, and I think the same would be true of capture. So the planet would have to interact with Jupiter or similar to be captured into a stable orbit around the sun. Taking 30AU encompasses the outer planets, and a small object passing beyond the orbit of Uranus would be pretty hard to spot, and fairly unlikely to interact. A volume of a sphere of 5 AU diameter might be better for our own system, as that brings it within the range of Jupiter.
14. ## Catching a rogue planet

Rogue, or free floating planets are probably quite common. The way planets interact as they form, some of them will get ejected and cast loose. The chance of one coming through our solar system is probably very very small. Mostly because space is very big, and planets are very tiny in relation to the separation. So the chance of a free planet encountering another system is pretty unlikely. Also for a planet to be ejected from the solar system is has to be going faster than about 42 km/s, so at this speed it is unlikely to be caught by another system maybe just deflected a bit. So being captured is pretty hard, it has to land in the right place, and then would need some special interactions with Jupiter and the Sun to slow it down enough. Not impossible, but far more likely it was made in place.
15. ## Wave-Particle Duality

Its Feynmans Multiple Paths theory. Basically if you do the maths and say "lets suppose it can take every possible path between A & B" you find that due to quantum interference most of them cancel out an amount to nothing, and the directish (to some fine tolerance) ones are the only ones left over. Put a double slit in the way, and you distort this and find there are a couple of ways it can go, and interfere with itself... well thats the 10 second version anyway!
16. ## Galaxy mergers can create spirals, perhaps?

Major mergers don't happen very often, well they do in galaxy clusters, but less so out in the field. Its an ongoing area of research, so ideas are still somewhat fluid.
17. ## Galaxy mergers can create spirals, perhaps?

It depends a lot on how much gas is involved in the collision. Dry mergers (galaxies with little gas) almost always seem to produce ellipticals. "Wet" mergers in contrast can produce spiral like or lenticular depending exactly on the interaction of the gas and if it ends up swirling around and making new stars.
18. ## measuring stellar distance for the intellectually challenged

Both Hertzsprung and Shapley tried to calibrate based on nearby Cepheids for which there were known parallax measurements. There were a lot of issues to solve about extinction and so on before a firm equation for the distance was worked out, and of course there was then the issue of Cepheid II's!
19. ## measuring stellar distance for the intellectually challenged

The Cepheid variable relationship was worked out by Henrietta Swan Leavitt observing Cepheids in the Magellanic Clouds. This is a smallish (astronomically speaking) cluster of stars a ways outside our galaxy, so to all intents and purposes are more or less all at the same distance. A bit like observing the lights of Manchester from London, they're all about the same distance away. So if there is a relationship between their brightening and dimming and their luminosity, you can tell that the relationship is pretty much fixed. It turned out there is a little more to it in the case of Cepheids, but that's mostly what it is. So once you have calibrated this and worked out how this equates with distance, you have a convenient yardstick
20. ## Observing From Earth

It's sort of impractical. You can't keep that hole through the atmosphere open - indeed I don't think you can open it in the first place, and there is a lot of pressure counteracting it so it would be a lot of effort. So - the solutions are place telescopes on very high mountainsput them in spaceremove the atmospheric shimmer by computer all of which are being done now.
21. ## Gravitational wave doubt

Yes - has been quite a popcorn fest
22. ## Black hole collision.

A bigger black hole, although a lot of gravitational waves produced just before hand.
23. ## DM detected?!

In other news - looks like possible resolution to the Fermi paradox http://arxiv.org/abs/1403.8146
24. ## DM detected?!

Interesting ... not heard anything about that one!
25. ## A Question about the SN in M82

Agreeing with the details above - mostly Not all type 1a are quite the same, they need to be normalised, which means taking the width of the light curve to reduce them down to the same thing - see below. They are nearly the same though. Also there is some suggestion that some type 1a may be two white dwarfs merging, which means more than 1.4 solar masses and a slightly different curve. Nothing is ever straight forward I also think to be strict they don't quite reach the Chandrasekhar limit, but go off just before, allowing the degenerate matter to burn, which it does in such a way as to not expand much, so goes off almost all at once. Nit picking I know... sorry!
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