Astronomy C

[iridium]

Oh... I thought you were supposed to use years and au.

[Crazy Puny Man]

(T^2)/(a^3) = (T^2)/(a^3)

For the formula above, you use Earth years and AU. For the following formula:

P^2=(4π^2)/GM a^3

Use SI units.

Be sure to keep your units straight when you do calculations, and know which units you use with which formulas

[asdfqwerzzz2]

Does anyone have any good resource on stellar evolution of variable stars? I can't really find much.

[syo_astro]

Does anyone have any good resource on stellar evolution of variable stars? I can't really find much.

That isn't exactly a bad question, but...variable star evolution encompasses A LOT >.>. So, if you had trouble with low/normal/high mass evolution maybe I could suggest something more specific. But variable stars in general...if you want a list of tons I could just point out like aavso.org or something. Trust me, a google search of "variable star evolution" or some variant will get something. Any specific problems?

[Crazy Puny Man]

The formula for circular velocity on the wiki...v = \sqrt{GM/R}...can it be used to calculate the orbital velocity of a single star within a binary system, if you assume its orbit is circular in shape?

[syo_astro]

The formula for circular velocity on the wiki...v = \sqrt{GM/R}...can it be used to calculate the orbital velocity of a single star within a binary system, if you assume its orbit is circular in shape?

In some cases! If you can assume circular orbits, and you know your binary math well, you can certainly use it! But with any formula have to get some problems and practice when to use it appropriately . So, I recommend knowing the derivations and other associated formulas.

For example, technically orbital speed comes from v=C/T meaning circumference divided by period (this is because it is the same as distance over time, but in a circle). The derivation takes a few more things, but I won't explain it all unless you need it. There are also elliptical orbits and other things to change it up.

I think the sqrt(GM/r) formula is more used for orbiting bodies at times (that is, a body rotating around another one like the Earth around the Sun or a satellite around the Earth). Those are technically binary systems, but I think some other formulas are useful for that stuff too (especially with stars). So, do your research and good luck! Any questions keep asking, and hope that helps .

[Crazy Puny Man]

What about in a binary of two stars orbiting around their barycenter? Would it be valid then?

[syo_astro]

What about in a binary of two stars orbiting around their barycenter? Would it be valid then?

Well, just to put it more in the context of a test it depends on what's given. With astrometric binaries I guess if they didn't give you a velocity graph, they could give you some info about mass, period, semi-major axis/radius, and then try to link that into barycentric math to find the mass of the other and/or the velocity.

But there are multiple ways to do these problems, so it depends on what's given (that's why I said to look into the derivation/use of everything...also, to check me/others because nobody is perfect XD). For some reason I haven't needed to use that equation much...but it's not bad to know. Know what I mean?

[Crazy Puny Man]

I talked to Luo about this, he said that in the scenario I mentioned I couldn't use the circular velocity formula to calculate the velocity of a star...

But what really bugs me is that (this question was on our state test...a little late to ask this, really ) on the test, I was given all the info about the system (i.e. period, avg distance between the stars) and...I was given that formula on the test, and one of the questions had me derive algebraically the ratio of the velocities of the stars in terms of the radius of the stars' orbits using that formula, which turned out to be v1/v2 = \sqrt{r2/r1} or something like that, and then I had to calculate the actual velocity given that ratio.

But in Carroll & Ostlie, the ratio given is v2/v1 = r2/r1...this relationship right here was also given on last year's nationals test (I think question 5b under Section C, if I remember correctly). So...

I mean, was the test wrong? I think the test writer was the head of the Astronomy department at the university where our state tournament was held.

I don't know if I did that section correctly, but if I screwed it up badly I don't think we would've placed as well as we did... I mean, it sure is possible...

EDIT: I'll try to contact the guy who ran this event and see what was up. But I'm afraid I'll have to wait a while

[asdfqwerzzz2]

I'm thinking of purchasing the "Astronomy Coach's Manual." If any of you have purchased or seen something similar, is it worth it?