Astronomy C

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Re: Astronomy C

Postby Crazy Puny Man » June 11th, 2013, 8:33 pm

Hi :) I'm working on astronomy over the summer, and I have some questions (several, really) about spectroscopic parallax:
1. Can you use this for any star? I understand that you can only really estimate absolute magnitude given spectral classification for main sequence stars, but you could still use the distance modulus for any star, right?

2. This website http://outreach.atnf.csiro.au/education ... allax.html mentions a B-V color index that they somehow use to do spectroscopic parallax for a red giant (Gamma Crucis, in the example). How does knowing the color index show that it's a red giant? (And for the example, what is the significance of Gamma Crucis being a M3.5 III star? I know that the M3.5 is the spectral classification, but what is the III? In the explanation part, they mention luminosity class--is that what this is by any chance?)

3. At the end of this webpage there's a little comment about how spectroscopic parallax "is not accurate for individual stars...[but] can yield statistically useful values" when done for many stars--why do we do spectroscopic parallax, then?

4. The Science Olympiad training handout for astronomy (http://www.soinc.org/sites/default/file ... andout.pdf) mentions that spectroscopic parallax tables might be a nice thing to have--what exactly is a spectroscopic parallax table? I googled it but couldn't find anything that seemed even vaguely useful.

Thanks so much for any help :).
1. As long as you have 2 of the three variables in the equation, you can use it for any star. You can even use it for supernovae (specifically, Type Ia-which is a standard candle)
2. Finding the color index of a star is an indirect measurement of its temperature (I think there's a way to convert-correct me if I'm wrong on this one, please), so you can correspond a star's color index with its temperature/class. There are some H-R diagrams that use a B-V color index along its horizontal axis instead of temperature or spectral class
3. I'm confused by this remark too. I can only think of two things: a) it won't work for optical binaries or b) (I think this is more probable) that spectroscopic parallax has only real application when used upon a variety of stars (i.e. to calculate distances to larger objects, such as clusters)?
4. I think what they're referring to is a table of spectral data. For each spectral class (like, say, B1, B2, B3...etc) they list the characteristics of stars at each respective class, such as luminosity, radius, surface temp, etc. I think you'll have to do a little digging to put one together

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Re: Astronomy C

Postby iridium » June 12th, 2013, 7:00 am

Thanks so much for the help :).

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Re: Astronomy C

Postby Crazy Puny Man » June 12th, 2013, 8:39 am

Sure, no problem :D

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Re: Astronomy C

Postby iridium » June 13th, 2013, 10:24 am

Ok, I have another question...

I was looking at tests in the wiki, and on the "2013 Invitationals Astronomy" test, question 13 is this:
If Earth is 3.156E7 seconds from the sun, and 1.4957E11 meters from the sun then use the Law of Harmonies and calculate the Earth’s T2/R3 (it has superscripts, but I don't know how to do that here) ratio.
a. 2.975E-19
b. 2.977E-19
c. 2.854E-19
d. 2.999E-19

I know that the Law of Harmonies is Kepler's Third Law, so wouldn't earth's T2/R3 ratio be (1year)2/(1au)3 = 1? How does one get anything to the negative 19th power from this? And (I might as well ask this, too...) why are you told that the earth is 3.156E7 seconds from the sun? It can't be light-seconds, so I'm a little lost.

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Re: Astronomy C

Postby cytokid101 » June 13th, 2013, 11:02 am

Ok, I have another question...

I was looking at tests in the wiki, and on the "2013 Invitationals Astronomy" test, question 13 is this:
If Earth is 3.156E7 seconds from the sun, and 1.4957E11 meters from the sun then use the Law of Harmonies and calculate the Earth’s T2/R3 (it has superscripts, but I don't know how to do that here) ratio.
a. 2.975E-19
b. 2.977E-19
c. 2.854E-19
d. 2.999E-19

I know that the Law of Harmonies is Kepler's Third Law, so wouldn't earth's T2/R3 ratio be (1year)2/(1au)3 = 1? How does one get anything to the negative 19th power from this? And (I might as well ask this, too...) why are you told that the earth is 3.156E7 seconds from the sun? It can't be light-seconds, so I'm a little lost.
I'm pretty sure it means that the Earth's orbital period is 3.156E7 s. You calculate the T2/R3 ratio in the SI units they give you (meters and seconds). I got B if I'm not mistaken.

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Re: Astronomy C

Postby iridium » June 13th, 2013, 11:18 am

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

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Re: Astronomy C

Postby Crazy Puny Man » June 13th, 2013, 11:21 am

(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

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Re: Astronomy C

Postby asdfqwerzzz2 » June 23rd, 2013, 9:06 am

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

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Re: Astronomy C

Postby syo_astro » June 23rd, 2013, 1:45 pm

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?
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Re: Astronomy C

Postby Crazy Puny Man » June 23rd, 2013, 2:11 pm

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?


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