Astronomy C

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Re: Astronomy C

Postby RandomPerson » February 16th, 2010, 3:48 pm

Was it about 25 DSO questions followed by 10 multiple choice followed by 5 short answer/FITB, and 2 math questions?
Yup, and one of the math questions was on the Hubble Law, and worded very badly I believe.
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Re: Astronomy C

Postby Glacierguy1 » February 16th, 2010, 9:58 pm

My test had no hubble law. The formulas used were Distance Modulus, Parallax distance formula, Small angle approximation, and modified Kepler's third law for binary mass.
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Re: Astronomy C

Postby walkingstyx » February 17th, 2010, 7:33 am

I have been doing a bunch of old Astronomy tests and I have noticed some kind of discrepancy between the tests. Some of the tests when using the Luminosity to Absolute magnitude formula use the 4.83 number, others use 4.7, and others I can't even guess what number they used to get their answer. Which number is the generally accepted value for "Absolute Magnitude of the sun" at least when using that formula?
It doesn't really matter which you use. The difference between the values you get with 4.7 and 4.83 is about .02, and you really don't need that kind of accuracy. Even the most extreme event supervisor will not require you to have an answer perfect to within .02.

If the spirit matters to you more than the placing, the absolute magnitude of the sun is generally accepted as 4.83.
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Re: Astronomy C

Postby sisomg11 » February 17th, 2010, 7:05 pm

3 questions:

1) Rules clarification: we are allowed to bring a binder AND a laptop, correct?
2) Does anyone know where to find how to determine if an image is taken in radio, visible, infrared, or UV light? I can't find anything on this
3) How does one determine the age of a cluster from its H-R plot? We've missed a few points on this topic lately that the answer keys haven't explained well and I'd like to know.
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Re: Astronomy C

Postby pjgscioisamazing » February 17th, 2010, 7:21 pm

3 questions:

1) Rules clarification: we are allowed to bring a binder AND a laptop, correct?
2) Does anyone know where to find how to determine if an image is taken in radio, visible, infrared, or UV light? I can't find anything on this
3) How does one determine the age of a cluster from its H-R plot? We've missed a few points on this topic lately that the answer keys haven't explained well and I'd like to know.
1) Yes, a binder and a laptop, or a binder and a binder, or a laptop and a laptop... any combination of the 2, as long as you only bring two resources.

2) I'll take a quick look and be back to you on this one.

3) It's turnoff point. You look at its main-sequence turnoff point, where stars literally turnoff and go into the giant branch. The age of the stars at the turnoff point is the age of the cluster. I know this is used for globular clusters, which are the type of clusters on the rules. Not sure about open cluster, but it is probably similar.

Hopefully this helps, and I'm sure someone on here can explain it better (I got the info up there, but I can't quite get it out hahaha) :D
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Re: Astronomy C

Postby sisomg11 » February 17th, 2010, 7:22 pm

Thanks for the tip, but as for point 3 - that basically is what the answer key said - I was more looking for how to determine the age of the stars at that turnoff point. Can anyone explain that?
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Re: Astronomy C

Postby pjgscioisamazing » February 17th, 2010, 7:31 pm

Thanks for the tip, but as for point 3 - that basically is what the answer key said - I was more looking for how to determine the age of the stars at that turnoff point. Can anyone explain that?
That one I'm not quite sure about... :lol:

And about your second point here's some tips I found in my binder:

Optical: Look like optical images and should be easily recognizable
X-Ray: Show phenomena in the millions of degree range, so only the most energetic features are seen, such as single point sources like a central star and shockwave features of Eta Carinae (the most luminous star in the galaxy)
Infrared: Radiated by dust and shows area where dust is concentrated, such as materials ejected from stars, or areas of star formation. (As name implies will often look a reddish color- thermal radiation)
Radio: Associated with magnetic field lines.

For these, i would just look up pictures of different wavelengths.
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Re: Astronomy C

Postby Glacierguy1 » February 17th, 2010, 10:24 pm

The thing about globular clusters is that all of the stars formed at the same time, so you have to assum that the stars at the end of that cluster's truncated main sequence are ridculously close to the end of the Main sequence lifetime for their particular luminosity and Mass and stuff. So the age of the cluster would be the same as the main sequence age for those stars at the end, so for a globular cluster where the main sequence "ends" at where the sun is on the main sequence, the age would be about 10 billion years old.

Generally, star lifetime is proportional to the lifetime of the sun by the equation Lifetime of the star in solar lifetimes(ie 10 Gyr)= Mass in solar masses^-2.5

NB:Source for relation is wikipedia, may not be entirely reliable, but I know that the general relation of Increased mass to decreased lifetime by an exponent greater than (less than -) 2 is correct.
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Re: Astronomy C

Postby sisomg11 » February 18th, 2010, 6:47 pm

Maybe I'm missing something, but how can you tell what the mass is just from its H-R position?
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Re: Astronomy C

Postby Glacierguy1 » February 18th, 2010, 10:12 pm

The main sequence is also a mass sequence. Luminosity in solar luminosities= Mass in solar masses^3.5
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Re: Astronomy C

Postby walkingstyx » February 19th, 2010, 3:32 pm

Also quite a few HR Diagrams have information of this nature already written on them. My personal favorite:
http://www.slackerastronomy.org/slacker ... iagram.png
It comes complete with mass, lifetime, and radius in addition to the normal. And it's pretty.
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Re: Astronomy C

Postby sisomg11 » February 21st, 2010, 11:49 am

Thanks to those answering those questions: now for another. At our last invitational, we got our test back but we haven't seen an answer key. A few of the questions that we missed I'm not really sure how to do (because we thought we did them right and got major points off on them - and still by some miracle managed a silver), so could someone explain these three to me?

1) Careful observations over the course of many years have shown that there seem to be stars orbiting Sagittarius A*. The period of the orbit of one of these stars is measured as 16 years. There is no chnage in the radial velocity of the star over the period of its orbit, and the motion of the star describes a circle with an angular diameter of .24" (arcseconds). For the purposes of this problem, we assume that the distance to Sagittarius A* is 8.3Kpc. What does this tell us about the mass of Sagittarius A*?

2) Careful observations of a number of RR Lyrae stars in this object (attached to a picture of Andromeda) yield an average visual apparent magnitude of mv = 25.3. What is the distance to this object?

3) (attached to a time plot of apparent magnitude in v, b, and r bands) - How far away is this star and by association the DSO that contains it? (The period is given as 13.87 days and it is a Type II Cepheid)
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Re: Astronomy C

Postby Glacierguy1 » February 21st, 2010, 7:35 pm

for number 2, the magnitude of all RR lyrae stars is about 0.7, sojust use that number in the distance modulus.

for number 3, use the v band apparent magnitude and you can find the absolute magnitude using the period luminosity relationship. then use the distance modulus.

I think for number 1, you can use either the small angle approximation or a tangent formula to find the semimajor axis of the star's orbit, then convert it to AUs and use newton's form of kepler's 3rd law. then it will give you an answer about 3.6E6 solar masses and you can neglect the mass of the star next to the black hole.
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Re: Astronomy C

Postby sisomg11 » February 22nd, 2010, 3:52 pm

Can you give me a more detailed explanation on number 1? I understand now on the others, but that one was a bit confusing still.
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Re: Astronomy C

Postby Glacierguy1 » February 22nd, 2010, 4:58 pm

Draw a triangle with the earth at one vertex and the two positions of thes star at the other points. the angle at point earth is .12 seconds and the side conecting earth to the original positon of the star is 8.3 kiloparsecs. use the formula tangent = opp./adj. and the tangent of .12 arcsec times the adjacent side 8.3 kiloparsecs. once you get the answer, about 1000 AU, you can plug it into Newtons version of kepler's third law. It should give you about 3.8-3.9 Million solar masses, comparable to accepted values for the mass of Sgr A*
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