Astronomy C

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raxu
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Re: Astronomy C

Postby raxu » February 2nd, 2017, 4:45 pm

What is the accepted value of Hubble's Constant, H_0? The most recent measurements have fluctuated between 67 and 73...

Would you expect the grader to accept any correct calculations with a reasonable H_0?
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Re: Astronomy C

Postby Avogadro » February 2nd, 2017, 5:25 pm

raxu wrote:What is the accepted value of Hubble's Constant, H_0? The most recent measurements have fluctuated between 67 and 73...

Would you expect the grader to accept any correct calculations with a reasonable H_0?

My notes have it as 73.8, based off of NASA. I'd expect the grader to accept it if it's reasonably close, but it's probably best to just keep up with the constant, since using a constant of 67 would yield a pretty different result than using 73.8 to the point where it may just appear completely wrong.
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Re: Astronomy C

Postby Unome » February 2nd, 2017, 5:30 pm

Avogadro wrote:
raxu wrote:What is the accepted value of Hubble's Constant, H_0? The most recent measurements have fluctuated between 67 and 73...

Would you expect the grader to accept any correct calculations with a reasonable H_0?

My notes have it as 73.8, based off of NASA. I'd expect the grader to accept it if it's reasonably close, but it's probably best to just keep up with the constant, since using a constant of 67 would yield a pretty different result than using 73.8 to the point where it may just appear completely wrong.

If they don't say anything, I tend to use 70 because that's mostly likely to hit all of the ranges somewhat closely (or as closely as one can get with ~5% variance...)
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Re: Astronomy C

Postby antoine_ego » February 3rd, 2017, 2:17 am

raxu wrote:What is the accepted value of Hubble's Constant, H_0? The most recent measurements have fluctuated between 67 and 73...

Would you expect the grader to accept any correct calculations with a reasonable H_0?

Often times, the test will tell you what value they want you to use to reduce the possible range of answers. For example, at MIT, they told us to use the value of 65.
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Re: Astronomy C

Postby Avogadro » February 3rd, 2017, 7:49 am

A question for anyone who was at the Mira Loma invite: what did you think of that test? I took it recently and it was quite different from what I've usually seen, though apparently it's fine according to the rules.
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Placement Record:

Code: Islip | Conestoga | Tiger | Regionals | States
Out of: 61 | 42 | 36 | 37 | 36

Chemistry Lab: 9 | - | - | 4 | 4
Astronomy: 14 | - | 5 | 10 | 3
Material Science: 12 | 19 | 9 | 5 | 9
Optics: 14 | 7 | 3 | 4 | 2

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Re: Astronomy C

Postby antoine_ego » February 4th, 2017, 4:06 pm

Avogadro wrote:A question for anyone who was at the Mira Loma invite: what did you think of that test? I took it recently and it was quite different from what I've usually seen, though apparently it's fine according to the rules.


My understanding is that typically the Mira Loma and Troy invitational tests are much more math heavy then other ones. Usually they stress the concepts really heavily, and don't bother with the standard regurgitation of Wikipedia articles. I like it.
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Re: Astronomy C

Postby embokim » February 5th, 2017, 4:45 am

Hey, just wondering if any tests would when solving Hubble's law, whether they would give Doppler effect portion of it to find velocity? instead of straight up them giving the values? Has the Hubble's Law questions been straight forward? Thanks!
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Re: Astronomy C

Postby antoine_ego » February 5th, 2017, 7:17 am

embokim wrote:Hey, just wondering if any tests would when solving Hubble's law, whether they would give Doppler effect portion of it to find velocity? instead of straight up them giving the values? Has the Hubble's Law questions been straight forward? Thanks!

I assume by Doppler effect portion you are referring to redshift. If so, yes, they appear routinely on most tests. For the most part they are straightforward, if anything, the trickiest part is figuring out whether it's moving towards or away from you, but that's still quite easy.
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Re: Astronomy C

Postby Ashernoel » February 5th, 2017, 7:40 pm

embokim wrote:Hey, just wondering if any tests would when solving Hubble's law, whether they would give Doppler effect portion of it to find velocity? instead of straight up them giving the values? Has the Hubble's Law questions been straight forward? Thanks!


I like combining this with distance modules for some good problems
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Re: Astronomy C

Postby embokim » February 6th, 2017, 6:27 pm

Suppose that now after a series of observations astronomers have deduced some properties of this pair of
stars. The parallax angle of this system is 0.0555…”. The mean separation of these two stars appears to
be 1.11”.

What is the semimajor axis of this binary star system, in AU?

How would you solve this question?
Thank You so much!
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Re: Astronomy C

Postby Ashernoel » February 6th, 2017, 8:44 pm

embokim wrote:Suppose that now after a series of observations astronomers have deduced some properties of this pair of
stars. The parallax angle of this system is 0.0555…”. The mean separation of these two stars appears to
be 1.11”.

What is the semimajor axis of this binary star system, in AU?

How would you solve this question?
Thank You so much!


Ok so its a few steps but nothing crazy.

First you want to take the parallax angle and find the distance in parsecs to the system. d =1/theta.
Once you have the distance to the system, you can use the Small Angle formula to find the D, diameter in parsecs (or AU ish, there is a trick here to avoid conversions but lets keep it simple). 1.11" = 206265D/d

Now that you have the diameter of the system, the semi major axis is half of that and in AU, so use parsec to AU conversion and divide by 2. This problem makes a lot of assumptions about the observers being on earth and that the system's major axis is moving along a "transverse" line, but this method should work for the information that is given.
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Re: Astronomy C

Postby Unome » February 7th, 2017, 6:34 am

On the same test: can anyone explain how to do 2.a and 2.c?

(that was a great test btw, I ended up scoring ~55 or so).
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Re: Astronomy C

Postby embokim » February 7th, 2017, 6:53 pm

Thank you so much! it helped me so much
But may I ask, what is the small angle formula?
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Re: Astronomy C

Postby Ashernoel » February 7th, 2017, 6:54 pm

Unome wrote:On the same test: can anyone explain how to do 2.a and 2.c?

(that was a great test btw, I ended up scoring ~55 or so).


for 2a you look at the peak in B and see that it is slightly above 2446560 JD. Convert a value of around 2446562 JD to the Gregorian calendar and you should get around may 11th, 1986, which is probably pretty close.

As For 2c, either use the paper to reference the material, or get it wrong. The distance modulus used in the paper is used from Surface Brightness Fluctuations to the galaxy, which aren't given in the problem.


Hope this helps!

embokim wrote:Thank you so much! it helped me so much


Awesome :D
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Re: Astronomy C

Postby embokim » February 8th, 2017, 3:13 am

For Newton's Law of Gravitation in what unit is R? (star separation distance)
Thanks
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