Astronomy C

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Re: Astronomy C

Postby UTF-8 U+6211 U+662F » March 10th, 2019, 7:57 am

Hello! I'm thinking of doing Astro next year, is there anything I can do to start preparing now even though we don't know the topics will be?

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Re: Astronomy C

Postby Unome » March 10th, 2019, 8:35 am

UTF-8 U+6211 U+662F wrote:Hello! I'm thinking of doing Astro next year, is there anything I can do to start preparing now even though we don't know the topics will be?

The 2017 and 2018 topics sort of form the baseline for other years, you could probably study that info in slightly less depth to get a good sense of the basics.
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Re: Astronomy C

Postby Selundar » March 10th, 2019, 8:46 am

UTF-8 U+6211 U+662F wrote:Hello! I'm thinking of doing Astro next year, is there anything I can do to start preparing now even though we don't know the topics will be?


A good chunk of Astronomy stays the same every year. The main parts that change are the DSOs and the sub-topic, but stellar evolution and the math portions are pretty consistent. So for stellar evolution, you should probably look into HR diagrams and spectral classes, low mass vs high mass evolution, supernovae, and stellar remnants (white dwarfs, neutron stars, and black holes). For the math, it would probably be best to look up all the terms listed in the rules to get the equations for them and you can use any test you have access to within the last several years to get practice. Hope this helps!
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Re: Astronomy C

Postby Zxcvbnm123 » March 14th, 2019, 8:52 pm

How do you do question 16 and 17 on the Golden Gate Invitational Test? https://drive.google.com/drive/folders/ ... MRmlHdJc2u

Also, how do you calculate distances to Cepheid and RR Lyrae Stars?

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Re: Astronomy C

Postby syo_astro » March 14th, 2019, 9:02 pm

Zxcvbnm123 wrote:How do you do question 16 and 17 on the Golden Gate Invitational Test? https://drive.google.com/drive/folders/ ... MRmlHdJc2u

Also, how do you calculate distances to Cepheid and RR Lyrae Stars?


Have you given those questions a start or have work you can share to check over? That usually makes the checking/explaining go faster.

In the mean time, I'll start things off with 16a. RR Lyrae stars have a period-luminosity relationship that indicates they have an ~constant absolute magnitude (or luminosity) across a range of periods. IF for whatever reason you know you have an RR Lyrae star, you therefore know its absolute magnitude.

Rant about finding this RR Lyrae abs mag
You can google for the value, but keep in mind usually the numbers listed will vary. Also, there are some finer details and different types that add a bit of an error bar on the value you'll find anyway. Practically for the test, some writers have answer ranges anyway, so just pick a value and go with it (unless someone has studied this very deeply..). You can also ask the proctor / explain the range of values...I haven't really run into issues with this, but usually others ask or worry more about that, so others can feel free to chime in.

Anyway, once you get your absolute magnitude, we remember that we need distance (in pc). We are also given apparent magnitude of the star. If you have apparent magnitude and the absolute magnitude of a star, you can use the distance modulus to find the distance in pc.

Does this help?
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Re: Astronomy C

Postby Zxcvbnm123 » March 14th, 2019, 10:10 pm

syo_astro wrote:
Zxcvbnm123 wrote:How do you do question 16 and 17 on the Golden Gate Invitational Test? https://drive.google.com/drive/folders/ ... MRmlHdJc2u

Also, how do you calculate distances to Cepheid and RR Lyrae Stars?


Have you given those questions a start or have work you can share to check over? That usually makes the checking/explaining go faster.

In the mean time, I'll start things off with 16a. RR Lyrae stars have a period-luminosity relationship that indicates they have an ~constant absolute magnitude (or luminosity) across a range of periods. IF for whatever reason you know you have an RR Lyrae star, you therefore know its absolute magnitude.

Rant about finding this RR Lyrae abs mag
You can google for the value, but keep in mind usually the numbers listed will vary. Also, there are some finer details and different types that add a bit of an error bar on the value you'll find anyway. Practically for the test, some writers have answer ranges anyway, so just pick a value and go with it (unless someone has studied this very deeply..). You can also ask the proctor / explain the range of values...I haven't really run into issues with this, but usually others ask or worry more about that, so others can feel free to chime in.

Anyway, once you get your absolute magnitude, we remember that we need distance (in pc). We are also given apparent magnitude of the star. If you have apparent magnitude and the absolute magnitude of a star, you can use the distance modulus to find the distance in pc.

Does this help?


I tried doing that, but the answer I am getting (around 70,000) is no where near close to the actual answer.

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Re: Astronomy C

Postby Unome » March 15th, 2019, 5:21 am

Zxcvbnm123 wrote:
syo_astro wrote:
Zxcvbnm123 wrote:How do you do question 16 and 17 on the Golden Gate Invitational Test? https://drive.google.com/drive/folders/ ... MRmlHdJc2u

Also, how do you calculate distances to Cepheid and RR Lyrae Stars?


Have you given those questions a start or have work you can share to check over? That usually makes the checking/explaining go faster.

In the mean time, I'll start things off with 16a. RR Lyrae stars have a period-luminosity relationship that indicates they have an ~constant absolute magnitude (or luminosity) across a range of periods. IF for whatever reason you know you have an RR Lyrae star, you therefore know its absolute magnitude.

Rant about finding this RR Lyrae abs mag
You can google for the value, but keep in mind usually the numbers listed will vary. Also, there are some finer details and different types that add a bit of an error bar on the value you'll find anyway. Practically for the test, some writers have answer ranges anyway, so just pick a value and go with it (unless someone has studied this very deeply..). You can also ask the proctor / explain the range of values...I haven't really run into issues with this, but usually others ask or worry more about that, so others can feel free to chime in.

Anyway, once you get your absolute magnitude, we remember that we need distance (in pc). We are also given apparent magnitude of the star. If you have apparent magnitude and the absolute magnitude of a star, you can use the distance modulus to find the distance in pc.

Does this help?


I tried doing that, but the answer I am getting (around 70,000) is no where near close to the actual answer.

Try an absolute magnitude in the range of 0.55. That said, I took a look at the problem and the answer key seems a bit off to me - it would imply that an RR Lyrae star has an absolute magnitude in the range of 11 to 12.
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Re: Astronomy C

Postby antoine_ego » March 15th, 2019, 7:23 am

Unome wrote:
Zxcvbnm123 wrote:
syo_astro wrote:
Have you given those questions a start or have work you can share to check over? That usually makes the checking/explaining go faster.

In the mean time, I'll start things off with 16a. RR Lyrae stars have a period-luminosity relationship that indicates they have an ~constant absolute magnitude (or luminosity) across a range of periods. IF for whatever reason you know you have an RR Lyrae star, you therefore know its absolute magnitude.

Rant about finding this RR Lyrae abs mag
You can google for the value, but keep in mind usually the numbers listed will vary. Also, there are some finer details and different types that add a bit of an error bar on the value you'll find anyway. Practically for the test, some writers have answer ranges anyway, so just pick a value and go with it (unless someone has studied this very deeply..). You can also ask the proctor / explain the range of values...I haven't really run into issues with this, but usually others ask or worry more about that, so others can feel free to chime in.

Anyway, once you get your absolute magnitude, we remember that we need distance (in pc). We are also given apparent magnitude of the star. If you have apparent magnitude and the absolute magnitude of a star, you can use the distance modulus to find the distance in pc.

Does this help?


I tried doing that, but the answer I am getting (around 70,000) is no where near close to the actual answer.

Try an absolute magnitude in the range of 0.55. That said, I took a look at the problem and the answer key seems a bit off to me - it would imply that an RR Lyrae star has an absolute magnitude in the range of 11 to 12.


In general, I use an average absolute magnitude of +0.75 for RR Lyrae variables based off of Wikipedia. I agree with Unome though, the answer key is wrong on this, since reverse engineering their answer gives me +11.58 as their absolute magnitude, which is less than even the Sun.
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Re: Astronomy C

Postby syo_astro » March 15th, 2019, 4:25 pm

Sent some emails, double checked myself, indeed the key has a mistake. The answers should be about
(a) 70,795 pc
(b) 0.014 mas

So the 70,000 pc is basically correct...the previous two posts also agree with my point about not just assuming "the wiki value is right" and searching around a little or asking. Besides, the citation for the wiki value still works, and it's important to note there is an errorbar even on that average value...statistics are tricky!

Hope that helps.
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Re: Astronomy C

Postby SciolyHarsh » March 18th, 2019, 8:14 pm

On the golden gate test, how would you do 17 c, d, and f?
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Re: Astronomy C

Postby Name » March 18th, 2019, 8:49 pm

SciolyHarsh wrote:On the golden gate test, how would you do 17 c, d, and f?

Explantion
17a use keplers third law- 9.965 solar mass
17b mass times distance = mass times distance- 5.97 solar mass
17c You know the distance of star b is 20 au. Convert to km, multiply by 2pi (it's a circular orbit). The period is 112 years, convert to seconds, and divide the circumference of the circle by the period in seconds. I got 5.98 km/s
17d Same thing except distance is now 30 au. I got 8.41 km/s
17e. Use the mass-lumo-radius relationship for lumo in solar lumo. Convert to absolute magnitude, use distance modulus
17f. In 17e you find the distance to be 7.9389 parsecs. 50 au divided by 7.9389 = 6.3. For an explanation- distance is 7.9 parsec. Convert to au is 1637521 au. This can be the radius- convert this to a circumference, which is 10288848 au. divide 50 by this to get 4.86E-6. This is in degrees. Convert to arc sec by muliplying by 360, 60, and 60 to get 6.3. I'm not sure about the if the seperation is visable or not, that's probably just some random piece of trivia. (Wiki says yes if seperation > 1 arc sec for telescopes, less for professional telescopes, interferometry, or space-based equipment.)
Edit: added in explanations for other parts of the question as well

For reference, the question
17. Star B and Star C orbit one another in a binary system with a separation of 50 AU. Assume
that the two stars have circular orbits. Star B has a radius that is twice that of the Sun and
an effective temperature of 3,500 K.
(a) The period of Star B’s orbit around their common barycenter is 112 years. What is the
combined mass of Star B and Star C, in Solar Masses?
(b) Star B lies 20 AU from the barycenter of the two orbits. What is the mass of Star B,
in Solar masses?
(c) What is the orbital velocity of Star B, in kilometers per second?
(d) What is the orbital velocity of Star C, in kilometers per second?
(e) The apparent magnitude of Star B is 5. How far away is this system, in parsecs?
(f) What is the maximum apparent separation of Star B and Star C, in arcseconds? Is this
separation visible to current telescopes (i.e., would this be a visual binary)?

I think this is right? I don't usually do the math stuff and im kinda rusty on astro math.
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Re: Astronomy C

Postby SciolyHarsh » March 18th, 2019, 9:45 pm

Name wrote:
SciolyHarsh wrote:On the golden gate test, how would you do 17 c, d, and f?

Explantion
17a use keplers third law- 9.965 solar mass
17b mass times distance = mass times distance- 5.97 solar mass
17c You know the distance of star b is 20 au. Convert to km, multiply by 2pi (it's a circular orbit). The period is 112 years, convert to seconds, and divide the circumference of the circle by the period in seconds. I got 5.98 km/s
17d Same thing except distance is now 30 au. I got 8.41 km/s
17e. Use the mass-lumo-radius relationship for lumo in solar lumo. Convert to absolute magnitude, use distance modulus
17f. In 17e you find the distance to be 7.9389 parsecs. 50 au divided by 7.9389 = 6.3. For an explanation- distance is 7.9 parsec. Convert to au is 1637521 au. This can be the radius- convert this to a circumference, which is 10288848 au. divide 50 by this to get 4.86E-6. This is in degrees. Convert to arc sec by muliplying by 360, 60, and 60 to get 6.3. I'm not sure about the if the seperation is visable or not, that's probably just some random piece of trivia. (Wiki says yes if seperation > 1 arc sec for telescopes, less for professional telescopes, interferometry, or space-based equipment.)
Edit: added in explanations for other parts of the question as well

For reference, the question
17. Star B and Star C orbit one another in a binary system with a separation of 50 AU. Assume
that the two stars have circular orbits. Star B has a radius that is twice that of the Sun and
an effective temperature of 3,500 K.
(a) The period of Star B’s orbit around their common barycenter is 112 years. What is the
combined mass of Star B and Star C, in Solar Masses?
(b) Star B lies 20 AU from the barycenter of the two orbits. What is the mass of Star B,
in Solar masses?
(c) What is the orbital velocity of Star B, in kilometers per second?
(d) What is the orbital velocity of Star C, in kilometers per second?
(e) The apparent magnitude of Star B is 5. How far away is this system, in parsecs?
(f) What is the maximum apparent separation of Star B and Star C, in arcseconds? Is this
separation visible to current telescopes (i.e., would this be a visual binary)?

I think this is right? I don't usually do the math stuff and im kinda rusty on astro math.


Sorry i don't really understand what you did for 17f. But thanks for the rest
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Re: Astronomy C

Postby Name » March 19th, 2019, 3:49 am

SciolyHarsh wrote:
Name wrote:
SciolyHarsh wrote:On the golden gate test, how would you do 17 c, d, and f?

Explantion
17a use keplers third law- 9.965 solar mass
17b mass times distance = mass times distance- 5.97 solar mass
17c You know the distance of star b is 20 au. Convert to km, multiply by 2pi (it's a circular orbit). The period is 112 years, convert to seconds, and divide the circumference of the circle by the period in seconds. I got 5.98 km/s
17d Same thing except distance is now 30 au. I got 8.41 km/s
17e. Use the mass-lumo-radius relationship for lumo in solar lumo. Convert to absolute magnitude, use distance modulus
17f. In 17e you find the distance to be 7.9389 parsecs. 50 au divided by 7.9389 = 6.3. For an explanation- distance is 7.9 parsec. Convert to au is 1637521 au. This can be the radius- convert this to a circumference, which is 10288848 au. divide 50 by this to get 4.86E-6. This is in degrees. Convert to arc sec by muliplying by 360, 60, and 60 to get 6.3. I'm not sure about the if the seperation is visable or not, that's probably just some random piece of trivia. (Wiki says yes if seperation > 1 arc sec for telescopes, less for professional telescopes, interferometry, or space-based equipment.)
Edit: added in explanations for other parts of the question as well

For reference, the question
17. Star B and Star C orbit one another in a binary system with a separation of 50 AU. Assume
that the two stars have circular orbits. Star B has a radius that is twice that of the Sun and
an effective temperature of 3,500 K.
(a) The period of Star B’s orbit around their common barycenter is 112 years. What is the
combined mass of Star B and Star C, in Solar Masses?
(b) Star B lies 20 AU from the barycenter of the two orbits. What is the mass of Star B,
in Solar masses?
(c) What is the orbital velocity of Star B, in kilometers per second?
(d) What is the orbital velocity of Star C, in kilometers per second?
(e) The apparent magnitude of Star B is 5. How far away is this system, in parsecs?
(f) What is the maximum apparent separation of Star B and Star C, in arcseconds? Is this
separation visible to current telescopes (i.e., would this be a visual binary)?

I think this is right? I don't usually do the math stuff and im kinda rusty on astro math.


Sorry i don't really understand what you did for 17f. But thanks for the rest


Basically apparent separation (arc sec)= separation in au/distance in parsecs. The other part was just how you can calculate it without knowing this equation.
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Re: Astronomy C

Postby PM2017 » March 19th, 2019, 7:59 am

Name wrote:
SciolyHarsh wrote:
Sorry i don't really understand what you did for 17f. But thanks for the rest


Basically apparent separation (arc sec)= separation in au/distance in parsecs. The other part was just how you can calculate it without knowing this equation.


For further explanation, here's why this is the case:

There's a thing called small angle approximation, that says that theta ~ tan(theta) for small angles, because as the angle gets smaller, sin theta is approximately the same as the arc length created by that angle, and cos theta approaches 1. (.: sin(theta) = theta; cos(theta) = 1). Since tan(theta) = sin(theta)/cos(theta), tan(theta) = theta/1.

Then, you know that you have a right triangle (assuming the object is face on), with the adjacent side being the distance to the object (let's call this "D"), and the opposite side being the diameter/length/whatnot of the object (let's call this "L" ). So, you get tan(theta) = L/D, and since we said earlier that tan(theta) = theta, you get your final equation as theta=L/D. Remember, of course, that this is in radians. You almost certainly have to convert to degrees, arcminutes, arcseconds, or milliarcseconds.

Since 1 rad = 206,625 arcseconds, you could just set your equation to theta = 206,625L/D. Make sure L and D are the same. I made the mistake of thinking that the 206625 was to convert from pc to AU (by definition, the conversion from AU to pc is the same as radians to arcseconds.), and that's why I had to make the edit below.

EDIT: I removed the other paragraph since it was wrong.
Last edited by PM2017 on March 19th, 2019, 8:08 am, edited 1 time in total.
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Re: Astronomy C

Postby Rossyspsce » April 17th, 2019, 8:52 am

getting thrown on this event for next year(2019-2020 season), because our astro people will all be graduating. What should I do to study for this event/where should I start?


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