## Astronomy C

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### Re: Astronomy C

ET2020 wrote:Do you know of any good resources to learn how to use JS9? It's seems confusing and I'm not sure how to answer questions about it.

I agree that it can be confusing/intimidating at first. Keep in mind that this year, you won't actually have to use the JS9 software in competition, just be able to understand screenshots of its various functionalities.

I wrote a very basic-level introduction to JS9 here. Beyond the guide, I think it's helpful to play around on the site and see what it can do. To perform well on JS9 questions at high-level tournaments, be sure to also understand fundamental concepts in multi-wavelength astronomy, energy spectra, light curves, etc. The goal of JS9 is to be able to apply theory to understand observational data! In case you haven't seen it, there's a JS9 question on the MIT exam (see #19) and detailed solutions here, so that might give you a feel for what sort of questions to expect.

M3335
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### Re: Astronomy C

Hi all, I was reviewing over past Upenn invitational math questions to practice, and I found one that seems impossible:

Particularly parts B) and C). I feel like there's certain information missing? I tried multiple approaches, such as using $F = \frac{GMm_c}{R^2} = \frac{m_cV_c^2}{R_c}$ , Kepler's third law for binary systems, and even integrating the cosine function of one of the curves with respect to time to find the distance it traveled in one period, but I couldn't seem to get the answer. It seems you need two known variables, whereas the problem only provides you with one. The given answer to B) is 1.5E7 km and the answer to C) is .09Msolar. I feel like the two parts could be answered in any order, but that's just a guess.

I'm assuming you need to use the information about the type M star to answer the question, but I don't know how.

Help would be much appreciated.

syo_astro
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### Re: Astronomy C

M3335 wrote:Hi all, I was reviewing over past Upenn invitational math questions to practice, and I found one that seems impossible:

Part B: Have you tried applying circular (orbital) velocity? If you know the radial velocity and the period, you should be able to figure out the radius of orbit for a given orbiting object.
Part C: If you know the mass ratio, the orbital separation, and the period, you should have two equations and two unknowns that should be solvable (the mass ratio and Kepler's third law). You really shouldn't need to do any integrals or fancy math in astro (or all events, really). I would say you'd need to do Part B first, at least it's more intuitive to.

Haven't tried the math yet, but I can try it in a bit. Not sure about the units on the y-axis though (I'd guess km/s of course)...binary star questions should be easy to write, but I've messed them up tons of times -_-. Luckily, invites have been good opportunities for me to get complaints about stuff like that (nobody lets me live down the math on two invites posted online XD), I actually do use the feedback and triple check for real tournies!
B: Crave the Wave, Environmental Chemistry, Robo-Cross, Meteorology, Physical Science Lab, Solar System, DyPlan (E and V), Shock Value
C: Microbe Mission, DyPlan (Earth's Fresh Waters), Fermi Questions, GeoMaps, Gravity Vehicle, Scrambler, Rocks, Astronomy

M3335
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### Re: Astronomy C

syo_astro wrote:
M3335 wrote:Hi all, I was reviewing over past Upenn invitational math questions to practice, and I found one that seems impossible:

Part B: Have you tried applying circular (orbital) velocity? If you know the radial velocity and the period, you should be able to figure out the radius of orbit for a given orbiting object.
Part C: If you know the mass ratio, the orbital separation, and the period, you should have two equations and two unknowns that should be solvable (the mass ratio and Kepler's third law). You really shouldn't need to do any integrals or fancy math in astro (or all events, really). I would say you'd need to do Part B first, at least it's more intuitive to.

Haven't tried the math yet, but I can try it in a bit. Not sure about the units on the y-axis though (I'd guess km/s of course)...binary star questions should be easy to write, but I've messed them up tons of times -_-. Luckily, invites have been good opportunities for me to get complaints about stuff like that (nobody lets me live down the math on two invites posted online XD), I actually do use the feedback and triple check for real tournies!

Ah thank you! Somehow that method slipped from my mind. I used circular velocity, and came up with around 1.49E5 km but .08Msolar, so I think the answer key may be wrong.

idislikeboomi
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### Re: Astronomy C

Anybody have the image set for the carnegie mellon test?

c0c05w311y
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### Re: Astronomy C

idislikeboomi wrote:Anybody have the image set for the carnegie mellon test?

I'm pretty sure this is the right version.
Let me know if you have any questions or comments about the exam!

Zxcvbnm123
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### Re: Astronomy C

Does anyone know where I can find the 2019 Golden Gate Invitational test?

EastStroudsburg13
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### Re: Astronomy C

Golden Gate Invitational tests will be released publicly on their website on March 9.
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Alke
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### Re: Astronomy C

Hi everyone!

I did astronomy two seasons ago but I'm back! I'm a little rusty and I am always running out of time. Thus, do you all have tips for time management/splitting up the work with your partner?

I'm trying to think through some strategies like having one person do math and the other do mulitple choice. However, I don't know how well that'll work!

-Thanks

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### Re: Astronomy C

Alke wrote:Hi everyone!

I did astronomy two seasons ago but I'm back! I'm a little rusty and I am always running out of time. Thus, do you all have tips for time management/splitting up the work with your partner?

I'm trying to think through some strategies like having one person do math and the other do mulitple choice. However, I don't know how well that'll work!

-Thanks

I'd suggest splitting the test. I usually take the DSOs (or everything besides math), while my partner takes all the math (even though math is my favorite part of astro ). We're usually capable of finishing the test without too much of a problem. Near the end we usually look into the other person's section to help potentially solve questions that the other person didn't get/check over a bit, so i'd still recommend being knowledgable in your partner's section. Figure out which sections you and your partner would rather do, and then split the test accordingly.
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PM2017
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### Re: Astronomy C

Alke wrote:Hi everyone!

I did astronomy two seasons ago but I'm back! I'm a little rusty and I am always running out of time. Thus, do you all have tips for time management/splitting up the work with your partner?

I'm trying to think through some strategies like having one person do math and the other do mulitple choice. However, I don't know how well that'll work!

-Thanks

We split the test right down the middle and take 30 seconds or so to scan our halves and see which pages we're most comfortable with/have the most points and get those done. I generally hand the in-depth DSO questions to my partner, while she gives me the majority of the calculations. We try to get our respective sections done by the 30-minute mark and then go onto finish the conceptual section, since that's the most luck-based. (unless I see HR diagrams or light curves or something else that's not really trivia).

Also, we circle the question numbers that we don't immediately get and move on. We come back after we finish everything we are comfortable with and then we discuss amongst each other.

Honestly, if you looked at us without realizing we were any good, you would probably laugh at "those two kids who were arguing with each other throughout the test," and wouldn't think we had any chemistry together. But, I would say our achievements say otherwise... .

(sorry, I don't normally boast, but it's a somewhat funny story.)
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ET2020
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### Re: Astronomy C

I've noticed that there seem to be two different equations used to calculate recessional velocity from redshift. The more commonly used one, v = Z*c, works fine for relatively close objects, but creates a problem for objects with redshifts > 1. The equation is problematic because it implies that we should not be able to see things with z > 1, since they would be receding faster than light, and therefore the light would not be able to reach us. However, there have been many objects observed to have z >> 1. The correct equation, V = c*[(z^2+2z)/(z^2+2z+2)], gives accurate answers even for distant objects. Unfortunately, I've gotten a few practice questions wrong because the test writer used the simplified version of the equation. Which one should I use?
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syo_astro
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### Re: Astronomy C

ET2020 wrote:I've noticed that there seem to be two different equations used to calculate recessional velocity from redshift. The more commonly used one, v = Z*c, works fine for relatively close objects, but creates a problem for objects with redshifts > 1. The equation is problematic because it implies that we should not be able to see things with z > 1, since they would be receding faster than light, and therefore the light would not be able to reach us. However, there have been many objects observed to have z >> 1. The correct equation, V = c*[(z^2+2z)/(z^2+2z+2)], gives accurate answers even for distant objects. Unfortunately, I've gotten a few practice questions wrong because the test writer used the simplified version of the equation. Which one should I use?

This is always a great question! Similar issues come up for other equations too (like whether to assume circular orbits, etc). One way is to ask the proctor something like "there are two possible equations to use for this question, should we account for relativistic effects?" But I'm aware on the spot that can take away precious time and be cumbersome, especially if the proctor can't answer or doesn't know.

I would guess most test writers shoot for the simpler equations, but I know that's not a guarantee either. Thoughts from others?
B: Crave the Wave, Environmental Chemistry, Robo-Cross, Meteorology, Physical Science Lab, Solar System, DyPlan (E and V), Shock Value
C: Microbe Mission, DyPlan (Earth's Fresh Waters), Fermi Questions, GeoMaps, Gravity Vehicle, Scrambler, Rocks, Astronomy

Unome
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### Re: Astronomy C

syo_astro wrote:
ET2020 wrote:I've noticed that there seem to be two different equations used to calculate recessional velocity from redshift. The more commonly used one, v = Z*c, works fine for relatively close objects, but creates a problem for objects with redshifts > 1. The equation is problematic because it implies that we should not be able to see things with z > 1, since they would be receding faster than light, and therefore the light would not be able to reach us. However, there have been many objects observed to have z >> 1. The correct equation, V = c*[(z^2+2z)/(z^2+2z+2)], gives accurate answers even for distant objects. Unfortunately, I've gotten a few practice questions wrong because the test writer used the simplified version of the equation. Which one should I use?

This is always a great question! Similar issues come up for other equations too (like whether to assume circular orbits, etc). One way is to ask the proctor something like "there are two possible equations to use for this question, should we account for relativistic effects?" But I'm aware on the spot that can take away precious time and be cumbersome, especially if the proctor can't answer or doesn't know.

I would guess most test writers shoot for the simpler equations, but I know that's not a guarantee either. Thoughts from others?

I've never seen a test writer use the relativistic version of the equation on a test, so I pretty much always use the non-relativistic equation. Although, at a particularly competitive tournament if the test doesn't specify, I'd probably ask.
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### Re: Astronomy C

Unome wrote:
syo_astro wrote:
ET2020 wrote:I've noticed that there seem to be two different equations used to calculate recessional velocity from redshift. The more commonly used one, v = Z*c, works fine for relatively close objects, but creates a problem for objects with redshifts > 1. The equation is problematic because it implies that we should not be able to see things with z > 1, since they would be receding faster than light, and therefore the light would not be able to reach us. However, there have been many objects observed to have z >> 1. The correct equation, V = c*[(z^2+2z)/(z^2+2z+2)], gives accurate answers even for distant objects. Unfortunately, I've gotten a few practice questions wrong because the test writer used the simplified version of the equation. Which one should I use?

This is always a great question! Similar issues come up for other equations too (like whether to assume circular orbits, etc). One way is to ask the proctor something like "there are two possible equations to use for this question, should we account for relativistic effects?" But I'm aware on the spot that can take away precious time and be cumbersome, especially if the proctor can't answer or doesn't know.

I would guess most test writers shoot for the simpler equations, but I know that's not a guarantee either. Thoughts from others?

I've never seen a test writer use the relativistic version of the equation on a test, so I pretty much always use the non-relativistic equation. Although, at a particularly competitive tournament if the test doesn't specify, I'd probably ask.

I'd say that there's a lot of variation and it'd hard to generalize in either direction. As a competitor, I only saw test writers use the relativistic form, which is the opposite of Unome's experience. I would recommend either asking (as syo_astro suggested) or calculating both and saying "this one is taking relativity into account, while this one does not" if the test doesn't specify.
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