## Astronomy C

alchzh
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### Re: Astronomy C

mcmn1619 wrote:Can someone help me with 17e on this test? I've tried using M_a + M_b = a^3 / p^2, but I don't know what to do to find a: https://scioly.org/tests/files/astronom ... h_test.pdf

You can use the relation $m_1v_1 = m_2v_2$ (since the system isn't perfectly circular this might be slightly off... how exact do you need to be?)
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potatopotato37
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### Re: Astronomy C

I was taking the UPenn (SOUP) Astronomy exam and came across this question. The answer key says the distance to this galaxy (when calculated using Hubble's Law) is 18-23 Mpc, but every time I calculate it, I get about 219 Mpc. Do any of you guys know how to do this problem?

UPenn Astronomy wrote:Galaxy E is a nearby galaxy that contains a Star F, a Type I Cepheid variable with a pulsation period of 34 days and an apparent magnitude of 26.3. Galaxy E’s H-ɑ spectral line is observed at 659.72 nm. Assume H​0​ is roughly 70 km/s/Mpc, and note that the rest wavelength of the H-ɑ line is 656.28 nm.

My thinking:

PM2017
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### Re: Astronomy C

potatopotato37 wrote:I was taking the UPenn (SOUP) Astronomy exam and came across this question. The answer key says the distance to this galaxy (when calculated using Hubble's Law) is 18-23 Mpc, but every time I calculate it, I get about 219 Mpc. Do any of you guys know how to do this problem?

UPenn Astronomy wrote:Galaxy E is a nearby galaxy that contains a Star F, a Type I Cepheid variable with a pulsation period of 34 days and an apparent magnitude of 26.3. Galaxy E’s H-ɑ spectral line is observed at 659.72 nm. Assume H​0​ is roughly 70 km/s/Mpc, and note that the rest wavelength of the H-ɑ line is 656.28 nm.

My thinking:

I think the problem is that people accidentally use d = v*H0, instead of v = d*H0.

EDIT: NVM, that isnt the problem here. The problem is that the your redshift is somehow ten times larger than it should be (check your decimal places) The actual redshift is 0.005241
Last edited by PM2017 on March 30th, 2018, 3:28 pm, edited 1 time in total.
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### Re: Astronomy C

potatopotato37 wrote:I was taking the UPenn (SOUP) Astronomy exam and came across this question. The answer key says the distance to this galaxy (when calculated using Hubble's Law) is 18-23 Mpc, but every time I calculate it, I get about 219 Mpc. Do any of you guys know how to do this problem?

UPenn Astronomy wrote:Galaxy E is a nearby galaxy that contains a Star F, a Type I Cepheid variable with a pulsation period of 34 days and an apparent magnitude of 26.3. Galaxy E’s H-ɑ spectral line is observed at 659.72 nm. Assume H​0​ is roughly 70 km/s/Mpc, and note that the rest wavelength of the H-ɑ line is 656.28 nm.

My thinking:

I got an answer within the range on the key
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potatopotato37
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### Re: Astronomy C

Unome wrote:
potatopotato37 wrote:I was taking the UPenn (SOUP) Astronomy exam and came across this question. The answer key says the distance to this galaxy (when calculated using Hubble's Law) is 18-23 Mpc, but every time I calculate it, I get about 219 Mpc. Do any of you guys know how to do this problem?

UPenn Astronomy wrote:Galaxy E is a nearby galaxy that contains a Star F, a Type I Cepheid variable with a pulsation period of 34 days and an apparent magnitude of 26.3. Galaxy E’s H-ɑ spectral line is observed at 659.72 nm. Assume H​0​ is roughly 70 km/s/Mpc, and note that the rest wavelength of the H-ɑ line is 656.28 nm.

My thinking:

I got an answer within the range on the key

I just realized my mistake T_T

When I calculated the redshift, I got the right answer, but when I used it to find the distance, I typed it in with one less 0. This made z 10 times bigger than it should be, making the final answer 10 times bigger than it should be. Without this mistake, I'd get 21.9 Mpc which is within the range of accepted answers. Thanks for the help!

PM2017
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### Re: Astronomy C

potatopotato37 wrote:
Unome wrote:
potatopotato37 wrote:I was taking the UPenn (SOUP) Astronomy exam and came across this question. The answer key says the distance to this galaxy (when calculated using Hubble's Law) is 18-23 Mpc, but every time I calculate it, I get about 219 Mpc. Do any of you guys know how to do this problem?

My thinking:

I got an answer within the range on the key

I just realized my mistake T_T

When I calculated the redshift, I got the right answer, but when I used it to find the distance, I typed it in with one less 0. This made z 10 times bigger than it should be, making the final answer 10 times bigger than it should be. Thanks for the help!

yeah, whenever the answer is off by a power of ten check your decimal places, lol
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### Re: Astronomy C

In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.
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Alex-RCHS
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### Re: Astronomy C

Unome wrote:In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.

I’m not sure either. The paper says that the symbol is proportional to rotational velocity via the conservation of angular momentum. The phrase “rotational velocity” always annoys me because I never know what exactly it’s referring to. Is it the same thing as angular velocity, or is it recessional velocity?
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### Re: Astronomy C

Alex-RCHS wrote:
Unome wrote:In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.

I’m not sure either. The paper says that the symbol is proportional to rotational velocity via the conservation of angular momentum. The phrase “rotational velocity” always annoys me because I never know what exactly it’s referring to. Is it the same thing as angular velocity, or is it recessional velocity?

Rotational velocity is typically the maximum linear velocity at the edge of the star (for whatever measure of "edge" is being used), since that's most relevant to spectral redshift and such.
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PM2017
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### Re: Astronomy C

Unome wrote:In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.

I think it just means stellar radius.
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Alex-RCHS
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### Re: Astronomy C

Unome wrote:
Alex-RCHS wrote:
Unome wrote:In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.

I’m not sure either. The paper says that the symbol is proportional to rotational velocity via the conservation of angular momentum. The phrase “rotational velocity” always annoys me because I never know what exactly it’s referring to. Is it the same thing as angular velocity, or is it recessional velocity?

Rotational velocity is typically the maximum linear velocity at the edge of the star (for whatever measure of "edge" is being used), since that's most relevant to spectral redshift and such.

Sorry if this is a dumb question, but maximum linear velocity of what?
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### Re: Astronomy C

Alex-RCHS wrote:
Unome wrote:
Alex-RCHS wrote:I’m not sure either. The paper says that the symbol is proportional to rotational velocity via the conservation of angular momentum. The phrase “rotational velocity” always annoys me because I never know what exactly it’s referring to. Is it the same thing as angular velocity, or is it recessional velocity?

Rotational velocity is typically the maximum linear velocity at the edge of the star (for whatever measure of "edge" is being used), since that's most relevant to spectral redshift and such.

Sorry if this is a dumb question, but maximum linear velocity of what?

I think in this context it means the tangential velocity ($v_t = \omega r$ ) of a point on the surface of the star
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### Re: Astronomy C

Alex-RCHS wrote:
Unome wrote:Rotational velocity is typically the maximum linear velocity at the edge of the star (for whatever measure of "edge" is being used), since that's most relevant to spectral redshift and such.

Sorry if this is a dumb question, but maximum linear velocity of what?

I think in this context it means the tangential velocity ($v_t = \omega r$ ) of a point on the surface of the star

Thanks - for some reason I forgot the word entirely.
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### Re: Astronomy C

PM2017 wrote:
Unome wrote:In this paper, I notice the use of $R_*$ - does anyone know what this symbol means? I think it's an "effective radius" of sorts, but I can't figure out where it comes from.

I think it just means stellar radius.

Essentially, yes but with a major "but". In the paper they say it is the "hydrostatic radius"...which has some specifics, but it's basically a more mathematical / physics-y (read:consistent) way of describing a star's radius. The formality comes in because of asking "Is this the radius to surface of the star, somewhere inside?" This is actually a very interesting thing to think about: What is the surface of a star?

If I were to be very formal, I would suspect this is the the radius of the star out to "where we can observe most of the star's radiation"...which is somewhere pretty close to the surface. Can't guarantee I'm right, the original paper discussing the physics doesn't really mention this, but that's what I'd suspect anyway. Some other relevant radii I guess would be the core radius or the radius where winds come off of from the star, but those don't seem as directly relevant for what the paper was talking about.
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### Re: Astronomy C

I also wrote this test! (syo also helped with this one.)

I think I made it a bit too difficult for a Regionals... again. Sorry.
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