Astronomy C

Locked
User avatar
alchzh
Member
Member
Posts: 27
Joined: January 3rd, 2018, 1:36 pm
Division: Grad
State: NY
Has thanked: 0
Been thanked: 2 times

Re: Astronomy C

Post by alchzh »

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 (since the system isn't perfectly circular this might be slightly off... how exact do you need to be?)
Columbia University. Maybe starting an invitational. No promises.
alchzh's Userpage
potatopotato37
Member
Member
Posts: 2
Joined: March 30th, 2018, 3:05 pm
Has thanked: 0
Been thanked: 0

Re: Astronomy C

Post by potatopotato37 »

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.
The redshift of this galaxy (z) is 0.05241. The recessional velocity is therefore 1.5311e4 km/s. Dividing this by 70 km/s/Mpc gives a distance of 219 Mpc
User avatar
PM2017
Member
Member
Posts: 524
Joined: January 20th, 2017, 5:02 pm
Division: Grad
State: CA
Has thanked: 23 times
Been thanked: 13 times

Re: Astronomy C

Post by PM2017 »

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.
The redshift of this galaxy (z) is 0.05241. The recessional velocity is therefore 1.5311e4 km/s. Dividing this by 70 km/s/Mpc gives a distance of 219 Mpc
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.
West High '19
UC Berkeley '23

Go Bears!
User avatar
Unome
Moderator
Moderator
Posts: 4338
Joined: January 26th, 2014, 12:48 pm
Division: Grad
State: GA
Has thanked: 235 times
Been thanked: 85 times

Re: Astronomy C

Post by Unome »

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.
The redshift of this galaxy (z) is 0.05241. The recessional velocity is therefore 1.5311e4 km/s. Dividing this by 70 km/s/Mpc gives a distance of 219 Mpc
Redshift of about 0.005 and a recessional velocity of ~1572, which gives me ~22.46 Mpc
Userpage

Opinions expressed on this site are not official; the only place for official rules changes and FAQs is soinc.org.
potatopotato37
Member
Member
Posts: 2
Joined: March 30th, 2018, 3:05 pm
Has thanked: 0
Been thanked: 0

Re: Astronomy C

Post by potatopotato37 »

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.
The redshift of this galaxy (z) is 0.05241. The recessional velocity is therefore 1.5311e4 km/s. Dividing this by 70 km/s/Mpc gives a distance of 219 Mpc
Redshift of about 0.005 and a recessional velocity of ~1572, which gives me ~22.46 Mpc
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!
User avatar
PM2017
Member
Member
Posts: 524
Joined: January 20th, 2017, 5:02 pm
Division: Grad
State: CA
Has thanked: 23 times
Been thanked: 13 times

Re: Astronomy C

Post by PM2017 »

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?


The redshift of this galaxy (z) is 0.05241. The recessional velocity is therefore 1.5311e4 km/s. Dividing this by 70 km/s/Mpc gives a distance of 219 Mpc
Redshift of about 0.005 and a recessional velocity of ~1572, which gives me ~22.46 Mpc
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
West High '19
UC Berkeley '23

Go Bears!
User avatar
Unome
Moderator
Moderator
Posts: 4338
Joined: January 26th, 2014, 12:48 pm
Division: Grad
State: GA
Has thanked: 235 times
Been thanked: 85 times

Re: Astronomy C

Post by Unome »

In this paper, I notice the use of - 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.
Userpage

Opinions expressed on this site are not official; the only place for official rules changes and FAQs is soinc.org.
User avatar
Alex-RCHS
Member
Member
Posts: 539
Joined: November 4th, 2016, 3:46 pm
Division: Grad
State: NC
Has thanked: 3 times
Been thanked: 1 time

Re: Astronomy C

Post by Alex-RCHS »

Unome wrote:In this paper, I notice the use of - 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?
About me!
Raleigh Charter HS (NC) 2018
UNC-Chapel Hill 2022
User avatar
Unome
Moderator
Moderator
Posts: 4338
Joined: January 26th, 2014, 12:48 pm
Division: Grad
State: GA
Has thanked: 235 times
Been thanked: 85 times

Re: Astronomy C

Post by Unome »

Alex-RCHS wrote:
Unome wrote:In this paper, I notice the use of - 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.
Userpage

Opinions expressed on this site are not official; the only place for official rules changes and FAQs is soinc.org.
User avatar
PM2017
Member
Member
Posts: 524
Joined: January 20th, 2017, 5:02 pm
Division: Grad
State: CA
Has thanked: 23 times
Been thanked: 13 times

Re: Astronomy C

Post by PM2017 »

Unome wrote:In this paper, I notice the use of - 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.
West High '19
UC Berkeley '23

Go Bears!
Locked

Return to “2018 Study Events”

Who is online

Users browsing this forum: No registered users and 4 guests