You can use the relation (since the system isn't perfectly circular this might be slightly off... how exact do you need to be?)
Re: Astronomy C
Posted: March 30th, 2018, 3:15 pm
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 H0 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
Re: Astronomy C
Posted: March 30th, 2018, 3:24 pm
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 H0 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
Re: Astronomy C
Posted: March 30th, 2018, 3:25 pm
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 H0 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
Re: Astronomy C
Posted: March 30th, 2018, 3:29 pm
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 H0 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!
Re: Astronomy C
Posted: March 30th, 2018, 3:30 pm
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
Re: Astronomy C
Posted: April 1st, 2018, 1:13 pm
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.
Re: Astronomy C
Posted: April 1st, 2018, 2:05 pm
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?
Re: Astronomy C
Posted: April 1st, 2018, 2:18 pm
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.
Re: Astronomy C
Posted: April 1st, 2018, 3:10 pm
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.
- 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.