Homework Help: What is the redshift and velocity of the galaxy relative to us

1. Mar 14, 2007

b_o3

hey everyone... so im tryna solve this problem :

a galaxy shows an emission line at a wavelenght of 6610.2 A, which we identify as the H-alpha line in the Balmer series of Hydrogen. what is the redshift and velocity of the galaxy relative to us.

period= 4.3 days
magnitude= 26.25
absolute magnitude= -3.21
distance= 7 800 000

okay so my main problem is trying to get planck's constant. I'm tryna use v(redshift velocity)=H0(planck's constant) * D

OR should i use Z(redshift)= (wavelenght observed-wavelenght emitted) / wavelenght emitted

thanks!

2. Mar 14, 2007

Mentz114

H0 is the Hubble constant, here.

3. Mar 14, 2007

b_o3

yes it is ....

4. Mar 14, 2007

Mentz114

Use
Z(redshift)= (wavelength observed-wavelength emitted) / wavelength emitted

5. Mar 14, 2007

b_o3

yea but i dont have 'wavelenght emitted'... i only have wavelenght observed :S

6. Mar 14, 2007

Mentz114

Wavelength emitted is given as

"H-alpha line in the Balmer series of Hydrogen"

look it up.

7. Mar 14, 2007

b_o3

the wavelenght is not exact... and thats not what they meant by that (i think?) cuz they gave the wavelenght and i think they just mentioned the balmer series for clarification... at a wavelenght of 6610.2 A, which we identify as the H-alpha line

8. Mar 14, 2007

b_o3

okay i re-did the problem and now i have these values:

distance = 29.46 m
M (absolute magnitude)= -3.21
period= 4.3 days
m(magnitude)= 26.25

i just cant seem to find an equation WITHOUT another value for wavelenght (emitted wavelenght value_)

9. Mar 14, 2007

Mentz114

The whole point about red-shift measurements is to get the frequency difference. The emitted frequency is different and we can measure it here on earth and compare with what we received.

Your post #7 is just wrong.

Last edited: Mar 14, 2007