<p>Redshift:</p>
<p><span class="math-container">$z=\frac{41}{656}=0.0625$</span></p>
<p>Speed</p>
<p><span class="math-container">$v\approx c\cdot z=299792458\cdot 0.0625=1.9\cdot 10^7\ {\rm m/s}$</span></p>
<p>Best regards.</p>

CC BY-SA 4.0
2022-01-19T12:33:18.423Z
2022-01-19T15:03:24.027Z
2022-01-19T15:03:24.027Z
Albert
<p>I was going through some old exercises but I could never figure out who to solve this one</p>
<p>The hydrogen absorption line (656 nm) of the galaxy NGC 77 is shifted by 41 nm into the reddish wavelength range. From this measured redshift, calculate the speed at which the galaxy is moving away from us. (c = <span class="math-container">$3 * 10^8$</span> m/s) .. Now the solution has been given as <span class="math-container">$1.9*10^7$</span> m/s</p>
<p>But as far as I know I'd calculate it with <span class="math-container">$\dfrac{\lambda_B-\lambda}{\lambda}=\dfrac{v_s}{c}$</span>, which would get me a speed of <span class="math-container">$4.5*10^9$</span> m/s</p>
<p>I never quiet understood how you'd get <span class="math-container">$1.9*10^7$</span> m/s ...</p>

CC BY-SA 4.0
2022-01-19T12:31:19.360Z
Using your formula, I get the 1.9e7 m/s (or 1.875e7, to be exact). You must have made an error. If I (wrongly) swap the numinator and denominator, I get closer to your result, 4.8e9 m/s. If you spell out your steps in the calculation, we can probably help.
pela
<redshift><speed><wavelength>
Calculate speed of a galaxy with redshift
Iks Deh
# Calculate speed of a galaxy with redshift

(Asked by: username_0 on 2022-01-19)

I was going through some old exercises but I could never figure out who to solve this one

The hydrogen absorption line (656 nm) of the galaxy NGC 77 is shifted by 41 nm into the reddish wavelength range. From this measured redshift, calculate the speed at which the galaxy is moving away from us. (c = $3 \* 10^8$ m/s) .. Now the solution has been given as $1.9\*10^7$ m/s

But as far as I know I'd calculate it with $\dfrac{\lambda\_B-\lambda}{\lambda}=\dfrac{v\_s}{c}$, which would get me a speed of $4.5\*10^9$ m/s

I never quiet understood how you'd get $1.9\*10^7$ m/s ...

**Tags:** redshift, speed, wavelength

> **Comments:**
> **username_1**: Using your formula, I get the 1.9e7 m/s (or 1.875e7, to be exact). You must have made an error. If I (wrongly) swap the numinator and denominator, I get closer to your result, 4.8e9 m/s. If you spell out your steps in the calculation, we can probably help.  

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## 1 answers

### Answer by username_2 on 2022-01-19. Score: 1

Redshift:

$z=\frac{41}{656}=0.0625$

Speed

$v\approx c\cdot z=299792458\cdot 0.0625=1.9\cdot 10^7\ {\rm m/s}$

Best regards.


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