<p>The light emitted by stars passes through various gases, these gases absorb very specific wavelengths of light.  For example Hydrogen will absorb light at exactly 6563 <a href="https://en.wikipedia.org/wiki/Angstrom" rel="nofollow noreferrer">Å</a>.  So we know that the spectrum of light would have a dark line at 6563 Å if the galaxy wasn't moving away from us.</p>
<p>If the light is redshifted, this line also gets redshifted.  So if this line appears at 7830 Å, we can calculate the redshift.</p>
<p>Because gases absorb (and emit) light at particular known wavelengths, we do know emitted wavelength of the light.</p>

CC BY-SA 4.0
2022-07-21T16:05:45.733Z
Thank you - makes sense! (Just googled to work out that Å is Angstroms https://en.wikipedia.org/wiki/Angstrom  - a way of representing very tiny lengths)
tomh
CC BY-SA 4.0
2023-04-23T00:42:36.333Z
Hi James.  If you don't mind, I'd love to ask couple of questions.  Question 1 - at which step would light go through hydrogen ? you mean hydrogen gas that exists in a space(clouds of it -before forming the star from it) or you mean when fusion happens inside the core, energy (photon) is released and before it gets out of the surface, it gets scattered through other hydrogen atoms inside core.
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T00:42:52.893Z
Question 2: if the answer from question 1 is both, then in terms of fusion in the core, released photon could scatter between multiple times in different hydrogen atoms, wouldn't this redshift the wavelength even more or once light is gone through hydrogen atom one time, it doesn't matter how many times it goes through with it later, its wavelength spectrum will stay the same ?
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T00:44:20.517Z
Question 3: in the core, light would definately not only scatter hydrogen, but some other stuff as well before it would reach the outer shell and start moving to us. What if it scattered first through hydrogen and then helium after that it came to us ?
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T00:45:26.103Z
Question 4: https://physics.stackexchange.com/questions/760857/why-does-redshift-happen.
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T00:46:01.833Z
Question 5: https://astronomy.stackexchange.com/questions/53504/how-redshift-can-be-detected
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T00:46:28.587Z
Thank you in advance if you could help a little bit.
Giorgi Lagidze
CC BY-SA 4.0
2023-04-23T13:58:48.630Z
@Giorgi Lagidze: No, that is not how Stack Exchange works (unless the intent is to prompt improvement of the answer). You need to consider the scary proposition of asking a new question. (It should also be one question at a time, unless they are very closely related (essentially one question).)
Peter Mortensen
CC BY-SA 4.0
2023-04-23T14:15:40.660Z
The [Lyman break](https://en.wikipedia.org/wiki/Lyman-break_galaxy) could [be covered](https://www.youtube.com/watch?v=2VVVADtXNBE&t=23m51s) as well.
Peter Mortensen
CC BY-SA 4.0
2022-07-21T07:57:08.363Z
2022-07-21T16:31:33.933Z
2022-07-21T16:31:33.933Z
James K
<p>If z is calculated using the ratio between <a href="https://wikimedia.org/api/rest_v1/media/math/render/svg/6a999cd6480ab61dddeeb5372c8037c0d533f2c9" rel="nofollow noreferrer">observable wavelength and emitted wavelength</a>, how do we know the emitted wavelength of a star that is moving away from us? Wouldn’t we have to be at a constant distance from the star to know that value?</p>

<redshift>
How is z calculated in red shift observations?
tomh
# How is z calculated in red shift observations?

(Asked by: username_0 on 2022-07-21)

If z is calculated using the ratio between [observable wavelength and emitted wavelength](https://wikimedia.org/api/rest_v1/media/math/render/svg/6a999cd6480ab61dddeeb5372c8037c0d533f2c9), how do we know the emitted wavelength of a star that is moving away from us? Wouldn’t we have to be at a constant distance from the star to know that value?

**Tags:** redshift

---

## 1 answers

### Answer by username_1 on 2022-07-21. Score: 3 | Accepted answer

The light emitted by stars passes through various gases, these gases absorb very specific wavelengths of light. For example Hydrogen will absorb light at exactly 6563 [Å](https://en.wikipedia.org/wiki/Angstrom). So we know that the spectrum of light would have a dark line at 6563 Å if the galaxy wasn't moving away from us.

If the light is redshifted, this line also gets redshifted. So if this line appears at 7830 Å, we can calculate the redshift.

Because gases absorb (and emit) light at particular known wavelengths, we do know emitted wavelength of the light.

> **Comments:**
> **username_0**: Thank you - makes sense! (Just googled to work out that Å is Angstroms https://en.wikipedia.org/wiki/Angstrom  - a way of representing very tiny lengths)  
> **username_2**: Hi James.  If you don't mind, I'd love to ask couple of questions.  Question 1 - at which step would light go through hydrogen ? you mean hydrogen gas that exists in a space(clouds of it -before forming the star from it) or you mean when fusion happens inside the core, energy (photon) is released and before it gets out of the surface, it gets scattered through other hydrogen atoms inside core.  
> **username_2**: Question 2: if the answer from question 1 is both, then in terms of fusion in the core, released photon could scatter between multiple times in different hydrogen atoms, wouldn't this redshift the wavelength even more or once light is gone through hydrogen atom one time, it doesn't matter how many times it goes through with it later, its wavelength spectrum will stay the same ?  
> **username_2**: Question 3: in the core, light would definately not only scatter hydrogen, but some other stuff as well before it would reach the outer shell and start moving to us. What if it scattered first through hydrogen and then helium after that it came to us ?  
> **username_2**: Question 4: https://physics.stackexchange.com/questions/760857/why-does-redshift-happen.  
> **username_2**: Question 5: https://astronomy.stackexchange.com/questions/53504/how-redshift-can-be-detected  
> **username_2**: Thank you in advance if you could help a little bit.  
> **username_3**: @Giorgi Lagidze: No, that is not how Stack Exchange works (unless the intent is to prompt improvement of the answer). You need to consider the scary proposition of asking a new question. (It should also be one question at a time, unless they are very closely related (essentially one question).)  
> **username_3**: The [Lyman break](https://en.wikipedia.org/wiki/Lyman-break_galaxy) could [be covered](https://www.youtube.com/watch?v=2VVVADtXNBE&t=23m51s) as well.  

---

