<blockquote>
<p>Star red by nature<br />
Example: red supergiants<br />
They emit red light because of their surface temperature.</p>
</blockquote>
<p>The redshift of a <a href="https://en.wikipedia.org/wiki/Black_body" rel="nofollow noreferrer">blackbody</a> is indeed completely degenerate with its temperature. However, galaxies also have spectral features arising at set (rest-frame) frequencies. Typically these are <a href="https://en.wikipedia.org/wiki/Spectral_line" rel="nofollow noreferrer">spectral emission or absorption lines</a> associated with the energy differences of particular atomic transitions. We can determine the redshift from these features.</p>
<p>However, many telescopes do not have spectrographs! Redshifts often have to be estimated just from galaxy colors (specifically, the ratios of light intensity received through a handful of different color filters). In this case, there is absolutely a possibility for erroneous redshift inference.</p>
<p>See also <a href="https://astronomy.stackexchange.com/q/53504/47607">How can redshifted light be detected?</a></p>
<blockquote>
<p>Object moving further away from us<br />
Let's assume there is a galaxy which is close to us but still moving away from us very fast so even that appear red.</p>
</blockquote>
<p>Such peculiar velocities give rise to <a href="https://en.wikipedia.org/wiki/Redshift-space_distortions" rel="nofollow noreferrer">redshift-space distortions</a>. These can be sources of error if you are trying to fix the locations of individual, cosmologically nearby objects. However, their importance drops at high redshift, because peculiar velocities do not grow with distance (indeed they are smaller at earlier times).</p>
<p>Also, when studying the Universe at a statistical level, redshift-space distortions are more of a tool than a hindrance. These distortions are characteristically aligned along or transverse to our line of sight. If we assume that Earth is not at a privileged location within the universe, it thus becomes clear which statistical features are due to redshift-space distortions, and then they are useful because they give us information about the Universe's velocity structure.</p>

CC BY-SA 4.0
2023-04-25T19:03:05.883Z
2023-04-25T19:21:55.923Z
2023-04-25T19:21:55.923Z
Sten
<p>There are different factors for red appearance of different objects in the night sky. Two reasons which I know are</p>
<ol>
<li>Star red by nature<br />
Example: red supergiants<br />
They emit red light because of their surface temperature.</li>
<li>Object moving further away from us<br />
Let's assume there is a galaxy which is close to us but still moving away from us very fast so even it appears red.</li>
</ol>
<p>So do we depend on only the red appearance of galaxies to find distance?</p>

CC BY-SA 4.0
2023-04-25T14:47:47.663Z
One measures the shift of specific emission lines.
Jon Custer
CC BY-SA 4.0
2023-04-25T14:55:50.687Z
Learn about the cosmic distance ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard_candles.
planetmaker
<distances><redshift>
How can we estimate distance of galaxies by only redshift when there are other several factors other than distance to cause redshift
Prajwal D M
# How can we estimate distance of galaxies by only redshift when there are other several factors other than distance to cause redshift

(Asked by: username_0 on 2023-04-25)

There are different factors for red appearance of different objects in the night sky. Two reasons which I know are

1. Star red by nature  
   Example: red supergiants  
   They emit red light because of their surface temperature.
2. Object moving further away from us  
   Let's assume there is a galaxy which is close to us but still moving away from us very fast so even it appears red.

So do we depend on only the red appearance of galaxies to find distance?

**Tags:** distances, redshift
> **Comments:**
> **username_1**: One measures the shift of specific emission lines.  
> **username_2**: Learn about the cosmic distance ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard_candles.  

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## 1 answers

### Answer by username_3 on 2023-04-25. Score: 4 | Accepted answer

> Star red by nature  
> Example: red supergiants  
> They emit red light because of their surface temperature.

The redshift of a [blackbody](https://en.wikipedia.org/wiki/Black_body) is indeed completely degenerate with its temperature. However, galaxies also have spectral features arising at set (rest-frame) frequencies. Typically these are [spectral emission or absorption lines](https://en.wikipedia.org/wiki/Spectral_line) associated with the energy differences of particular atomic transitions. We can determine the redshift from these features.

However, many telescopes do not have spectrographs! Redshifts often have to be estimated just from galaxy colors (specifically, the ratios of light intensity received through a handful of different color filters). In this case, there is absolutely a possibility for erroneous redshift inference.

See also [How can redshifted light be detected?](https://astronomy.stackexchange.com/q/53504/47607)

> Object moving further away from us  
> Let's assume there is a galaxy which is close to us but still moving away from us very fast so even that appear red.

Such peculiar velocities give rise to [redshift-space distortions](https://en.wikipedia.org/wiki/Redshift-space_distortions). These can be sources of error if you are trying to fix the locations of individual, cosmologically nearby objects. However, their importance drops at high redshift, because peculiar velocities do not grow with distance (indeed they are smaller at earlier times).

Also, when studying the Universe at a statistical level, redshift-space distortions are more of a tool than a hindrance. These distortions are characteristically aligned along or transverse to our line of sight. If we assume that Earth is not at a privileged location within the universe, it thus becomes clear which statistical features are due to redshift-space distortions, and then they are useful because they give us information about the Universe's velocity structure.


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