<p>It's an observational fact that many disk galaxies have a color gradient with redder (or as you say 'yellow') stars in the central regions and bluer stars in the disk.
There are two things that can cause this central redness.</p>
<ul>
<li>Age, older stars being redder than younger stars</li>
<li>Metallicity, with red stars being more metal rich than blue stars</li>
</ul>
<p>Whether it's age or metallicity or both causing these gradients is topic of current research. It's a popular <a href="https://iopscience.iop.org/article/10.1088/2041-8205/764/1/L1/pdf" rel="noreferrer">theory</a> that galaxies grow inside-out, which causes a red center.</p>

CC BY-SA 4.0
2020-12-17T15:47:17.307Z
2020-12-17T15:47:17.307Z
theWrongAlice
<p>So we've seen millions of galaxies, and all of them have a bright, yellow nucleus. What is the cause of the yellowness? Is it because of aging stars, redshift from the SMBH, or something else?</p>

<star><galaxy><redshift><supermassive-black-hole>
Why are galactic nuclei yellow?
WarpPrime
# Why are galactic nuclei yellow?

(Asked by: username_0 on 2020-12-17)

So we've seen millions of galaxies, and all of them have a bright, yellow nucleus. What is the cause of the yellowness? Is it because of aging stars, redshift from the SMBH, or something else?

**Tags:** star, galaxy, redshift, supermassive-black-hole


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## 1 answers

### Answer by username_1 on 2020-12-17. Score: 5 | Accepted answer

It's an observational fact that many disk galaxies have a color gradient with redder (or as you say 'yellow') stars in the central regions and bluer stars in the disk.
There are two things that can cause this central redness.

* Age, older stars being redder than younger stars
* Metallicity, with red stars being more metal rich than blue stars

Whether it's age or metallicity or both causing these gradients is topic of current research. It's a popular [theory](https://iopscience.iop.org/article/10.1088/2041-8205/764/1/L1/pdf) that galaxies grow inside-out, which causes a red center.


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