<p>Only a few percent of galaxies in the local universe have active galactic nuclei (e.g., <a href="https://ui.adsabs.harvard.edu/abs/2020AJ....159...69M/abstract" rel="nofollow noreferrer">Mishra &amp; Dai 2020</a>). The fraction with <em>optically visible</em> AGN (e.g., Seyfert nuclei) is lower than this, since in some galaxies only the X-ray or radio emission gets through the dust in the nuclear region of the galaxy. Even when the AGN is directly visible in optical light, it is usually significantly fainter than the rest of the galaxy. True quasars are quite rare.</p>
<p>So in most cases, the &quot;very bright spot at the center&quot; is produced by a dense accumulation of stars, either in a bulge of some kind or in a compact nuclear star cluster. This is the case for M101 (e.g., <a href="https://ui.adsabs.harvard.edu/abs/2010ApJ...723...54K/abstract" rel="nofollow noreferrer">Kormendy et al. 2010</a>), the galaxy whose image you presented in your question.</p>

CC BY-SA 4.0
2020-12-15T19:31:26.657Z
2020-12-15T19:31:26.657Z
Peter Erwin
<p>Active Galactic Nuclei (AGNs) are very luminous galactic nuclei. In Quasars, a type of an AGN, the nucleus outshines the host galaxy. Therefore the main source of radiation is the nucleus, not the stars and the host.</p>

CC BY-SA 4.0
2020-12-13T13:36:49.740Z
While typically true, this answer would benefit from some numbers (with references) to back up the claim.
pela
CC BY-SA 4.0
2020-12-13T15:27:58.713Z
The majority of nearby galaxies do not have active nuclei. So for galaxies in general, the nuclear light is from stars.
Peter Erwin
CC BY-SA 4.0
2020-12-13T15:50:54.257Z
@PeterErwin I _think_ the question is about galaxies that do show AGN activity though.
pela
CC BY-SA 4.0
2020-12-14T02:01:41.150Z
Wow! That's amazing, to think that one object can really outshine entire galaxies. If quasars account for the majority of radiation of active galaxies, then would that make galaxies with no active quasar harder to detect?
Kinorana
CC BY-SA 4.0
2020-12-14T06:16:01.340Z
@Kinorana It depends on a lot of factors, such as Star Formation Rate, Mergers, Instruments, etc. Also, there are galaxies that are AGNs but do not outshine their galaxies.
AstroTeen
CC BY-SA 4.0
2020-12-15T19:10:37.283Z
@pela I thought it was ambiguous, since it referred to "the bright spot at the center of galaxies", and seemed to imply that all galaxies had "a quasar", which is obviously not the case.
Peter Erwin
CC BY-SA 4.0
2020-12-15T19:12:05.687Z
The majority of AGN in the local universe do not outshine their host galaxies; some AGNs are quite faint, or only visible in X-rays, for example.
Peter Erwin
CC BY-SA 4.0
2020-12-15T20:14:32.517Z
Yes, re-reading the question I agree it's ambiguous @PeterErwin.
pela
CC BY-SA 4.0
2020-12-13T10:10:50.510Z
2020-12-13T10:10:50.510Z
AstroTeen
<p>From what I've been able to gather, the very bright spot at the center of galaxies is produced by both the central quasar and a dense cluster of millions of stars that surround it. How much of the central light is produced by the quasar, and how much comes from the star cluster?</p>
<p><a href="https://i.sstatic.net/6UkZr.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6UkZr.png" alt="enter image description here" /></a></p>

<light><quasars><active-galaxy>
How much light comes from quasars in an AGN?
Kinorana
# How much light comes from quasars in an AGN?

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

From what I've been able to gather, the very bright spot at the center of galaxies is produced by both the central quasar and a dense cluster of millions of stars that surround it. How much of the central light is produced by the quasar, and how much comes from the star cluster?

[![enter image description here](https://i.sstatic.net/6UkZr.png)](https://i.sstatic.net/6UkZr.png)

**Tags:** light, quasars, active-galaxy


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## 2 answers

### Answer by username_1 on 2020-12-15. Score: 2 | Accepted answer

Only a few percent of galaxies in the local universe have active galactic nuclei (e.g., [Mishra & Dai 2020](https://ui.adsabs.harvard.edu/abs/2020AJ....159...69M/abstract)). The fraction with *optically visible* AGN (e.g., Seyfert nuclei) is lower than this, since in some galaxies only the X-ray or radio emission gets through the dust in the nuclear region of the galaxy. Even when the AGN is directly visible in optical light, it is usually significantly fainter than the rest of the galaxy. True quasars are quite rare.

So in most cases, the "very bright spot at the center" is produced by a dense accumulation of stars, either in a bulge of some kind or in a compact nuclear star cluster. This is the case for M101 (e.g., [Kormendy et al. 2010](https://ui.adsabs.harvard.edu/abs/2010ApJ...723...54K/abstract)), the galaxy whose image you presented in your question.


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### Answer by username_2 on 2020-12-13. Score: 2

Active Galactic Nuclei (AGNs) are very luminous galactic nuclei. In Quasars, a type of an AGN, the nucleus outshines the host galaxy. Therefore the main source of radiation is the nucleus, not the stars and the host.

> **Comments:**
> **username_3**: While typically true, this answer would benefit from some numbers (with references) to back up the claim.  
> **username_1**: The majority of nearby galaxies do not have active nuclei. So for galaxies in general, the nuclear light is from stars.  
> **username_3**: @username_1 I _think_ the question is about galaxies that do show AGN activity though.  
> **username_0**: Wow! That's amazing, to think that one object can really outshine entire galaxies. If quasars account for the majority of radiation of active galaxies, then would that make galaxies with no active quasar harder to detect?  
> **username_2**: @username_0 It depends on a lot of factors, such as Star Formation Rate, Mergers, Instruments, etc. Also, there are galaxies that are AGNs but do not outshine their galaxies.  
> **username_1**: @username_3 I thought it was ambiguous, since it referred to "the bright spot at the center of galaxies", and seemed to imply that all galaxies had "a quasar", which is obviously not the case.  
> **username_1**: The majority of AGN in the local universe do not outshine their host galaxies; some AGNs are quite faint, or only visible in X-rays, for example.  
> **username_3**: Yes, re-reading the question I agree it's ambiguous @username_1.  

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