<p>Quasars have a very low density at the present epoch, but were much more common in the past. Their co-moving space density peaks at redshifts of 2-3.</p>
<p><a href="https://i.sstatic.net/NriiW.gif" rel="noreferrer"><img src="https://i.sstatic.net/NriiW.gif" alt="Bright QSOs space density" /></a></p>
<p><em>The evolving space density of bright quasars. From <a href="https://ned.ipac.caltech.edu/level5/Madau/Madau7.html" rel="noreferrer">https://ned.ipac.caltech.edu/level5/Madau/Madau7.html</a></em>.</p>
<p>The fact that the quasar population density evolves was one of the key arguments against steady state cosmology.</p>

CC BY-SA 4.0
2020-06-26T05:35:02.030Z
2020-06-26T05:35:02.030Z
ProfRob
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Quasar" rel="nofollow noreferrer">Quasar</a> includes the following in its introduction:</p>
<blockquote>
<p>The term quasar originated as a contraction of quasi-stellar (star-like) radio source, because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, and when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some host-galaxies are strongly interacting or merging galaxies.</p>
</blockquote>
<p>So today instead of &quot;quasi-stellar&quot; we might simply say &quot;unresolved&quot;. <s>The light is star-like because the light from galaxies has a strong stellar component.</s> (this second sentence is argued against in comments)</p>
<p>Why are quasars so far away that they required high resolution techniques (i.e. space telescopes or adaptive optics) to resolve them? Is it just that they are fairly rare and a roll of the dice resulted in none close enough to be resolved in older large telescopes of the 1950's, or are there some cosmological effects in play as well?</p>

CC BY-SA 4.0
2020-06-26T15:19:19.210Z
... and now Motorola doesn't make them any more :-) .   //happy Friday
Carl Witthoft
CC BY-SA 4.0
2020-06-26T15:40:33.723Z
"The light is star-like because the light from galaxies has a strong stellar component." No, the light is "star-like" because it looks like a point source from this distance. Once you look at the redshift of this "star" things become less "star-like". There was discussion on whether these are really stars with new physical characteristics embedded in host galaxies (it looked like they were embedded in a foreground galaxy on some plates).
David Tonhofer
CC BY-SA 4.0
2020-06-27T01:10:18.273Z
@CarlWitthoft I have never been able to not hear that jingle in my head every time I see or hear the word quasar. For old time's sake: https://youtu.be/CWhn2BLyM1o?t=24 and here's a strange one I've never seen: https://youtu.be/OdzQC3uInug
uhoh
CC BY-SA 4.0
2020-06-27T01:41:54.113Z
@DavidTonhofer indeed! I've edited and added strikethrough to that sentence, read further, and then asked [In the 1950's how were radio-astrometric positions with portable dishes so precise they could be assigned to their dim optical counterparts (Quasars)?](https://astronomy.stackexchange.com/q/36699/7982) The quote there is quite informative.
uhoh
<observational-astronomy><cosmology><quasars>
Why are quasars so far away that they couldn't be optically resolved in the 1950's?
uhoh
# Why are quasars so far away that they couldn't be optically resolved in the 1950's?

(Asked by: username_0 on 2020-06-25)

Wikipedia's [Quasar](https://en.wikipedia.org/wiki/Quasar) includes the following in its introduction:

> The term quasar originated as a contraction of quasi-stellar (star-like) radio source, because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, and when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some host-galaxies are strongly interacting or merging galaxies.

So today instead of "quasi-stellar" we might simply say "unresolved". ~~The light is star-like because the light from galaxies has a strong stellar component.~~ (this second sentence is argued against in comments)

Why are quasars so far away that they required high resolution techniques (i.e. space telescopes or adaptive optics) to resolve them? Is it just that they are fairly rare and a roll of the dice resulted in none close enough to be resolved in older large telescopes of the 1950's, or are there some cosmological effects in play as well?

**Tags:** observational-astronomy, cosmology, quasars
> **Comments:**
> **username_1**: ... and now Motorola doesn't make them any more :-) .   //happy Friday  
> **username_2**: "The light is star-like because the light from galaxies has a strong stellar component." No, the light is "star-like" because it looks like a point source from this distance. Once you look at the redshift of this "star" things become less "star-like". There was discussion on whether these are really stars with new physical characteristics embedded in host galaxies (it looked like they were embedded in a foreground galaxy on some plates).  
> **username_0**: @CarlWitthoft I have never been able to not hear that jingle in my head every time I see or hear the word quasar. For old time's sake: https://youtu.be/CWhn2BLyM1o?t=24 and here's a strange one I've never seen: https://youtu.be/OdzQC3uInug  
> **username_0**: @DavidTonhofer indeed! I've edited and added strikethrough to that sentence, read further, and then asked [In the 1950's how were radio-astrometric positions with portable dishes so precise they could be assigned to their dim optical counterparts (Quasars)?](https://astronomy.stackexchange.com/q/36699/7982) The quote there is quite informative.  

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## 1 answers

### Answer by username_3 on 2020-06-26. Score: 10 | Accepted answer

Quasars have a very low density at the present epoch, but were much more common in the past. Their co-moving space density peaks at redshifts of 2-3.

[![Bright QSOs space density](https://i.sstatic.net/NriiW.gif)](https://i.sstatic.net/NriiW.gif)

*The evolving space density of bright quasars. From <https://ned.ipac.caltech.edu/level5/Madau/Madau7.html>*.

The fact that the quasar population density evolves was one of the key arguments against steady state cosmology.


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