<strong>Even &quot;round&quot; galaxies look different from stars</strong>
<p>cphyc's answers the question excellently: Spectroscopy is the answer, although since — as explained below — galaxies are <em>not</em> point sources, the morphology of stars and galaxies is also different: even elliptical galaxies observed along one of their axes look different from stars. Although both are round, the way that their light falls off radially is different; stars' light decrease roughly as a normal distribution from the center and out (with some extra profile folded in which depends on the instrument), while the <em>surface brightness profile</em> of galaxies decrease in a somewhat more complicated fashion (e.g. a <a href="https://en.wikipedia.org/wiki/Sersic_profile" rel="noreferrer">Sérsic profile</a>).</p>
<strong>Can galaxies be point sources?</strong>
<p>Wrt. the fraction of galaxies that are point source, the answer is virtually none. Galaxies can almost always be resolved although, as cphyc also correctly says, not with any instrument. Radio and gamma-ray telescopes have very poor resolution, and at these wavelengths the sources usually cannot be resolved unless they're relatively nearby. But at optical wavelengths, as well as UV and IR, telescopes like the Hubble Space Telescope and even good ground-based telescopes can resolve ~all galaxies, unless they're so small that they're too dim to be seen anyway.</p>
<strong>Angular diameter in an expanding Universe</strong>
<p>The reason is a rather peculiar feature of the expanding Universe: A galaxy will look smaller and smaller, the farther away it is (as expected from everyday life), but only out to a certain distance, after which they will appear larger and larger. Why is this so? Because light moves with a finite speed, we observe galaxies as they were in the past — the more distant, the longer time ago. And since in an expanding Universe, &quot;long time ago&quot; also means closer, the angle that a galaxy spans on the sky is the angle that it spanned when it <em>emitted</em> the light, not the angle it spans <em>today</em>. That is, very distant galaxies emitted the light we see today when they were so close that they spanned a large angle.</p>
<p>The exact relation between distance and the solid angle of a galaxy depends on the cosmology (i.e. the values of density parameters, Hubble constant, etc.). For the latest <a href="http://arxiv.org/abs/1502.01589" rel="noreferrer">Planck measurements (2015)</a>, a galaxy that is 1 kpc (~3000 lightyears) across — which would be considered a  small galaxy — spans an angle given by this figure:</p>
<p><a href="https://i.sstatic.net/5UPyK.png" rel="noreferrer"><img src="https://i.sstatic.net/5UPyK.png" alt="arcsec_kpc" /></a></p>
<p>You'll see that galaxies look smaller and smaller the farther there are away, until at a distance of roughly 15 billion lightyears, after which they look larger again. The most distant galaxy observed, <a href="https://en.wikipedia.org/wiki/GN-z11" rel="noreferrer">GN-z11</a>, is so far away that its light was emitted less than half a billion years after the Big Bang. With a radius of <span class="math-container">$0.6\pm0.3\,\mathrm{kpc}$</span> (<a href="http://arxiv.org/abs/1603.00461" rel="noreferrer">Oesch et al. 2016</a>) it still spans 0.15 arcsec, which is resolvable by HST.</p>
<strong>Decreasing surface brightness</strong>
<p>Unfortunately, this effect also makes distant galaxies more difficult to detect. A galaxy only emits so much light, so distributing its light over, say, twice the angular diameter, makes it four times less bright.</p>
<p>Thus, the problem of observing very distant galaxies is not that they're small, but that they're <em>dim</em>.</p>

CC BY-SA 3.0
2017-04-13T00:39:47.083Z
@pela Will it make sense to think of this as follows: when new galaxies become visible in our Cosmological Horizon, they would span across a larger angular diameter and thus appear bigger, even if very faint?
Dhruv Saxena
CC BY-SA 3.0
2017-04-13T08:06:41.927Z
@DhruvSaxena: In principle yes, but if you could see all the way to the horizon, you would look back in time all the way to the Big Bang ($z\rightarrow\infty$) where no galaxies had yet formed. We cannot really see any farther than the CMB ($z\simeq1100$), but even then there were no galaxies. We already _are_ able to see some of the very first galaxies, which formed a few 100 million years after BB ($z\sim10$). In principle they should look large and faint, but the effect is somewhat counteracted by the fact that galaxies at that time simply had not grown to be so large as they are today.
pela
CC BY-SA 3.0
2016-09-15T12:39:37.923Z
2017-04-12T20:29:18.853Z
2020-06-17T09:47:40.653Z
pela
<p>Good answers have already been given, but I wanted to provide another way of looking at it. Take a look at the image below, which is the <a href="https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field#Hubble_eXtreme_Deep_Field" rel="noreferrer">Hubble Extreme Deep Field</a> (XDF) $-$ for those who don't know, this is a small patch of the sky that Hubble has stared at for a total of 23 days over 10 years $-$ and you'll notice something interesting. It is clear to see that many of the larger objects are galaxies, but you'll see a great number of smaller pinpoints of light (nearly 5,500 of them) which are galaxies so far away that Hubble can barely resolve their extent and size. Now take a look at the bright object in the lower right quadrant. You should see that it has blue and red spikes around it, referred to as <a href="https://en.wikipedia.org/wiki/Diffraction_spike" rel="noreferrer">diffraction spikes</a>. This object is clearly a star and you can primarily tell because of the diffraction spikes. You don't see these diffraction spikes on the galaxies, even the galaxies that are tiny pinpoints. This is a relatively easy way to visually distinguish between a star and a galaxy when you're looking at it through a telescope where such diffraction spikes are expected to occur.</p>

<p><a href="https://i.sstatic.net/icAUQ.jpg" rel="noreferrer"><img src="https://i.sstatic.net/icAUQ.jpg" alt="Hubble Extreme Deep Field"></a></p>

<p>This implies that visually, stars and galaxies look different, even if they're both tiny spots on the image. There will also be differences in the way they look in less perceptible ways. This concept is capitalized upon by a program widely used by astronomers, <a href="http://www.astromatic.net/software/sextractor" rel="noreferrer">SExtractor</a>, designed to be given an image of the sky and be able to differentiate between stars and galaxies. It uses these small differences between the ways galaxies and stars appear in images to figure out which is which. If you want more detailed information on how this program distinguishes between stars and galaxies, take a look at their <a href="http://adsabs.harvard.edu/abs/1996A%26AS..117..393B" rel="noreferrer">published paper</a>.</p>

CC BY-SA 3.0
2016-09-15T15:49:00.843Z
Fantastic additional information. Thank you.
MaxW
CC BY-SA 3.0
2016-09-15T17:40:25.487Z
Why don't galaxies have diffraction spikes?
John Dvorak
CC BY-SA 3.0
2016-09-15T17:53:33.377Z
@JanDvorak It is not so much that galaxies don't produce diffraction spikes, it's more than you can't see diffraction spikes on galaxies. Galaxies are extended objects, whereas stars are point sources. For a galaxy, every point produces a (dim) diffraction spike, but for the entire image, those spikes smear together so you'll never see nice diffraction spikes for a galaxy like you would for a "point-like" star. Secondly, galaxies are often dimmer than stars. Any resulting diffraction spikes are going to be exceedingly hard to see.
zephyr
CC BY-SA 3.0
2016-09-15T22:00:47.273Z
If that star with diffraction spikes was far enough away, would you still see the diffraction spikes, or would it be so small that it would look the same as the supposed galaxy points do? That's what I pictured when I read the question. Obviously the diffraction spikes make it easy to tell the difference, but is there a point where the light source is so small you can't tell the difference? Or is SExtractor that accurate that it can tell from the tiniest of light sources?
Zack
CC BY-SA 3.0
2016-09-16T06:20:02.543Z
@Zack A single star that far away essentially wouldn't be seen. But even then, it would have a single set of spikes where a galaxy will have billions of overlapping sets. In 2015, [Hubble photos resolved individual stars](http://www.nasa.gov/content/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy) in Andromeda. I don't think individual stars have otherwise ever been imaged outside the Milky Way (and maybe in a couple closer dwarf galaxies).
user2338816
CC BY-SA 3.0
2016-09-16T08:00:12.000Z
Also, it is worth noting that diffraction spikes are due to the fact that your instrument sees the object as a point / as an under-resolved extended source. For example, you you stare at the sky with your naked eyes, stars are 'shining' (you see theses spikes) but planets like Mars and Jupiter don't. This is because our eye sees them as an under-resolved extended source, whereas stars are just points (for your eye).
cphyc
CC BY-SA 3.0
2016-09-16T08:34:25.360Z
@user2338816 I thought seeing individual stars in Andramada “nebula“ was the final proof of it being an “island universe“, and measuring individual cephid variables found the distance and scale to other galaxies.
JDługosz
CC BY-SA 3.0
2016-09-16T09:59:52.263Z
@JDługosz Detecting the light variation of a Cepheid in a cloudy haze of light from many stars doesn't require actual resolution of a single star. He saw a sharp increase in brightness in what he thought was a nebula and assumed he'd witnessed a nova. After comparing against earlier photos by others, he recognized the variability as a Cepheid. There was math that could calculate the distance of a Cepheid, and the result put it at least a million light-years outside our galaxy. That's when he realized it wasn't a nebula, not because if individual stars. Still looked like a haze.
user2338816
CC BY-SA 3.0
2016-09-16T13:23:53.113Z
@cphyc I think your comment is misleading. You see diffraction spikes for stars because the light is diffracting off the struts holding the secondary mirror in place. You don't see it for galaxies and planets for the reason I outlined above. It's not that they don't exist, they just blend together and so clear diffraction spikes can't be made out. Also, I don't know about you, but I don't see diffraction spikes on stars I see with my naked eye. Not unless I squint (in which case the diffraction is due to your eyelashes). There needs to be something for the light to defract off of.
zephyr
CC BY-SA 3.0
2016-09-16T13:26:24.767Z
@Zack You seem to be misunderstanding how it works. Stars have diffraction spikes precisely because they are so far away. Their physical size to distance ratio is so incredibly tiny (compared to say, galaxies) that they're effectively point sources. Because they are a single point of light, they have strong diffraction spikes created by that single point. The brightness of the spikes dims with the distance, but the spikes are so well defined because the stars are so point-like due to their distance.
zephyr
CC BY-SA 3.0
2016-09-15T15:47:24.517Z
2016-09-15T15:47:24.517Z
zephyr
<p>To distinguish galaxies from stars, you can use the spectrum. Roughly, stars have a black-body like spectrum with features depending on the absorption and emission on the line of sight and in the chromosphere of the star.</p>

<p>Galaxies on the other hand of a spectrum that is the composite of tons of stars. The spectrum will for example be much wider (ranging from smaller to larger wavelengths) because of the diversity in the spectra of stars.</p>

<p>Take a look at <a href="http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra_astro_types.html">http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra_astro_types.html</a> if you want a quick overview of the differences.</p>

<p>I don't have a precise number about the number of galaxies we see as point source, but the answer varies greatly from one instrument to another one. If you try to observe a galaxy using radiotelescopes in interferometry, you can resolve much better scales than an Earth-based small visible telescope, etc…</p>

CC BY-SA 3.0
2016-09-15T18:04:25.020Z
Has it been possible to actually reach that high resolution  to detect different bands from the highly redshifted spectrum?
Lelouch
CC BY-SA 3.0
2016-09-16T05:16:55.997Z
Also, the stars in a galaxy have more motion than the surface of a star, the lines will be more blurred out by doppler shifting.
Loren Pechtel
CC BY-SA 3.0
2016-09-16T05:39:30.500Z
You can also poke around in images produced by the Sloan Digital Sky Survey (SDSS) that have a resolution of about 1 arcsec and compare them with images from the WISE Atlas, that has a resolution of about 10 arcsec (6 arcsec native, convolved with the PSF to improve sensitivity of detecting point-like objects). Compare the galaxy at (179.710668548, -0.438511083) - nice and resolved in [SDSS](http://skyserver.sdss.org/dr13/en/tools/chart/navi.aspx), featureless dot in [AllWISE](http://irsa.ipac.caltech.edu/applications/wise).
Sean Lake
CC BY-SA 3.0
2016-09-16T07:56:13.990Z
@Lelouch because the whole spectrum is uniformly redshifted, you can actually resolve bands, etc… on Earth even for far away galaxies. However, having the bands in the *emitted* visible spectrum of the galaxy is increasingly difficult.
cphyc
CC BY-SA 3.0
2016-09-15T09:48:20.370Z
2016-09-15T09:48:20.370Z
cphyc
<p>Obviously a star would be a point source. A galaxy should be an irregular blob if close, but if it is far away then it would seem that a galaxy too would be just a point source. </p>

<p>Given that the star and galaxy were both only detectable as point sources can astronomers tell them apart with redshift? By some other method?</p>

<p>A followup question...</p>

<p>What percentage of galaxies if our universe can we only detect as point sources? </p>

CC BY-SA 3.0
2016-09-15T09:49:18.373Z
The percentage of galaxies we see as point sources depends on the instrument, so which one are you thinking of?
cphyc
<star><galaxy><redshift>
Can the difference between a star and a galaxy which are point sources be detected?
MaxW
# Can the difference between a star and a galaxy which are point sources be detected?

(Asked by: username_0 on 2016-09-15)

Obviously a star would be a point source. A galaxy should be an irregular blob if close, but if it is far away then it would seem that a galaxy too would be just a point source.

Given that the star and galaxy were both only detectable as point sources can astronomers tell them apart with redshift? By some other method?

A followup question...

What percentage of galaxies if our universe can we only detect as point sources?

**Tags:** star, galaxy, redshift
> **Comments:**
> **username_1**: The percentage of galaxies we see as point sources depends on the instrument, so which one are you thinking of?  

---

## 3 answers

### Answer by username_1 on 2016-09-15. Score: 15 | Accepted answer

To distinguish galaxies from stars, you can use the spectrum. Roughly, stars have a black-body like spectrum with features depending on the absorption and emission on the line of sight and in the chromosphere of the star.

Galaxies on the other hand of a spectrum that is the composite of tons of stars. The spectrum will for example be much wider (ranging from smaller to larger wavelengths) because of the diversity in the spectra of stars.

Take a look at <http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra_astro_types.html> if you want a quick overview of the differences.

I don't have a precise number about the number of galaxies we see as point source, but the answer varies greatly from one instrument to another one. If you try to observe a galaxy using radiotelescopes in interferometry, you can resolve much better scales than an Earth-based small visible telescope, etc…

> **Comments:**
> **username_9**: Has it been possible to actually reach that high resolution  to detect different bands from the highly redshifted spectrum?  
> **username_10**: Also, the stars in a galaxy have more motion than the surface of a star, the lines will be more blurred out by doppler shifting.  
> **username_11**: You can also poke around in images produced by the Sloan Digital Sky Survey (SDSS) that have a resolution of about 1 arcsec and compare them with images from the WISE Atlas, that has a resolution of about 10 arcsec (6 arcsec native, convolved with the PSF to improve sensitivity of detecting point-like objects). Compare the galaxy at (179.710668548, -0.438511083) - nice and resolved in [SDSS](http://skyserver.sdss.org/dr13/en/tools/chart/navi.aspx), featureless dot in [AllWISE](http://irsa.ipac.caltech.edu/applications/wise).  
> **username_1**: @Lelouch because the whole spectrum is uniformly redshifted, you can actually resolve bands, etc… on Earth even for far away galaxies. However, having the bands in the *emitted* visible spectrum of the galaxy is increasingly difficult.  

---

### Answer by username_2 on 2016-09-15. Score: 13

**Even "round" galaxies look different from stars**

cphyc's answers the question excellently: Spectroscopy is the answer, although since — as explained below — galaxies are *not* point sources, the morphology of stars and galaxies is also different: even elliptical galaxies observed along one of their axes look different from stars. Although both are round, the way that their light falls off radially is different; stars' light decrease roughly as a normal distribution from the center and out (with some extra profile folded in which depends on the instrument), while the *surface brightness profile* of galaxies decrease in a somewhat more complicated fashion (e.g. a [Sérsic profile](https://en.wikipedia.org/wiki/Sersic_profile)).

**Can galaxies be point sources?**

Wrt. the fraction of galaxies that are point source, the answer is virtually none. Galaxies can almost always be resolved although, as cphyc also correctly says, not with any instrument. Radio and gamma-ray telescopes have very poor resolution, and at these wavelengths the sources usually cannot be resolved unless they're relatively nearby. But at optical wavelengths, as well as UV and IR, telescopes like the Hubble Space Telescope and even good ground-based telescopes can resolve ~all galaxies, unless they're so small that they're too dim to be seen anyway.

**Angular diameter in an expanding Universe**

The reason is a rather peculiar feature of the expanding Universe: A galaxy will look smaller and smaller, the farther away it is (as expected from everyday life), but only out to a certain distance, after which they will appear larger and larger. Why is this so? Because light moves with a finite speed, we observe galaxies as they were in the past — the more distant, the longer time ago. And since in an expanding Universe, "long time ago" also means closer, the angle that a galaxy spans on the sky is the angle that it spanned when it *emitted* the light, not the angle it spans *today*. That is, very distant galaxies emitted the light we see today when they were so close that they spanned a large angle.

The exact relation between distance and the solid angle of a galaxy depends on the cosmology (i.e. the values of density parameters, Hubble constant, etc.). For the latest [Planck measurements (2015)](http://arxiv.org/abs/1502.01589), a galaxy that is 1 kpc (~3000 lightyears) across — which would be considered a small galaxy — spans an angle given by this figure:

[![arcsec_kpc](https://i.sstatic.net/5UPyK.png)](https://i.sstatic.net/5UPyK.png)

You'll see that galaxies look smaller and smaller the farther there are away, until at a distance of roughly 15 billion lightyears, after which they look larger again. The most distant galaxy observed, [GN-z11](https://en.wikipedia.org/wiki/GN-z11), is so far away that its light was emitted less than half a billion years after the Big Bang. With a radius of $0.6\pm0.3\,\mathrm{kpc}$ ([Oesch et al. 2016](http://arxiv.org/abs/1603.00461)) it still spans 0.15 arcsec, which is resolvable by HST.

**Decreasing surface brightness**

Unfortunately, this effect also makes distant galaxies more difficult to detect. A galaxy only emits so much light, so distributing its light over, say, twice the angular diameter, makes it four times less bright.

Thus, the problem of observing very distant galaxies is not that they're small, but that they're *dim*.

> **Comments:**
> **username_3**: @pela Will it make sense to think of this as follows: when new galaxies become visible in our Cosmological Horizon, they would span across a larger angular diameter and thus appear bigger, even if very faint?  
> **username_2**: @DhruvSaxena: In principle yes, but if you could see all the way to the horizon, you would look back in time all the way to the Big Bang ($z\rightarrow\infty$) where no galaxies had yet formed. We cannot really see any farther than the CMB ($z\simeq1100$), but even then there were no galaxies. We already _are_ able to see some of the very first galaxies, which formed a few 100 million years after BB ($z\sim10$). In principle they should look large and faint, but the effect is somewhat counteracted by the fact that galaxies at that time simply had not grown to be so large as they are today.  

---

### Answer by username_4 on 2016-09-15. Score: 12

Good answers have already been given, but I wanted to provide another way of looking at it. Take a look at the image below, which is the [Hubble Extreme Deep Field](https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field#Hubble_eXtreme_Deep_Field) (XDF) $-$ for those who don't know, this is a small patch of the sky that Hubble has stared at for a total of 23 days over 10 years $-$ and you'll notice something interesting. It is clear to see that many of the larger objects are galaxies, but you'll see a great number of smaller pinpoints of light (nearly 5,500 of them) which are galaxies so far away that Hubble can barely resolve their extent and size. Now take a look at the bright object in the lower right quadrant. You should see that it has blue and red spikes around it, referred to as [diffraction spikes](https://en.wikipedia.org/wiki/Diffraction_spike). This object is clearly a star and you can primarily tell because of the diffraction spikes. You don't see these diffraction spikes on the galaxies, even the galaxies that are tiny pinpoints. This is a relatively easy way to visually distinguish between a star and a galaxy when you're looking at it through a telescope where such diffraction spikes are expected to occur.

[![Hubble Extreme Deep Field](https://i.sstatic.net/icAUQ.jpg)](https://i.sstatic.net/icAUQ.jpg)

This implies that visually, stars and galaxies look different, even if they're both tiny spots on the image. There will also be differences in the way they look in less perceptible ways. This concept is capitalized upon by a program widely used by astronomers, [SExtractor](http://www.astromatic.net/software/sextractor), designed to be given an image of the sky and be able to differentiate between stars and galaxies. It uses these small differences between the ways galaxies and stars appear in images to figure out which is which. If you want more detailed information on how this program distinguishes between stars and galaxies, take a look at their [published paper](http://adsabs.harvard.edu/abs/1996A%26AS..117..393B).

> **Comments:**
> **username_0**: Fantastic additional information. Thank you.  
> **username_5**: Why don't galaxies have diffraction spikes?  
> **username_4**: @JanDvorak It is not so much that galaxies don't produce diffraction spikes, it's more than you can't see diffraction spikes on galaxies. Galaxies are extended objects, whereas stars are point sources. For a galaxy, every point produces a (dim) diffraction spike, but for the entire image, those spikes smear together so you'll never see nice diffraction spikes for a galaxy like you would for a "point-like" star. Secondly, galaxies are often dimmer than stars. Any resulting diffraction spikes are going to be exceedingly hard to see.  
> **username_6**: If that star with diffraction spikes was far enough away, would you still see the diffraction spikes, or would it be so small that it would look the same as the supposed galaxy points do? That's what I pictured when I read the question. Obviously the diffraction spikes make it easy to tell the difference, but is there a point where the light source is so small you can't tell the difference? Or is SExtractor that accurate that it can tell from the tiniest of light sources?  
> **username_7**: @Zack A single star that far away essentially wouldn't be seen. But even then, it would have a single set of spikes where a galaxy will have billions of overlapping sets. In 2015, [Hubble photos resolved individual stars](http://www.nasa.gov/content/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy) in Andromeda. I don't think individual stars have otherwise ever been imaged outside the Milky Way (and maybe in a couple closer dwarf galaxies).  
> **username_1**: Also, it is worth noting that diffraction spikes are due to the fact that your instrument sees the object as a point / as an under-resolved extended source. For example, you you stare at the sky with your naked eyes, stars are 'shining' (you see theses spikes) but planets like Mars and Jupiter don't. This is because our eye sees them as an under-resolved extended source, whereas stars are just points (for your eye).  
> **username_8**: @user2338816 I thought seeing individual stars in Andramada “nebula“ was the final proof of it being an “island universe“, and measuring individual cephid variables found the distance and scale to other galaxies.  
> **username_7**: @JDługosz Detecting the light variation of a Cepheid in a cloudy haze of light from many stars doesn't require actual resolution of a single star. He saw a sharp increase in brightness in what he thought was a nebula and assumed he'd witnessed a nova. After comparing against earlier photos by others, he recognized the variability as a Cepheid. There was math that could calculate the distance of a Cepheid, and the result put it at least a million light-years outside our galaxy. That's when he realized it wasn't a nebula, not because if individual stars. Still looked like a haze.  
> **username_4**: @cphyc I think your comment is misleading. You see diffraction spikes for stars because the light is diffracting off the struts holding the secondary mirror in place. You don't see it for galaxies and planets for the reason I outlined above. It's not that they don't exist, they just blend together and so clear diffraction spikes can't be made out. Also, I don't know about you, but I don't see diffraction spikes on stars I see with my naked eye. Not unless I squint (in which case the diffraction is due to your eyelashes). There needs to be something for the light to defract off of.  
> **username_4**: @Zack You seem to be misunderstanding how it works. Stars have diffraction spikes precisely because they are so far away. Their physical size to distance ratio is so incredibly tiny (compared to say, galaxies) that they're effectively point sources. Because they are a single point of light, they have strong diffraction spikes created by that single point. The brightness of the spikes dims with the distance, but the spikes are so well defined because the stars are so point-like due to their distance.  

---

