<ol>
<li><p>The individual stars can be resolved in galaxies close to us...including our own, the Milky Way Galaxy.</p></li>
<li><p>More distant galaxies are morphological similar to those that can be resolved into stars, so a natural induction would be that they are similar in nature to those we can resolve into stars.</p></li>
<li><p>Galactic spectra are consistent with them being composed of stars in the sort of relative motion we expect.</p></li>
<li><p>Supernovae are observed in distant galaxies, and these are similar to those seen close up which we know are stellar in origin.</p></li>
<li><p>... etc</p></li>
</ol>

CC BY-SA 3.0
2015-05-17T07:46:13.337Z
And I would add: the galaxy we inhabit clearly contains stars. The OP only has to look at the sky on a clear night to see this.
andy256
CC BY-SA 3.0
2015-05-17T07:51:00.997Z
But you need an extra step to establish that the Milky Way is in fact an entity of the same general class as the close by entities we label as galaxies. Which can/has of course be/en done but is not essential here .. maybe :)
Conrad Turner
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2015-05-17T07:53:43.623Z
I agree, and up voted your answer. I was suggesting an extra point.
andy256
CC BY-SA 3.0
2015-05-17T08:02:28.013Z
OK, reference to the Milky Way added
Conrad Turner
CC BY-SA 3.0
2015-05-17T09:26:49.470Z
And one might add that the spectra of galaxies contain absorption lines attributable to heavy elements. The only places we know of in the universe that can produce significant quantities of heavy elements are stars. So stars must be present. A similar argument applies to dust. Also, whether we can resolve individual stars or not does correlate with the estimated distance. i.e. If there were a class of objects that didn't consist of stars, there are certainly no nearby examples..
ProfRob
CC BY-SA 3.0
2015-05-17T13:25:08.473Z
@RobJeffries Heavy elements indicate that there was a super nova (or several) explosions. They do not indicate that stars are currently present. (But it would change our current belief if only heavy stars made up a galaxy and have all exploded so there are no stars in that galaxy currently. Even the most distant galaxies appear to have the full range of stellar types.)
LDC3
CC BY-SA 3.0
2015-05-17T13:42:57.230Z
@LDC3 Many of the heavy elements are NOT made in supernovae, they are made and disseminated into the ISM mostly in lower mass AGB stars (for example carbon). Supernovae do not only occur in massive stars. Type Ia supernovae occur in what were relatively low mass stars with long lifetimes. "Several explosions"?? You need a billion to enrich a galaxy! Yes I suppose you could hypothesise that distant galaxies produce no low-mass stars. But most of the light actually comes from relatively low-mass giant stars in most galaxies.
ProfRob
CC BY-SA 3.0
2015-05-17T13:56:21.113Z
@RobJeffries Are you saying that our sun is producing significant quantities of heavy elements (heavier than iron)? (Are you saying carbon is a heavy element?) Also, from what I read, when our sun dies, it will expel the outer hydrogen layer and most of the core will be compressed into a neutron star. I don't see how this will enrich the ISM with heavy elements (although it will redistribute the dust in the ISM).
LDC3
CC BY-SA 3.0
2015-05-17T16:41:41.813Z
@LDC3 No the sun is not currently producing heavy elements. But it will. Read about AGB stars, 3rd dredge up, thermal pulsations and the s-process. Yes I am classing carbon as a heavy element in the sense it is non-primordial, and must have been produced in a star. The rest of what you have said about the fate of the Sun is plain wrong. What do you think dust is made of?
ProfRob
CC BY-SA 3.0
2015-05-17T07:41:30.317Z
2015-05-17T13:15:39.163Z
2015-05-17T13:15:39.163Z
Conrad Turner
<p>What is the evidence that galaxies are made of billions of stars? </p>

<p>Even faint galaxies?  </p>

<p>Can spectral analysis distinguish galaxies from individual stars?</p>

CC BY-SA 3.0
2015-05-17T07:42:44.953Z
Welcome to [Astronomy.se] @Edwin. Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!
andy256
CC BY-SA 3.0
2015-05-18T22:18:58.753Z
I did search Astronomy.StackExchange and Googled around a bit, but didn't find the answer to my question. I know that it is only since about 1920 that "nebula" were identified as collections of stars and called galaxies. Apparently this was not trivial to figure out. And, star counts of galaxies are given in Wikipedia as from 10^3 to 10^14.  I am curious about how such  counts are made in modern astronomy.  I was just able to locate this article
W. Edwin Clark
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2015-05-18T22:30:08.323Z
I was just able to locate this article: http://www.space.com/25959-how-many-stars-are-in-the-milky-way.html#  which gives a little more detail about how it is done. It seems that the first step is to estimate the mass of the galaxy and then use a model of the galaxy to figure out the number of stars. This means that one must try to subtract out the mass that comes from dark matter. All in all a pretty tricky business.
W. Edwin Clark
<galaxy>
Evidence that galaxies are made of billions of stars?
W. Edwin Clark
# Evidence that galaxies are made of billions of stars?

(Asked by: username_0 on 2015-05-17)

What is the evidence that galaxies are made of billions of stars?

Even faint galaxies?

Can spectral analysis distinguish galaxies from individual stars?

**Tags:** galaxy

> **Comments:**
> **username_1**: Welcome to [Astronomy.se] @Edwin. Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!  
> **username_0**: I did search Astronomy.StackExchange and Googled around a bit, but didn't find the answer to my question. I know that it is only since about 1920 that "nebula" were identified as collections of stars and called galaxies. Apparently this was not trivial to figure out. And, star counts of galaxies are given in Wikipedia as from 10^3 to 10^14.  I am curious about how such  counts are made in modern astronomy.  I was just able to locate this article  
> **username_0**: I was just able to locate this article: http://www.space.com/25959-how-many-stars-are-in-the-milky-way.html#  which gives a little more detail about how it is done. It seems that the first step is to estimate the mass of the galaxy and then use a model of the galaxy to figure out the number of stars. This means that one must try to subtract out the mass that comes from dark matter. All in all a pretty tricky business.  

---

## 1 answers

### Answer by username_2 on 2015-05-17. Score: 7

1. The individual stars can be resolved in galaxies close to us...including our own, the Milky Way Galaxy.
2. More distant galaxies are morphological similar to those that can be resolved into stars, so a natural induction would be that they are similar in nature to those we can resolve into stars.
3. Galactic spectra are consistent with them being composed of stars in the sort of relative motion we expect.
4. Supernovae are observed in distant galaxies, and these are similar to those seen close up which we know are stellar in origin.
5. ... etc

> **Comments:**
> **username_1**: And I would add: the galaxy we inhabit clearly contains stars. The OP only has to look at the sky on a clear night to see this.  
> **username_2**: But you need an extra step to establish that the Milky Way is in fact an entity of the same general class as the close by entities we label as galaxies. Which can/has of course be/en done but is not essential here .. maybe :)  
> **username_1**: I agree, and up voted your answer. I was suggesting an extra point.  
> **username_2**: OK, reference to the Milky Way added  
> **username_3**: And one might add that the spectra of galaxies contain absorption lines attributable to heavy elements. The only places we know of in the universe that can produce significant quantities of heavy elements are stars. So stars must be present. A similar argument applies to dust. Also, whether we can resolve individual stars or not does correlate with the estimated distance. i.e. If there were a class of objects that didn't consist of stars, there are certainly no nearby examples..  
> **username_4**: @RobJeffries Heavy elements indicate that there was a super nova (or several) explosions. They do not indicate that stars are currently present. (But it would change our current belief if only heavy stars made up a galaxy and have all exploded so there are no stars in that galaxy currently. Even the most distant galaxies appear to have the full range of stellar types.)  
> **username_3**: @username_4 Many of the heavy elements are NOT made in supernovae, they are made and disseminated into the ISM mostly in lower mass AGB stars (for example carbon). Supernovae do not only occur in massive stars. Type Ia supernovae occur in what were relatively low mass stars with long lifetimes. "Several explosions"?? You need a billion to enrich a galaxy! Yes I suppose you could hypothesise that distant galaxies produce no low-mass stars. But most of the light actually comes from relatively low-mass giant stars in most galaxies.  
> **username_4**: @RobJeffries Are you saying that our sun is producing significant quantities of heavy elements (heavier than iron)? (Are you saying carbon is a heavy element?) Also, from what I read, when our sun dies, it will expel the outer hydrogen layer and most of the core will be compressed into a neutron star. I don't see how this will enrich the ISM with heavy elements (although it will redistribute the dust in the ISM).  
> **username_3**: @username_4 No the sun is not currently producing heavy elements. But it will. Read about AGB stars, 3rd dredge up, thermal pulsations and the s-process. Yes I am classing carbon as a heavy element in the sense it is non-primordial, and must have been produced in a star. The rest of what you have said about the fate of the Sun is plain wrong. What do you think dust is made of?  

---

