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

Even faint galaxies?

Can spectral analysis distinguish galaxies from individual stars?

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    $\begingroup$ Welcome to Astronomy @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! $\endgroup$
    – andy256
    Commented May 17, 2015 at 7:42
  • $\begingroup$ 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 $\endgroup$ Commented May 18, 2015 at 22:18
  • $\begingroup$ I was just able to locate this article: 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. $\endgroup$ Commented May 18, 2015 at 22:30

1 Answer 1

  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

  • $\begingroup$ 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. $\endgroup$
    – andy256
    Commented May 17, 2015 at 7:46
  • $\begingroup$ 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 :) $\endgroup$ Commented May 17, 2015 at 7:51
  • $\begingroup$ I agree, and up voted your answer. I was suggesting an extra point. $\endgroup$
    – andy256
    Commented May 17, 2015 at 7:53
  • $\begingroup$ OK, reference to the Milky Way added $\endgroup$ Commented May 17, 2015 at 8:02
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    $\begingroup$ @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? $\endgroup$
    – ProfRob
    Commented May 17, 2015 at 16:41

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