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Not all galaxies are spiral in shape[1][2], some nebulae are huge[3] and nebula are the nursery of stars[4]. How to tell them apart?

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I have already compare the contents for galaxy and a nebula, they are very similar as both have stars, planets and dusts.

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  • $\begingroup$ It may be worth pointing out that, originally, there was no distinction between the two, and scientists of the time called both objects nebulae. It took some time before they could be distinguished, which is an issue of gravitational binding. The smallest (or at least least massive) known galaxy Segue 2 has a significantly smaller expanse than the biggest nebula. $\endgroup$
    – zibadawa timmy
    Commented Apr 24, 2015 at 14:17

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Galaxies are a large organised collection of stars (& nebulae) outside of our own galaxy, very distant. They shine by their own light.

Nebulae are clouds of gas & debris from, usually, a stellar explosion (Nova, supernova, etc), within our own galaxy, lit up by nearby stars (maybe internal ones) - but the gas is not glowing with it's own light, generally.

So a galaxy is going to be a bright clump of glowing stars, regular in shape - be that elliptical, barred or other, generally they will appear symmetrical & regular. A nebula will be much more amorphous & patchy, possibly with shapes from the clouds of gas (c.f. Horsehead, Orion), and you will be able to see individual stars in it - unlike a galaxy where they are too tightly packed & distant to make out individual stars.

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  • $\begingroup$ Many galaxies, including the two closest to the Sun, do not fit your definition of regularity. $\endgroup$
    – ProfRob
    Commented Apr 22, 2015 at 5:55
  • $\begingroup$ Not compared to your average nebula? Really? $\endgroup$
    – Mark Williams
    Commented Apr 22, 2015 at 7:55
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    $\begingroup$ Is this a galaxy or a nebula? astropixels.com/galaxies/images/SMC-02w.jpg Is it symmetrical and regular? The thing on the right fitst that definition, but that isn't a galaxy. $\endgroup$
    – ProfRob
    Commented Apr 22, 2015 at 11:31
  • $\begingroup$ Nebulae can exist in other galaxies. They pretty much always do. Nothing is special about the Milky Way in this regard. $\endgroup$
    – zibadawa timmy
    Commented Apr 24, 2015 at 14:09
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That is a good question, and before it was known that Andromeda is a galaxy, it was indeed assumed to be a nebula.

What really gives away a galaxy is its distance. Even Andromeda, with is the closest galaxy to ours, is over 30 times farther away than any object within our galaxy. If you know the distance, you can tell pretty clearly if it's something within our galaxy, or a completely different galaxy farther out.

Just from looking through a telescope it can be really difficult to tell them apart.

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  • $\begingroup$ Andromeda is not the closest galaxy. $\endgroup$
    – ProfRob
    Commented Apr 23, 2015 at 12:07

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