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For instance, If a large globular cluster were somehow ejected from its galaxy, and was observed by astronomers, would they call it a galaxy, or would there be characteristics of the star cluster that make it obviously not a galaxy by definition? And why are the Magellanic Clouds dwarf galaxies, and not large star clusters? Could we remove stars until the Magellanic clouds became star clusters, or is there something intrinsically galaxy like about these objects?

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I am only using globular clusters as an example, since, at least in photographs, globular clusters look like elliptical galaxies. –  eraticus Mar 19 at 0:57
    
There are also some astronomical objects which are in doubt between stellar clusters and elliptical galaxies, I will try to find the reference. –  Py-ser Mar 19 at 1:10
    
@py-ser I already have, in my answer –  Jeremy Mar 22 at 6:01
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Omega Centauri is an interesting edge case between globular clusters and dwarf galaxies. It may even have a central black hole. –  Keith Thompson Mar 25 at 17:10
    
Yes, Omega Centauri is was the reason why I asked this question. It seemed to have an ambiguously "in between" mass between a small galaxy, and a large cluster. Also, I have seen it stated that this is possibly a small galaxy's center, whose surrounding stars were stolen by the Milky Way. As a non-astronomer, it got me to wondering if there was an intrinsic difference between a small galaxy nucleus (terminology?), and a large globular cluster. –  eraticus Mar 27 at 19:03

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There are a couple of important distinctions between the two types of objects. Galaxies are objects which range in mass from about $10^{9}-10^{12} M_{\odot}$, and contain 'halos' of dark matter which represent the majority of the mass of the object. Now, there are things called dwarf galaxies (which are less massive than regular galaxies; I'd imagine that there really could be a bit of a blurred distinction between these two kinds of objects), however dwarf galaxies still seem to have dark matter components to them.

Globular clusters are tightly bound collections of about $10^{6}$ stars, and do not have significant (if any) dark matter contained within or around them. So for these reasons in practice it is not hard to confuse the two.

Some References:

1) Evidence that dwarf galaxies are dominated by a dark-matter component.

2) Dark matter present in globular clusters are either unlikely, or incredibly highly concentrated and approximately the same mass as the stellar component.

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So the main differences between a galaxy object and a star cluster object is the galaxy's association with dark matter? Is mass a requirement to make the distinction? I am curious if mass is an absolute requirement. Stars can vary in mass, but do not become different things. Also, both references were extremely interesting. Thank you! –  eraticus Mar 24 at 20:24
    
However, your answer makes it sound as if an assessment of dark matter is made to determine whether an object is a galaxy or a cluster. Isn't it more correct to note that when we look at things that have historically been classed as a galaxy or a cluster, and we look at how much dark matter appears to be associated with them, we see a correlation? While we might now use a measure of dark matter present to make a distinction in otherwise unclear cases, surely the amount of gas and the ages of the stars present will be more relevant in the classification of an object? –  Jeremy Mar 25 at 10:52
    
astromax, since we don't yet fully understand the mechanics of dark matter, isn't it possible that a mechanism exists by which globular cluster ejecta attract dark matter to themselves? Do we know any instances of free-standing astronomical objects of the mass of globular clusters that don't have the dark-matter requirements of dwarf galaxies? –  called2voyage Mar 25 at 13:54
    
@eraticus Well, whether total mass is an absolute distinction which can be made to determine an object's classification is not something I couldn't say is universally true. Stars are a different case - they are discrete packages of gas which at one point either collapsed due to an overdensity in a region or did not because it simply didn't have enough mass (I know there are other effects here that determine star fomation; from a purely Newtonian perspective, I think my statement is not unreasonable). When it comes to the formation of galaxies and galaxy-type objects, you have to remember where –  astromax Mar 26 at 12:42
    
... it all came from. Since DM doesn't interact electromagnetically, it was able to cool off and form overdensities for which baryons would later fall into - basically, the baryons do trace the dark matter. For a sufficiently massive dark matter overdensity, it is able to collect enough gas for there to be a galaxy within; for smaller overdensities, there may not be enough gas within to form a galaxy, and to that end, there may not be enough to even form any stars. This is the idea behind dark companion galaxies, and has been proposed as a solution to the 'missing halo problem'. –  astromax Mar 26 at 12:48

An overly simplistic distinction might be that globular clusters do not have any cloudy/gaseous regions, with no current star forming activity occurring. Globulars tend to have very similar stars, which together with their lack of gas regions, indicates that they formed early on in a single star forming period - though actually some globulars show evidence of additional period(s) of star forming. A galaxy will tend to have ongoing star forming activity and have evidence of stars of various ages.

But that is over-simplifying; there isn't always that clear-cut a difference between globulars and galaxies. At least one recent paper suggests that globular clusters and dwarf spheroidal galaxies may not be clearly distinct groups.

However, the Magellanic Clouds do make themselves quite distinct from globular clusters in being rich in dust and gas. There is a lot of current star-formation in progress, and there is a lot of variety in the stars in these galaxies.

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Don't all galaxies also have black holes at their centers as well? –  TheBluegrassMathematician Mar 19 at 1:11
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@RyanMcGaha no, that is not necessarily a defining characteristic of a galaxy. It appears that some, maybe most but not necessarily all astronomy.com/news/2014/01/…, galaxies do have a supermassive black hole in the centre universetoday.com/13732/…, but some galaxies apparently do not. adsabs.harvard.edu/abs/2001AJ....122.2469G –  Jeremy Mar 19 at 3:19
    
So if globular clusters and spheroidal galaxies are the same thing, would then the difference between the two only be "context"? And there is no intrinsic difference between a star cluster and a galaxy? –  eraticus Mar 21 at 23:08
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@eraticus that isn't what I said. I said they may not be clearly distinct groups, not 'the same thing'. That is, it may be difficult to tell in a few cases if one is one or the other. But generally, they will be catalogued as either a globular or a dwarf spheroidal galaxy. –  Jeremy Mar 22 at 6:00
    
@Jeremy, understood. I am trying to understand if there is some sort of diagnostic attribute that causes the cataloguer to mark one object as a globular cluster, and one as a spheroidal galaxy. Maybe my question is not a good question. I assumed that there was something intrinsically different, like the presence of a large black hole. –  eraticus Mar 24 at 20:32

Main difference (beyond mass, which we can extrapolate to make giant clusters and small galaxies touch) is star age.

In a cluster, all stars were formed in a single burst. Moreover this uses to happen when a pressure wave travels through the proto-cluster cloud inside a galaxy (This gives us a secondary difference: if it is inside another galaxy or not).

In a galaxy, pressure waves move forward and backward through the gas, causing constand bursts of star formation, so you get stars of varied ages (This gives us another difference: if there is gas or not, as mentioned by Jeremy).

Another difference is how tightly gravitationally bound are the stars of the body, as mentioned by astromax.

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Though some clusters do have stars of varying ages, apparently having undergone more than one star forming period... –  Jeremy Mar 25 at 10:44

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