<p>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.</p>

<p>But that is over-simplifying; there isn't always that clear-cut a difference between globulars and galaxies. <a href="http://arxiv.org/pdf/0711.4795v1.pdf" rel="nofollow">At least one recent paper</a> suggests that globular clusters and dwarf spheroidal galaxies may not be clearly distinct groups. </p>

<p>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.</p>

CC BY-SA 3.0
2014-03-19T01:11:01.093Z
Don't all galaxies also have black holes at their centers as well?
TheBluegrassMathematician
CC BY-SA 3.0
2014-03-19T03:19:19.320Z
@RyanMcGaha no, that is not necessarily a defining characteristic of a galaxy. It appears that some, maybe most but not necessarily all http://www.astronomy.com/news/2014/01/dwarf-galaxies-give-clues-to-origin-of-supermassive-black-holes, galaxies do have a supermassive black hole in the centre http://www.universetoday.com/13732/why-are-there-black-holes-in-the-middle-of-galaxies/, but some galaxies apparently do not. http://adsabs.harvard.edu/abs/2001AJ....122.2469G
Jeremy
CC BY-SA 3.0
2014-03-21T23:08:36.713Z
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
CC BY-SA 3.0
2014-03-22T06:00:08.553Z
@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
CC BY-SA 3.0
2014-03-24T20:32:30.460Z
@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
CC BY-SA 3.0
2014-03-25T10:59:38.243Z
The authors of the paper I pointed you to suggest the use of cluster flattening / ellipticity to distinguish between clusters and dwarf spheroidal galaxies where other distinguishing parameters (mass, luminosity, size, mass-to-light ratio, and spread of metallicity) do not conclusively categorise the object.
Jeremy
CC BY-SA 3.0
2014-03-25T11:06:12.673Z
Is there something intrinsically different? A cluster will appear to have no gas, and mostly similarly aged stars. That is it. It only gets complicated when you consider edge cases or notice 'rule-breakers'.
Jeremy
CC BY-SA 3.0
2014-03-18T20:33:07.957Z
2014-03-18T20:33:07.957Z
Jeremy
<p>Main difference (beyond mass, which we can extrapolate to make giant clusters and small galaxies touch) is star age.</p>

<p>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).</p>

<p>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).</p>

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

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2014-03-25T10:44:13.923Z
Though some clusters do have stars of varying ages, apparently having undergone more than one star forming period...
Jeremy
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2014-03-25T10:04:13.733Z
2014-03-25T10:04:13.733Z
Envite
<p>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 <em>could</em> 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.</p>

<p>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.</p>

<p>Some References:</p>

<p>1) <a href="http://arxiv.org/abs/1310.0828">Evidence that dwarf galaxies are dominated by a dark-matter component.</a> </p>

<p>2) <a href="http://arxiv.org/abs/1210.7787">Dark matter present in globular clusters are either unlikely, or incredibly highly concentrated and approximately the same mass as the stellar component.</a></p>

CC BY-SA 3.0
2014-03-24T20:24:23.163Z
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
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2014-03-25T10:52:41.447Z
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
CC BY-SA 3.0
2014-03-25T13:54:11.390Z
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
CC BY-SA 3.0
2014-03-26T12:42:49.817Z
@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
CC BY-SA 3.0
2014-03-26T12:48:58.583Z
... 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
CC BY-SA 3.0
2014-03-26T12:53:19.627Z
@Jeremy In practice I don't think people go looking for dark matter in these star clusters/globular clusters. Lensing is not really something I've seen be used to determine total mass of these objects (I don't think the geometry works out for these objects), but one could use the velocity dispersion to infer total mass. But no, I do not think an assessment of dark matter is made in order for people to say if an object is truly a star cluster or a galaxy. You could be right about people using stellar population age and gas content to make this type of statement - I simply do not know.
astromax
CC BY-SA 3.0
2014-03-26T13:00:02.760Z
@called2voyage I'm not entirely sure what you mean by mechanism - if you mean gravity, then yes, these types of objects do attract dark matter from their host galaxies/clusters in the form a gravitational wake (http://en.wikipedia.org/wiki/Dynamical_friction) - this is the cause of some dynamical friction of, say, a star cluster orbiting around the center of a massive galaxy. The question remains though: Do these objects contain some known radial distribution of dark matter intrinsically, or is the dark matter present simply from the parent object? I think you'd be hard pressed...
astromax
CC BY-SA 3.0
2014-03-26T13:04:12.190Z
... to find objects of this mass unassociated with galaxies or clusters. I don't know this for sure of course, but my intuition tells me that if it is able to be flung out of the galaxy entirely, chances are it has been disturbed in the process. It is hard for me to imagine a situation where a globular cluster could be flung out of a galaxy and remain the size and shape it was to begin with. If any have been found, please update this post!
astromax
CC BY-SA 3.0
2014-04-08T02:01:09.953Z
I just saw this in Scientific American. There is a satellite galaxy to the Milky Way that has only about 100 stars. [Fossil Galaxy May Be One of First Ever Formed](http://www.scientificamerican.com/article/fossil-galaxy-segue1/)
eraticus
CC BY-SA 3.0
2014-03-23T21:06:09.643Z
2014-03-23T21:13:44.623Z
2014-03-23T21:13:44.623Z
astromax
<p>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?  </p>

CC BY-SA 3.0
2014-03-19T00:57:51.547Z
I am only using globular clusters as an example, since, at least in photographs, globular clusters look like elliptical galaxies.
eraticus
CC BY-SA 3.0
2014-03-19T01:10:24.390Z
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
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2014-03-22T06:01:52.530Z
@py-ser I already have, in my answer
Jeremy
CC BY-SA 3.0
2014-03-25T17:10:27.910Z
[Omega Centauri](http://en.wikipedia.org/wiki/Omega_Centauri) is an interesting edge case between globular clusters and dwarf galaxies. It may even have a central black hole.
Keith Thompson
CC BY-SA 3.0
2014-03-27T19:03:24.303Z
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
<galaxy><star-cluster><globular-clusters>
Is there a fundamental difference between a small galaxy, and a large star cluster?
eraticus
# Is there a fundamental difference between a small galaxy, and a large star cluster?

(Asked by: username_0 on 2014-03-18)

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?

**Tags:** galaxy, star-cluster, globular-clusters

> **Comments:**
> **username_0**: I am only using globular clusters as an example, since, at least in photographs, globular clusters look like elliptical galaxies.  
> **username_1**: There are also some astronomical objects which are in doubt between stellar clusters and elliptical galaxies, I will try to find the reference.  
> **username_2**: @username_1 I already have, in my answer  
> **username_3**: [Omega Centauri](http://en.wikipedia.org/wiki/Omega_Centauri) is an interesting edge case between globular clusters and dwarf galaxies. It may even have a central black hole.  
> **username_0**: 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.  

---

## 3 answers

### Answer by username_6 on 2014-03-23. Score: 6 | Accepted answer

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.](http://arxiv.org/abs/1310.0828)

2) [Dark matter present in globular clusters are either unlikely, or incredibly highly concentrated and approximately the same mass as the stellar component.](http://arxiv.org/abs/1210.7787)

> **Comments:**
> **username_0**: 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!  
> **username_2**: 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?  
> **username_7**: username_6, 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?  
> **username_6**: @username_0 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  
> **username_6**: ... 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'.  
> **username_6**: @username_2 In practice I don't think people go looking for dark matter in these star clusters/globular clusters. Lensing is not really something I've seen be used to determine total mass of these objects (I don't think the geometry works out for these objects), but one could use the velocity dispersion to infer total mass. But no, I do not think an assessment of dark matter is made in order for people to say if an object is truly a star cluster or a galaxy. You could be right about people using stellar population age and gas content to make this type of statement - I simply do not know.  
> **username_6**: @username_7 I'm not entirely sure what you mean by mechanism - if you mean gravity, then yes, these types of objects do attract dark matter from their host galaxies/clusters in the form a gravitational wake (http://en.wikipedia.org/wiki/Dynamical_friction) - this is the cause of some dynamical friction of, say, a star cluster orbiting around the center of a massive galaxy. The question remains though: Do these objects contain some known radial distribution of dark matter intrinsically, or is the dark matter present simply from the parent object? I think you'd be hard pressed...  
> **username_6**: ... to find objects of this mass unassociated with galaxies or clusters. I don't know this for sure of course, but my intuition tells me that if it is able to be flung out of the galaxy entirely, chances are it has been disturbed in the process. It is hard for me to imagine a situation where a globular cluster could be flung out of a galaxy and remain the size and shape it was to begin with. If any have been found, please update this post!  
> **username_0**: I just saw this in Scientific American. There is a satellite galaxy to the Milky Way that has only about 100 stars. [Fossil Galaxy May Be One of First Ever Formed](http://www.scientificamerican.com/article/fossil-galaxy-segue1/)  

---

### Answer by username_2 on 2014-03-18. Score: 1

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](http://arxiv.org/pdf/0711.4795v1.pdf) 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.

> **Comments:**
> **username_4**: Don't all galaxies also have black holes at their centers as well?  
> **username_2**: @RyanMcGaha no, that is not necessarily a defining characteristic of a galaxy. It appears that some, maybe most but not necessarily all http://www.astronomy.com/news/2014/01/dwarf-galaxies-give-clues-to-origin-of-supermassive-black-holes, galaxies do have a supermassive black hole in the centre http://www.universetoday.com/13732/why-are-there-black-holes-in-the-middle-of-galaxies/, but some galaxies apparently do not. http://adsabs.harvard.edu/abs/2001AJ....122.2469G  
> **username_0**: 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?  
> **username_2**: @username_0 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.  
> **username_0**: @username_2, 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.  
> **username_2**: The authors of the paper I pointed you to suggest the use of cluster flattening / ellipticity to distinguish between clusters and dwarf spheroidal galaxies where other distinguishing parameters (mass, luminosity, size, mass-to-light ratio, and spread of metallicity) do not conclusively categorise the object.  
> **username_2**: Is there something intrinsically different? A cluster will appear to have no gas, and mostly similarly aged stars. That is it. It only gets complicated when you consider edge cases or notice 'rule-breakers'.  

---

### Answer by username_5 on 2014-03-25. Score: 0

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 username_2).

Another difference is how tightly gravitationally bound are the stars of the body, as mentioned by username_6.

> **Comments:**
> **username_2**: Though some clusters do have stars of varying ages, apparently having undergone more than one star forming period...  

---

