<p>I should clear this up: the Milky Way isn't a completely well-defined body. Galaxies shouldn't be thought of as a single body — they are more like a collection of various material and stars. That said, I can only give you a wonky estimate of what <em>might</em> be the composition.</p>

<p>By "Milky Way", I'm assuming you mean only the Milky Way's disc and halo (and not <em>everything</em> that orbits the Galactic Center, like the satellite galaxies). That alone is hard to define, but let's get a rough estimate. The entire mass of the Milky Way is a bit uncertain, but it falls between $0.8–1.5×10^{12}M_☉$. Let's average that at about $1.2×10^{12}$.</p>

<p>Sagittarius A*, the black hole at the center of our galaxy, has a mass of roughly $4,000,000M_☉$. That would make it only ~0.0003% of the galaxy! A few sources state that dark matter composes about 90% of the Milky Way's mass.
So it has a mass of approximately $1.08×10^{12}M_☉$.</p>

<p>If we want to dissect the rest, we'll have to make generalizations. From some sources like <a href="http://members.nova.org/~sol/chview/chv5.htm" rel="nofollow">this</a> one, about 75% of MW stars are M type stars, 15% are F-K stars and the remaining 10% are mostly degenerate stars. Knowing that there are 100 billion stars in our galaxy, we can take their average masses and <em>kind of</em> conclude that 3% of the Milky Way's mass is from stellar objects.</p>

<p>Regarding the mass of gases in the Milky Way, <a href="http://www.astro.cornell.edu/academics/courses/astro1101/lectures/21MilkyWay.pdf" rel="nofollow">this source</a> states that both atomic gas and molecular gas would individually make up 0.25% of the Milky Way's mass. Combined, we get 0.5%. The rest is buried in a sea of contradictions and mystery.</p>

<p>That said, here's the best breakdown I could make:</p>

<pre><code>Sagittarius A*: 0.0003%
Dark matter: 90%
Stellar objects: 3%
Gases and nebulae: 0.5%
Everything else: 6.5%
</code></pre>

<p>Again, it is extremely difficult to break down the mass of the components of a galaxy. This is the best you can really do without combining tremendous generalizations with shoddy math and sources.</p>

CC BY-SA 3.0
2016-06-25T08:26:22.450Z
And the gas... ? The dark matter content is not established "theoretically".
ProfRob
CC BY-SA 3.0
2016-06-25T11:38:21.173Z
@RobJeffries Getting the gas would require combining generalizations with shoddy math and sources. I could not find a reliable source for it, but if you could, you're more than welcome to share it.
Sir Cumference
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2016-06-25T11:42:17.313Z
The gas mass (fraction) in the disk and halo of the Milky Way is something that has certainly been measured/estimated before.
ProfRob
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2016-06-25T11:42:35.743Z
@RobJeffries Er, can I have a source? I can only find the halo.
Sir Cumference
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2016-06-25T11:44:49.247Z
It's your answer mate; your bounty.
ProfRob
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2016-06-25T11:51:06.913Z
@RobJeffries Ah, I actually found some sources (though they're lectures, so hopefully they're accurate).
Sir Cumference
CC BY-SA 3.0
2016-06-25T12:47:59.980Z
Wait, when you say "mass is $1.2×10^{12}$", OK.  You then say "dark matter composes about 90% of the Milky Way's mass".  Do you mean then "dark matter is 90% of the 1.2", or do you mean: "dark matter is 90%. regarding the rest, the mass is 1.2"  These are (obviously) incredibly different concepts, which is meant here?
Fattie
CC BY-SA 3.0
2016-06-25T12:50:00.167Z
@JoeBlow The former. The *entire* mass is 1.2. Dark matter is 90% of that, so it is ~1.08
Sir Cumference
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2016-06-25T12:53:01.450Z
OK.  again, thanks for the answer - would it be worth clearing that up in the answer?  (I might repeat - - this is information that is "just obvious" to professionals and experts.  But to say me, it's just utterly unclear within many orders of magnitude.)
Fattie
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2016-06-25T12:55:10.397Z
@JoeBlow All right, hopefully this is more clear.
Sir Cumference
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2016-06-25T13:00:45.877Z
I must say, @RobJeffries and SirCumf, when many sources state "gas is 0.25% of the mass" it is very unclear if, in that discussion, the writer meant ***percentage of 'conventional' mass*** or whether the writer meant ***percentage of all mass including the overwhelming dark matter***.  Obviously the two are totally different.
Fattie
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2016-06-25T13:02:31.580Z
@JoeBlow When astronomers say "0.25% of the total mass", it usually means the *total mass*, including dark matter.
Sir Cumference
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2016-06-25T13:08:47.460Z
Hmm. Please scroll to page 28 of the  excellent Cornell kid's presentation. It says "The MW has two primary constituents: a, Stars b, Gas and dust (ISM)."  So: that is ***utter nonsense***.  The only primary constituent is "dark matter". ***Given*** that utter nonsense, ***it then immediately states:*** "ISM is 3% of the mass of the galaxy".  Gven that they had JUST IMMEDIATELY TYPED *"The MW has two primary constituents: a, Stars b, ISM"*, surely the immediately following statement ("3%") refers ONLY TO that 'conventional matter' discussion???????????
Fattie
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2016-06-25T13:13:14.380Z
@JoeBlow I don't generally trust presentations. Often their facts can be oversimplified or inaccurate. Regardless, even if we exclude dark matter, the Milky Way certainly is not 97% stars and 3% gas.
Sir Cumference
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2016-06-25T13:15:28.087Z
Right, that's a good point.
Fattie
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2016-06-25T13:17:17.777Z
I might say @SirCumference your first paragraph and the first two sentences of the second paragraph  is, well, wholly unnecessary and only distracts from your otherwise excellent (and we can presently state, unrivaled!) answer.  (You might note the edit I just put in the OP - in braces.)
Fattie
CC BY-SA 3.0
2016-06-26T01:06:13.083Z
I'm happy to see the bounty go to an answer, and thanks!  I feel this question could be explored more clearly in future episodes.  Cheers!  Thanks again for this info.
Fattie
CC BY-SA 3.0
2016-06-25T06:06:20.560Z
2016-06-25T12:54:50.700Z
2016-06-25T12:54:50.700Z
Sir Cumference
<p>I'd like to know a <strong>breakdown of the mass of the Milky Way, by the major categories</strong>. So, something like</p>

<pre><code>Stars       ... 3%
Cool nebula ... 7%
Gray matter ... 12%
Etc
Etc
</code></pre>

<p>This <strong>order-of-magnitude knowledge</strong> that everyone should know.</p>

<p>(Of course, when talking of the galaxy, we can, obviously, think of either just <strong>the visible disk-bulge</strong>, or perhaps, <strong>a greater unit</strong> of the galaxy sitting in a "bulb" (of - perhaps - dark matter (?), gas (?), dust (?), halo stars (?) or something else): a brief clear answer in a few sentences would expose these concepts.)</p>

<p><strong>Surprisingly, this information is nowhere to be found in one place as an overview.</strong></p>

<p>I appreciate that the largest chunk may be dark matter (is that right?); but I don't even have a <em>ballpark idea</em> for the rest. It's the sort of knowledge that is obvious to professionals, but surprisingly not outlined in any of the usual popular science sources.</p>

CC BY-SA 3.0
2016-06-10T13:53:02.140Z
This kind of question has been asked before at http://physics.stackexchange.com/questions/149266/how-much-of-a-galaxys-mass-is-in-stars
Dean
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2016-06-10T14:40:40.673Z
Hi @Dean, say, that question is "How much of a galaxy's mass is in stars?" (there are very many questions about various aspects of one particular component of galactic mass). It has nothing on breaking down galactic mass in to components, as we ask here.  Like I say a "popular science reader" I'm really surprised to have been unable to find such a thing. (The general breakdown of the mass of the galaxy, should be common knowledge!) If you know of or can find anything like I ask in the question - be sure to alert me, I've probably missed something obvious or an obvious google!)
Fattie
CC BY-SA 3.0
2016-06-10T16:25:29.003Z
Silly question but have you read the [wiki page](https://en.wikipedia.org/wiki/Milky_Way) all the way through yet? Search it for "total mass" and **a few** percentages pop up, with references. I tried a general search of the [ADS](http://adswww.harvard.edu/) but "distribution" often means the proportion of normal and dark matter, so no good here. (I think a lot of trawling through papers will be needed to get a good set of figures.)
Andy
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2016-06-10T17:57:24.790Z
Hi Andy!  I know the paragraph you mean on the wiki page: the figures seem a bit bogus!  It sort of offers the mass of certain gasses *in relation to* star fraction - whatever that is (and then curiously says that "interstellar dust" is only a small percentage of that derived fraction).
Fattie
CC BY-SA 3.0
2016-06-10T20:04:42.297Z
90% dark matter, a few percent stars, 1-2% stellar remnants and brown dwarfs and a few percent gas and dust. The exact figures are matters of contemporary research.
ProfRob
CC BY-SA 3.0
2016-06-13T19:03:34.423Z
The usual estimates for dust put it at about 1% of the mass of the gas, and the gas at about 10% of the mass of the stars. Typical estimates for the latter are ~ 5 x 10^10 solar masses, with about 15-20 times that for the dark matter. So -- about what Rob Jeffries said, with the addition that the dust mass is a small fraction of the gas mass.
Peter Erwin
CC BY-SA 3.0
2016-06-14T01:04:56.553Z
I thought that the dark matter percentage was about 80-85%. Does the galaxy have a higher percentage than the universe as a whole?
Jack R. Woods
CC BY-SA 3.0
2016-06-19T06:54:45.390Z
Please specify out to what radius. Or perhaps you want it as a function of galactocentric radius too? And of course it varies massively as a function of height above the disc.
ProfRob
CC BY-SA 3.0
2016-06-19T12:18:38.757Z
Hi @RobJeffries - the question you pose just there, is a huge insight.  I've been staring at your comment, thinking how I would phrase an answer, in a book for interested 12year olds. Maybe it should be something like this: "The galaxy you can see (diagram) is made up something like this. (table.)  But.  The unseen galaxy is actually about this big (diagram, in which the seen galaxy is only small in the center). In fact, the overall unseen galaxy is made up something like this.  (table.)"  I wonder if that is the way to come to terms with this?
Fattie
CC BY-SA 3.0
2016-06-19T12:22:31.820Z
Note for example, when I wrote there... *"(diagram, in which the seen galaxy is only small in the center)."*.  As I emphasize here, to you guys it's probably **completely obvious** roughly what that looks like. But to me, I am totally clueless: is the "galaxy in the middle" about half the size of the "actual overall unseen galaxy"?  or is it 1/10000th the size, is it a dot in the middle? Is the whole thing completely ball-like - or extremely flat?  (Of course - as with anything - one can ask where does it end, what's the falloff - but I'm sure a reasonable "for 12 yr olds" answer exists!!)
Fattie
<galaxy><milky-way><mass>
Milky Way mass fraction breakdown review
Fattie
# Milky Way mass fraction breakdown review

(Asked by: username_0 on 2016-06-10)

I'd like to know a breakdown of the mass of the Milky Way, by the major categories. So, something like
Stars       ... 3%
Cool nebula ... 7%
Gray matter ... 12%
Etc
Etc

This order-of-magnitude knowledge that everyone should know.
(Of course, when talking of the galaxy, we can, obviously, think of either just the visible disk-bulge, or perhaps, a greater unit of the galaxy sitting in a "bulb" (of - perhaps - dark matter (?), gas (?), dust (?), halo stars (?) or something else): a brief clear answer in a few sentences would expose these concepts.)
Surprisingly, this information is nowhere to be found in one place as an overview.
I appreciate that the largest chunk may be dark matter (is that right?); but I don't even have a ballpark idea for the rest. It's the sort of knowledge that is obvious to professionals, but surprisingly not outlined in any of the usual popular science sources.

**Tags:** galaxy, milky-way, mass
> **Comments:**
> **username_1**: This kind of question has been asked before at http://physics.stackexchange.com/questions/149266/how-much-of-a-galaxys-mass-is-in-stars  
> **username_0**: Hi @Dean, say, that question is "How much of a galaxy's mass is in stars?" (there are very many questions about various aspects of one particular component of galactic mass). It has nothing on breaking down galactic mass in to components, as we ask here.  Like I say a "popular science reader" I'm really surprised to have been unable to find such a thing. (The general breakdown of the mass of the galaxy, should be common knowledge!) If you know of or can find anything like I ask in the question - be sure to alert me, I've probably missed something obvious or an obvious google!)  
> **username_2**: Silly question but have you read the [wiki page](https://en.wikipedia.org/wiki/Milky_Way) all the way through yet? Search it for "total mass" and **a few** percentages pop up, with references. I tried a general search of the [ADS](http://adswww.harvard.edu/) but "distribution" often means the proportion of normal and dark matter, so no good here. (I think a lot of trawling through papers will be needed to get a good set of figures.)  
> **username_0**: Hi Andy!  I know the paragraph you mean on the wiki page: the figures seem a bit bogus!  It sort of offers the mass of certain gasses *in relation to* star fraction - whatever that is (and then curiously says that "interstellar dust" is only a small percentage of that derived fraction).  
> **username_3**: 90% dark matter, a few percent stars, 1-2% stellar remnants and brown dwarfs and a few percent gas and dust. The exact figures are matters of contemporary research.  
> **username_4**: The usual estimates for dust put it at about 1% of the mass of the gas, and the gas at about 10% of the mass of the stars. Typical estimates for the latter are ~ 5 x 10^10 solar masses, with about 15-20 times that for the dark matter. So -- about what Rob Jeffries said, with the addition that the dust mass is a small fraction of the gas mass.  
> **username_5**: I thought that the dark matter percentage was about 80-85%. Does the galaxy have a higher percentage than the universe as a whole?  
> **username_3**: Please specify out to what radius. Or perhaps you want it as a function of galactocentric radius too? And of course it varies massively as a function of height above the disc.  
> **username_0**: Hi @RobJeffries - the question you pose just there, is a huge insight.  I've been staring at your comment, thinking how I would phrase an answer, in a book for interested 12year olds. Maybe it should be something like this: "The galaxy you can see (diagram) is made up something like this. (table.)  But.  The unseen galaxy is actually about this big (diagram, in which the seen galaxy is only small in the center). In fact, the overall unseen galaxy is made up something like this.  (table.)"  I wonder if that is the way to come to terms with this?  
> **username_0**: Note for example, when I wrote there... *"(diagram, in which the seen galaxy is only small in the center)."*.  As I emphasize here, to you guys it's probably **completely obvious** roughly what that looks like. But to me, I am totally clueless: is the "galaxy in the middle" about half the size of the "actual overall unseen galaxy"?  or is it 1/10000th the size, is it a dot in the middle? Is the whole thing completely ball-like - or extremely flat?  (Of course - as with anything - one can ask where does it end, what's the falloff - but I'm sure a reasonable "for 12 yr olds" answer exists!!)  

---

## 1 answers

### Answer by username_6 on 2016-06-25. Score: 5 | Accepted answer

I should clear this up: the Milky Way isn't a completely well-defined body. Galaxies shouldn't be thought of as a single body — they are more like a collection of various material and stars. That said, I can only give you a wonky estimate of what might be the composition.
By "Milky Way", I'm assuming you mean only the Milky Way's disc and halo (and not everything that orbits the Galactic Center, like the satellite galaxies). That alone is hard to define, but let's get a rough estimate. The entire mass of the Milky Way is a bit uncertain, but it falls between $0.8–1.5×10^{12}M_☉$. Let's average that at about $1.2×10^{12}$.
Sagittarius A*, the black hole at the center of our galaxy, has a mass of roughly $4,000,000M_☉$. That would make it only ~0.0003% of the galaxy! A few sources state that dark matter composes about 90% of the Milky Way's mass.
So it has a mass of approximately $1.08×10^{12}M_☉$.
If we want to dissect the rest, we'll have to make generalizations. From some sources like this one, about 75% of MW stars are M type stars, 15% are F-K stars and the remaining 10% are mostly degenerate stars. Knowing that there are 100 billion stars in our galaxy, we can take their average masses and kind of conclude that 3% of the Milky Way's mass is from stellar objects.
Regarding the mass of gases in the Milky Way, this source states that both atomic gas and molecular gas would individually make up 0.25% of the Milky Way's mass. Combined, we get 0.5%. The rest is buried in a sea of contradictions and mystery.
That said, here's the best breakdown I could make:
Sagittarius A*: 0.0003%
Dark matter: 90%
Stellar objects: 3%
Gases and nebulae: 0.5%
Everything else: 6.5%

Again, it is extremely difficult to break down the mass of the components of a galaxy. This is the best you can really do without combining tremendous generalizations with shoddy math and sources.

> **Comments:**
> **username_3**: And the gas... ? The dark matter content is not established "theoretically".  
> **username_6**: @RobJeffries Getting the gas would require combining generalizations with shoddy math and sources. I could not find a reliable source for it, but if you could, you're more than welcome to share it.  
> **username_3**: The gas mass (fraction) in the disk and halo of the Milky Way is something that has certainly been measured/estimated before.  
> **username_6**: @RobJeffries Er, can I have a source? I can only find the halo.  
> **username_3**: It's your answer mate; your bounty.  
> **username_6**: @RobJeffries Ah, I actually found some sources (though they're lectures, so hopefully they're accurate).  
> **username_0**: Wait, when you say "mass is $1.2×10^{12}$", OK.  You then say "dark matter composes about 90% of the Milky Way's mass".  Do you mean then "dark matter is 90% of the 1.2", or do you mean: "dark matter is 90%. regarding the rest, the mass is 1.2"  These are (obviously) incredibly different concepts, which is meant here?  
> **username_6**: @JoeBlow The former. The *entire* mass is 1.2. Dark matter is 90% of that, so it is ~1.08  
> **username_0**: OK.  again, thanks for the answer - would it be worth clearing that up in the answer?  (I might repeat - - this is information that is "just obvious" to professionals and experts.  But to say me, it's just utterly unclear within many orders of magnitude.)  
> **username_6**: @JoeBlow All right, hopefully this is more clear.  
> **username_0**: I must say, @RobJeffries and SirCumf, when many sources state "gas is 0.25% of the mass" it is very unclear if, in that discussion, the writer meant ***percentage of 'conventional' mass*** or whether the writer meant ***percentage of all mass including the overwhelming dark matter***.  Obviously the two are totally different.  
> **username_6**: @JoeBlow When astronomers say "0.25% of the total mass", it usually means the *total mass*, including dark matter.  
> **username_0**: Hmm. Please scroll to page 28 of the  excellent Cornell kid's presentation. It says "The MW has two primary constituents: a, Stars b, Gas and dust (ISM)."  So: that is ***utter nonsense***.  The only primary constituent is "dark matter". ***Given*** that utter nonsense, ***it then immediately states:*** "ISM is 3% of the mass of the galaxy".  Gven that they had JUST IMMEDIATELY TYPED *"The MW has two primary constituents: a, Stars b, ISM"*, surely the immediately following statement ("3%") refers ONLY TO that 'conventional matter' discussion???????????  
> **username_6**: @JoeBlow I don't generally trust presentations. Often their facts can be oversimplified or inaccurate. Regardless, even if we exclude dark matter, the Milky Way certainly is not 97% stars and 3% gas.  
> **username_0**: Right, that's a good point.  
> **username_0**: I might say @SirCumference your first paragraph and the first two sentences of the second paragraph  is, well, wholly unnecessary and only distracts from your otherwise excellent (and we can presently state, unrivaled!) answer.  (You might note the edit I just put in the OP - in braces.)  
> **username_0**: I'm happy to see the bounty go to an answer, and thanks!  I feel this question could be explored more clearly in future episodes.  Cheers!  Thanks again for this info.  

---

