# Milky Way mass fraction breakdown review

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.

• This kind of question has been asked before at physics.stackexchange.com/questions/149266/… – Dean Jun 10 '16 at 13:53
• Silly question but have you read the wiki page 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 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 Jun 10 '16 at 16:25
• 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. – Rob Jeffries Jun 10 '16 at 20:04
• 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 Jun 14 '16 at 1:04
• 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. – Rob Jeffries Jun 19 '16 at 6:54

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.

• And the gas... ? The dark matter content is not established "theoretically". – Rob Jeffries Jun 25 '16 at 8:26
• @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 Jun 25 '16 at 11:38
• The gas mass (fraction) in the disk and halo of the Milky Way is something that has certainly been measured/estimated before. – Rob Jeffries Jun 25 '16 at 11:42
• @RobJeffries Er, can I have a source? I can only find the halo. – Sir Cumference Jun 25 '16 at 11:42