Traditionally, when asked what percentage of matter in the Milky Way galaxy is matter that we can see -- i.e. "luminous matter" -- we're given a figure of 10%. The other 90%, we are told, is dark matter.

But it occurs to me that the calculation of how much luminous matter there is in the galaxy is based on a measurement of how much light the galaxy puts out, multiplied by a factor based on certain assumptions about what percentage of stars fall into the super-bright category and what percentage of stars fall into the super-dim category. A hot star like Sirius A, for example, will contribute 23 times as much light as the sun but only 2.3 times as much mass as the sun. A red dwarf star like Proxima Centauri, on the other hand, will contribute only 1/10,000 as much visible light as the sun, but will weigh in at a sizeable 1/10 the sun's mass. Therefore, an accurate estimate of how much luminous matter there is in the galaxy depends severely on an accurate survey of how many red dwarfs there are. An enormous number of red dwarfs would only make a tiny contribution to the total luminosity of the galaxy.


Well ... according to this article, which came out earlier this year, it seems we may indeed have underestimated the percentage of stars in the galaxy that are red dwarfs. Look at this passage:
How does this new, upwardly-revised estimate of the number of red dwarfs affect our calculations as to what percentage of the galaxy's mass is made up of luminous matter? Does it mean that the old 10% figure should be revised upward to 15%? 20%? 30%?! And since this also means that the percentage of dark matter in the galaxy must now be revised downward, how far does this go toward solving the Dark Matter riddle?

Well, the amount of exotic dark matter vs. regular matter (luminous and dark) in the universe was found independent of galactic observations, so how much DM we see (well, don't see) in the galaxy may not help us a whole lot in figuring out what it is.

What this could do is give us a better idea on the amount of luminous matter in the universe. It should be noted that that amount is ~10%, which is very likely related to your galactic figure above.

Now, as to how this figure should be changed (and we really need a bigger survey, IMO, that would get a good sample of O/B stars in addition to M-dwarfs to get a good feel for numbers of stars), I don't know what the previous M-dwarf ratio was, so I don't know how it changed. That would tell us how much the percentage of luminous matter should increase.

This isn't news at all. Astronomers have long known that about 80 percent of all stars in the Galaxy are red dwarfs. See, for example, the stellar pyramid on page 79 of Ken Croswell's book Planet Quest, which gives the following statistics:

Red (M) dwarfs: 80 percent
Orange (K) dwarfs: 9 percent
G main-sequence stars (like the Sun): 4 percent
White dwarfs: 5 percent
All other stars: 2 percent.

D'OH!

Confound you, Ken Croswell! First The Alchemy of the Heavens, and now this! You're always one step ahead of my nefarous plans.

Also note that stars do not make up the majority of the normal matter in the galaxy. The majority of non-dark matter in the galaxy is in the form of dust and gas.

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Just to clarify what I think I know...

The ~ 10% estimate for matter does include all the above, right?

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Actually, stars DO make up most of the normal (baryonic) matter of the Milky Way. Interstellar gas and dust account for only about 5 to 10 billion solar masses of material in the Galaxy.

Aren't brown dwarves supposed to be a large portion of the total mass?

Are you sure? Can you provide a citation? It's not that I don't believe you, it's just that you have contradicted what I had come to belive and I would like confirmation before I shift my paradigm!

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They make up some of the mass, but they would go in the stellar category, not the interstellar gas/dust category.

"The space between the stars looks empty, which is what it mostly is. A cubic centimeter of terrestrial air packs 25 million million million molecules, but the same volume of interstellar space in the Galactic disk typically holds just a single atom--by laboratory standards, a perfect vacuum. However, the Galaxy is so big that if you added up all this tenuous material, its total mass would equal 5 to 10 billion stars like the Sun--approximately 5 to 10 percent of the mass of the Galactic disk."
--Magnificent Universe by Ken Croswell, page 54.

Thanks. I stand corrected! ops:

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But that citation describes the typical cubic centimeter of space. That doesn't mean that nebulas and gas clouds cannot be much denser than this, and contribute quite a bit to the total baryonic mass.

What we need is an average interstellar density number, not a typical one.

The luminous matter seems pretty well accounted for, but a related question is whether or not we have underestimated the baryonic dark matter in galaxies.

I'm wondering about something. Has evidence of dark matter really been found or is it just used to account for the missing mass?

I assume that you're referring to tthe galaxy from the way your question is stated. We know two things: galactic rotation curves (velocity vs. radius) are flat through the disk into the halo and we do not see enough mass in luminous matter (i.e. stars). We know that dark matter exits in some form, since *we* are dark matter. (No telescope somewhere else will pick up the radiation we and the earth are emitting.) However, we also do not see evidence around here for enough normal (or baryonic, like drguss said) dark matter to make up the missing mass that must be there to make the galaxies rotate at the observed velocities. This leads us to believe that there must be something else out there making up the dark mass.

As a final note, going beyond galaxies, WMAP observations of the cosmic microwave background have told us how much baryonic matter, (exotic) dark matter, and dark energy are in the universe. These observation corroborate the dark matter theory originially based on the galactic evidence.

Many spiral galaxies have flat rotation curves . If the visible mass was all there was you would expect a fall-off of velocity with distance from the core. That is what is not observed so there is evidence for some unseen matter that is "dark".

Broadly speaking two types of dark matter have been proposed. One type of dark matter is baryonic which is composed of normal matter (protons, neutrons, electrons). A second type of dark matter proposed is called non-baryonic because it is composed of exotic particles that have yet to be detected but may exist based upon particle physics. The "Cold Dark Matter" models you often hear about are referring to the non-baryonic forms of dark matter.

Tobin Dax noted that WMAP and Big Bang parameters in general seem to restrict most dark matter to the non-baryonic forms. But is important to stress that this conclusion is based upon a combination of observations and Big Bang requirements. The universe is not obligated to conform to those requirements. Some researchers have pointed to observations that suggest the dark matter may actually be entirely baryonic in nature - which conflicts with the currently preferred version of the Big Bang (concordance model --> dark energy = 73%, non-baryonic dark matter =23%, baryonic matter = 4%). So the actual census of baryonic dark matter is an important test for the concordance model.

One interesting recent result is that many elliptical galaxies have a dearth of dark matter. Other problems include observations that indicate a coupling between dark matter halos and luminous matter unexpected in CDM models.

That's why it is important to keep in mind that what the observations indicate about dark matter are more important than what any one theory says the dark matter must be.

Gaaah! I thought I'd finally gotten the different varieties of dark matter sorted out, but I had the impression that the cold dark matter was baryonic, while hot dark matter was non-baryonic, perhaps neutrinos or something like that. And where does exotic dark matter fit into this, or is it just a synonym for one of the above, or a more specific variety?
As for dark energy, that's what might eventually rend the universe apart, right? Does it seem to be exerting a gravitational attraction meanwhile? If so, does that seem to be part of what's keeping galaxies together?

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The expansion rate may be the sum of two forces. As the universe expands, the gravitational force becomes less effective allowing the dark energy force to dominate. Acceleration would be the result.

If this is true, would the "Hubble Constant" become refined to an equation and not a "constant"?

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There are several independent classes of observation attesting to the existence of dark matter; this paper gives an overview, from both observational and theoretical perspective.

In brief:
- rotation curves of galaxies (as already noted by several posters)
- gravitational lensing, both strong and weak (here is an excellent recent, HST + X-ray + ground-based study of the distribution of dark matter in an Abell cluster)
- motion of galaxies in the Local Group
- X-ray emission from some clusters (-> high-temperature gas; if in equilbrium, needs far more mass than that implied by light from galaxies in the cluster)
- large-scale streaming (e.g. towards the Great Attractor)

An average is a "typical" value.