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?The most surprising result of RECONS so far is the realization of the extent to which red dwarfs are the real rulers of the universe, both in numbers and total mass. Within 10 parsecs of the Sun there are no hot, bright O and B stars, just 4 white A stars and 6 yellow-white F stars, 21 G stars similar to our Sun, 45 orange K dwarfs — and a whopping 236 cool, orange-red M dwarfs like Proxima Centauri (which still ranks as the Sun's closest neighbor). There are also 20 white dwarfs. That means that for every other star in the universe, there are no less than 2.5 red M dwarfs.