We’ve often speculated here about how many stars exist in the Milky Way. Earlier estimates have ranged from one hundred billion up to four hundred billion, with a few wildcard guesses in the range of one trillion. The number is still, of course, inexact, but recent work has led to a serious misunderstanding of the subject. As reported in this earlier post, Harvard’s Mark Reid and colleagues have discovered that the Milky Way is likely to be as massive as the Andromeda galaxy, which means that it could have the mass of three trillion stars like our own Sun.

Does that mean that the Milky Way contains three trillion stars? Absolutely not. I’m seeing the three trillion star number popping up all over the Internet, and almost reported it that way here when I first encountered the work. The misunderstanding comes from making mistaken assumptions about galactic mass. Reid used the Very Long Baseline Array to examine regions of intense star formation across the galaxy, a study the scientist reported at the American Astronomical Society’s winter meeting this past January. The Milky Way does indeed turn out to have much more mass than earlier studies had indicated.

Mass on the Galactic Scale

Science News ran a story on Reid’s work with this headline: “This Just In: Milky Way as Massive as Three Trillion Suns.” The headline is catchy, but read it carefully. It does not say the Milky Way contains three trillion stars. What it does say is that the galaxy has been found to be as massive as three trillion suns. In other words, it has the mass, at the upper limit, of three trillion G-class stars like the Sun.

Now factor in our understanding of galactic mass. Current thinking says that dark matter accounts for nine-tenths of a galaxy’s mass, and perhaps more. What Reid’s work shows us is that the galaxy is massive indeed, about fifty percent heavier than previously thought. But bumping up the mass estimate also bumps up the estimate of dark matter, and does not imply that the galaxy contains three trillion stars, or anywhere near it. Again, most of the mass in a galaxy is in the form of dark matter, which is what most of that additional mass would be made of.

The Dominance of Dark Matter

Because I wanted to be sure I had this right, I wrote to Dr. Reid asking whether the idea that the galaxy contained three trillion stars wasn’t a serious error, one that did not follow from his work. He agreed:

What we’ve found is that the Milky Way is rotating about 15% faster (254 km/s) than previously assumed (220 km/s). The faster rotation speed matches that of the nearby Andromeda galaxy (M31). The simplest interpretation is then that the Milky Way and Andromeda have a similar overall mass and size (that is dominated by dark matter halos). Estimates of the total mass of Andromeda are typically between 1 and 3 trillion solar masses. However, only a small fraction of that is in normal matter (eg stars), probably only about 0.1 trillion [italics mine].

A tenth of a trillion — a hundred billion — solar masses is what we have to work with in terms of normal matter. That figure would then need to be adjusted to reflect the relative abundance of stellar types, from the tiniest brown dwarf to the largest blue giant, to get a star count. There is still room for a range of estimates, but we’ve learned that while the Milky Way is crammed with stars, it’s a long way from having three trillion of them.