What is the best educated guess for the number of stars in the Milky Way? I’ve seen figures ranging from “some 20 billion” to “just under 600 billion.” The first seems too low, whereas the latter too high.
The short answer is, nobody knows. But your reasoning is sound: 20 billion is too low, and 600 billion is likely — though not certainly — too high. According to University of Massachusetts astronomer Martin Weinberg, the Milky Way probably contains 200 billion stars, though that number “could be larger or smaller by a factor of 2 or 3.”
Astronomers have imaged only a small fraction of these stars. They guesstimate the total using a multi-pronged approach. First, they tally very nearby stars as a baseline. Then they observe other galaxies to see how starlight is distributed (most of the stars in our own galaxy are hidden behind clouds of dust). Finally, they apply the locally determined relationship between stellar numbers and light output galaxy-wide.
Each step is plagued by uncertainty. For example, the most common stars (late-type M dwarfs) are the least luminous and hence can be seen only nearby. Yet censuses are incomplete even in our backyard, says Georgia State University astronomer Todd Henry. In the last five years, Henry and other researchers have discovered 51 new stars within 33 light-years (10 parsecs) of our Sun, a 17 percent increase.
— Joshua Roth