Most Stars Are Single

By Ken Croswell

January 19, 2006

The Sun: normal after all. Image by SOHO (ESA and NASA).

Sooner or later, the typical astronomy enthusiast learns a shocking truth: most star systems are double or triple, with two or three stars orbiting one another. Thus, our single Sun is in the minority. Now, however, an astronomer in Massachusetts says this "truth" is false--because most star systems in the Galaxy are in fact single.

Certainly double and triple stars seem common. Sirius, the brightest star in the night, is double, and Alpha Centauri, the nearest star system to the Sun, is triple. Furthermore, most yellow G-type stars like the Sun have stellar companions.

But Charles Lada of the Harvard-Smithsonian Center for Astrophysics says what really counts are red dwarfs--stars much fainter and cooler than the Sun--because they make up the vast majority of stars. In recent years, red dwarf surveys have found that most of these dim stars are single.

This trend shows up even among the Sun's nearest neighbors (see table below). The closest red dwarf, Proxima Centauri, is part of a triple star, but the next nearest red dwarfs--Barnard's Star, Wolf 359, and Lalande 21185--are all single.

When Lada works out the numbers, he reaches a conclusion you won't find in any astronomy textbook: two thirds of all star systems in the Milky Way are single. Moreover, that fraction would be still higher had he included brown dwarfs. Born with even less mass than red dwarfs, brown dwarfs can't sustain hydrogen fusion, and they are even likelier than red dwarfs to be loners.

"In general, I agree with what Charles did," says Helmut Abt, an astronomer at Kitt Peak National Observatory in Tucson, Arizona, who began studying the frequency of binaries three decades ago. Abt says a key issue is what these results imply about how binaries form, and he notes that Lada's results agree with computer simulations that model the motions of young stars in dense star clusters. In these simulations, massive stars tend to acquire massive companions and eject low-mass stars, such as red dwarfs. Says Abt, "It seems to provide a very clear, simple explanation for why you have a high binary frequency for massive stars, like B stars; a moderate one for G stars; and a very low binary frequency for M dwarfs."

Lada says the high single-star fraction of red dwarfs is good news for planet hunters, because planets may form most easily around single stars. In contrast, if a star system is multiple, one star's gravity may perturb the disk of gas and dust that's trying to give birth to planets around another star, preventing planets from developing. Thus, says Lada, red dwarfs may have solar systems with the same architecture and stability as our own. This, of course, raises the chance that red dwarfs could support life-bearing worlds.

Lada has submitted his work to Astrophysical Journal Letters.

Ken Croswell is an astronomer and the author of Magnificent Universe and See the Stars: Your First Guide to the Night Sky.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.

"Finally! An astronomy guide that the reader can actually follow without being a rocket scientist!" See all reviews of See the Stars here.

THE NEAREST RED DWARFS

Star	Single?
Proxima Centauri	NO
Barnard's Star	YES
Wolf 359	YES
Lalande 21185	YES
Luyten 726-8	NO
Ross 154	YES
Ross 248	YES
Lacaille 9352	YES
Ross 128	YES
Luyten 789-6	NO
Struve 2398	NO
Groombridge 34	NO
Giclas 51-15	YES

Adapted from page 189 of Magnificent Universe. All these stars are less than 12 light-years from the Sun.

KenCroswell.com

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