Some well-known binary star systems are in good view as night falls. Antares, the heart of the scorpion, is due south. Blue-white Spica is at about the same height in the southwest. And Polaris, the pole star, stands due north, just as it does every night of the year.

Many of the other stars visible to the naked eye are binaries as well — two stars that orbit each other, bound by their mutual gravitational pull.

In fact, binaries are quite common. But astronomers aren’t sure just how common. One study put the fraction of stars that are in binaries at about half. But others have come up with higher numbers.

Binaries probably are born in the same way as single stars — from the collapse of giant clouds of gas and dust. As the cloud gets especially tight, though, it splits apart. That creates two newborn stars — and sometimes three, four, or even more.

Most of the biggest and heaviest stars have binary companions. Binaries are less common among stars that are roughly the mass of the Sun. And they’re least common among the smallest and faintest of stars, known as red dwarfs. Because they’re so puny, they’re hard to study, so we’re not sure just how likely they are to be members of binaries. But red dwarfs account for most of the stars in the Milky Way Galaxy. So zeroing in on that likelihood will give us a better idea of just how many stars have companions — and how many travel through the galaxy alone.
 

Script by Damond Benningfield

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