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Seeing Double

June 5, 2021

When you look at some of the bright stars in June’s evening sky, you’re seeing double. That’s because the stars are binaries — two stars bound by their mutual gravity. The list includes Antares, which is low in the southeast after nightfall; Spica, which is higher in the south; and Regulus, over in the west.

In fact, most of the stars in the galaxy are members of binaries. The stars in a binary were born together, from the same cloud of gas and dust, so they’re siblings. And it’s easier to measure the masses of binaries than single stars, so they’re popular targets for astronomers.

One recent study, for example, identified 1.3 million binaries within about 3,000 light-years of Earth — many times more systems than were known before.

They were found by Kareem El-Badry, a graduate student at UC-Berkeley. He sifted through observations from the Gaia space telescope. He was looking for binaries where the two stars are far apart — at least 10 times the distance from Earth to the Sun. It’s easier to see their individual stars than in systems where the stars are close together.

Many of the binaries include at least one white dwarf — the “corpse” of a once-normal star. It’s easier to measure the age of a white dwarf than other types of stars, which then reveals the age of the companion. Having the masses of the stars and other details helps astronomers understand how all stars evolve — a big advantage to seeing double.

Script by Damond Benningfield

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