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The central black hole in the Milky Way was able to turn nearby red giant stars into blue ones

Countless stars reside 1.6 light-years from the central black hole in the Milky Way. But this same crowded neighborhood has fewer red giants – bright stars that are big, cool – than expected.

Now astrophysicists have a new theory of why: the supermassive black hole, Sagittarius A *, launched a powerful jet of gas that ripped the outer layers of the red giants. That turned the stars into smaller red giants or hotter, blue stars, Michal Zajaček, an astrophysicist at the Polish Academy of Sciences in Warsaw, and his colleagues suggest in an article published online on November 12 in the Astrophysical Journal.

Today Sagittarius A * is quiet, but two huge bubbles of gamma-ray emitting gas rooted in the center of the Milky Way tower far above and below the plane of the galaxy (SN: 12/9/20). These gas bubbles imply that the black hole came to life about 4 million years ago when something fell into it.

At that moment, a gas disk around the black hole fired a powerful jet of material at his star-studded neighborhood, Zajaček and his colleagues propose. “The jet acts preferentially on large red giants,” he says. "They can be effectively ablated by the jet." Zajaček says the largest, brightest red giants are missing near the galactic center.

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Red giants are vulnerable because they are large and their gas envelopes faint. A red giant forms from a smaller star after the center of the star is so full of helium that it can no longer burn its hydrogen fuel there. Instead, the star begins to burn hydrogen in a layer around the center, which causes the outer layers of the star to expand, causing its surface to cool and turn red. As a result, some red giants are more than a hundred times the diameter of the sun, making them easy tracks for the jet.

Still, Zajaček says that when red giants orbit the black hole, they must pass through the jet hundreds or thousands of times before they become hot blue stars. The equipment is most effective at removing red giants 0.13 light years from the black hole.

“The idea is plausible,” says Farhad Yusef-Zadeh, an astronomer at Northwestern University in Evanston, Illinois, who did not participate in the study.

Tuan Do, a UCLA astronomer, adds that "it may take a combination of several of these types of mechanisms to fully explain the lack of red giants." In particular, he says, something other than a jet probably explains the scarcity of red giants farthest from the black hole.

One of the candidates, say Zajaček and Do, is a large disk of gas that surrounded the black hole a few million years ago. This disk generated stars that now orbit around the black hole in a single plane. These new stars exist up to 1.6 light-years from the black hole, which is also the extension of the red giant space. When red giants spin around the black hole and dive repeatedly through the disk, their gas can rip off their outer layers, explaining another part of the red star's scarcity of the galactic center.

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