Countless stars are located within 1.6 light years of the central black hole of the Milky Way. But there are fewer red giants – shining stars that are big and cool – in this crowded neighborhood than expected.
Now astrophysicists have a new theory as to why: The supermassive black hole, Sagittarius A *, launched a powerful jet of gas that tore off the outer layers of the red giants. That turned the stars into smaller red giants, or stars that are hotter and bluer, suggest Michal Zajaček, astrophysicist at the Polish Academy of Sciences in Warsaw, and colleagues in an article published online on November 12th Astrophysical Journal.
Today Sagittarius A * is calm, but two huge bubbles of gamma-ray emitting gas are rooted in the center of the Milky Way Tower, well above and below the plane of the galaxy (SN: 09.12.20). These gas bubbles imply that the black hole came to life about 4 million years ago when something fell into it.
At that time, a gas disk around the black hole shot a powerful stream of material into its starry neighborhood. Zajaček and colleagues propose. “The jet works preferentially on large red giants,” he says. “They can be effectively removed by the jet.” The largest and brightest red giants appear to be missing near the galactic center, says Zajaček.
Red giants are vulnerable because they are tall and their gas shells are thin. A red giant forms from a smaller star after the center of the star is so filled with helium that it can no longer burn its hydrogen fuel there. Instead, the star begins to burn hydrogen in a layer around the center, causing the star’s outer layers to expand and its surface to cool and turn red. As a result, some red giants are more than 100 times the diameter of the Sun, making them easy choices for the jet.
Still, Zajaček says that when red giants orbit the black hole they have to go through the jet hundreds or thousands of times before they become hot blue stars. The jet is most effective at removing red giants within 0.13 light years of the black hole, the team calculates.
“The idea is plausible,” says Farhad Yusef-Zadeh, an astronomer at Northwestern University in Evanston, Illinois, who was not involved in the study.
Tuan Do, an astronomer at UCLA, adds, “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 likely explains the lack of red giants farther from the black hole.
One candidate, say Zajaček and Do, is a large disk of gas that orbited the black hole a few million years ago. This disk produced stars that now orbit the black hole in a single plane. These young stars exist up to 1.6 light years from the black hole, which is the size of the giant red gap. As red giants spun around the black hole and plunged through the disk repeatedly, their gas may have torn off their outer layers, explaining another part of the lack of red stars in the galactic center.