Scientists: Stars orbiting close to Milky Way’s supermassive black hole do it alone

The stars orbiting Sag A*
The stars orbiting Sag A*. Click for original image.

Based on a ten year study of the motions of nine stars orbiting close to Sagittarius A* (pronounced A-star), the supermassive black hole at the center of the Milky Way, scientists have concluded that they are single stars, not binaries as would be expected.

Using W. M. Keck Observatory on Maunakea, Hawaiʻi Island, Devin Chu of Hilo, an astronomer with the UCLA Galactic Center Orbits Initiative, led a 10-year survey that found these ‘S-stars,’ where ‘S’ stands for Sagittarius A*, the name of the monster black hole at our galaxy’s core, are all single.

The result is surprising given the S-stars Chu’s team observed included young, massive main-sequence stars that are only about six million years old. Normally, stars at this age that are 10 times more massive than our Sun spend their childhood years paired with a twin in a binary system, or sometimes even as triplets.

This finding suggests that the black hole’s massive gravitational field causes the binaries to be pulled apart, or somehow to merge during their formation.

This data point and the questions it raises pales before the more fundamental question that astronomers have been asking since these stars were first discovered in the 1990s: How is it possible for any stars to form so close to such a disruptive gravitational field?

Part of the gas cloud being ripped apart by the supermassive black hole at the center of the Milky Way have already swung past the black hole.

Part of a gas cloud, being ripped apart by the super massive black hole at the center of the Milky Way, has already swung past the black hole.

“The ionised gas at the head of the cloud is now stretched over more than 150 light-hours (about 160 billion kilometres) at the pericentre of the orbit around the black hole, with the closest approach being about 25 light-hours (or a bit more than 25 billion kilometres)”, explains Stefan Gillessen from MPE, who led the observing team. “The pericentre approach however is not a singular event but rather a process that will be stretching over a period of at least one year.”

The black hole, dubbed Sagittarius A* (pronounced A-star), is more than 4 billion times the mass of our Sun, but emits very little energy for its size. (Most super massive black holes emit energy as they swallow the mass around them.) Astronomers are hoping that they will see some action when it eats this cloud sometime next year.

The supermassive black hole in the center of the Milky Way is about to get a snack.

The supermassive black hole in the center of the Milky Way is about to get a snack.

Update: The recently launched NuStar telescope in July detected its first flare from the central black hole (which by the way is called Sagittarius A* and is pronounced Sagittarius A-star). If the gas cloud produces any fireworks as it whips past the black hole in the coming year then NuStar should see it.