Our Galaxy's Central Black Hole Has Become Suddenly Ravenous

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©. Star S0-2 orbiting the supermassive black hole at the center of the Milky Way. (Artist's rendering by Nicolle Fuller/National Science Foundation)

Astronomers are stunned and stumped by the brightest light seen in 24 years of observations of the black hole at the center of our galaxy.

In the middle of the galaxy we call home, the Milky Way, is a black hole called Sagittarius A*, or Sgr A*. It is usually a pretty calm black hole, just doing its black hole things. But during the spring, astronomers noticed some strange behavior. As Stuart Wolpert writes for UCLA, Sgr A* has been having an "unusually large meal of interstellar gas and dust, and researchers don't yet understand why."

The new mystery is based on observations of the black hole during four nights in April and May at the W.M. Keck Observatory in Hawaii. The brightness surrounding the black hole is generally not perfectly consistent, but during the nights in question, the scientists were "stunned" by the extreme variations in brightness.

"We have never seen anything like this in the 24 years we have studied the supermassive black hole," said Andrea Ghez, UCLA professor of physics and astronomy and a co-senior author of a study on the topic. "It's usually a pretty quiet, wimpy black hole on a diet. We don't know what is driving this big feast."

Looking at more than 13,000 observations of the black hole since 2003, the researchers found that on May 13, the area just outside the black hole's "point of no return" was two times brighter as the second-brightest observation, explains Wolpert. The "point of no return" is just what it sounds like (cue foreboding soundtrack music): the point where once matter enters, it can never escape.

Dramatic changes also occurred on two other nights; the three were "unprecedented," Ghez said.

Periods of brightness are the result of radiation caused by gas and dust entering the black hole, but the team didn't know if this was a dramatic singular occasion or the beginning of something more and/or something bigger.

"The big question is whether the black hole is entering a new phase -- for example if the spigot has been turned up and the rate of gas falling down the black hole 'drain' has increased for an extended period -- or whether we have just seen the fireworks from a few unusual blobs of gas falling in," said Mark Morris, UCLA professor of physics and astronomy and the paper's co-senior author.

"The first image I saw that night, the black hole was so bright I initially mistook it for the star S0-2, because I had never seen Sagittarius A* that bright," said UCLA research scientist Tuan Do, the study's lead author. "But it quickly became clear the source had to be the black hole, which was really exciting."

The scientists are not sure what the cause of increased activity is; it could be gas from a passing star, or it could have something to do with several large asteroids that were consumed by the hole. Ghez said another possibility is that the black hole skinned G2, a "bizarre object" that is most likely a pair of binary stars.

As for how Sagittarius A* may affect our own little orb – like, are we about to be lunch for the Milky Way's bottomless pit of a black hole? – there's nothing to worry about, say the astronomers. The black hole is some 26,000 light-years away and poses no threat to our planet. But if you happen to be an asteroid flying around up there, take care when passing by.

The research has been published by the UCLA Galactic Center Group in Astrophysical Journal Letters.