Milky Way's Supermassive Black Hole Seems to Be Getting Hungrier and Scientists Aren't Sure Why

The supermassive black hole at the center of our galaxy seems to be getting hungrier and scientists aren't sure why.

In a study published in Astrophysical Journal Letters, a team of researchers from the University of California, Los Angeles (UCLA,) examined more than 13,000 observations of the supermassive black hole—known as Sagittarius A*—dating back to 2003.

These observations, from the Very Large Telescope in Chile and the W. M. Keck Observatory in Hawaii, revealed at least three "unprecedented" spikes in brightness earlier this year at regions just outside the black hole's event horizon—the point beyond which nothing, not even light, can escape.

Scientists say the increases in brightness around the black hole are caused by radiation as it sucks in an unusually large quantity of gas and dust

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

While the brightness of Sagittarius A* always varies to a certain degree, the unusual observations—recorded in April and May this year—were extreme in nature, particularly the measurement made on May 13.

"The brightness of Sagittarius A* varies all the time, getting brighter and fainter on the timescale of minutes to hours—it basically flickers like a candle," lead author of the study Tuan Do, also from UCLA, told Newsweek. "We think that something unusual might be happening this year because the black hole seems to vary in brightness more, reaching brighter levels than we've ever seen in the past. On May 13, 2019, the brightness of Sagittarius A* changed by a factor of 75 in about two hours."

At present, it is not clear whether the latest observations indicate a one-off event or the beginning of a period of increased activity for the black hole.

"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," UCLA's Mark Morris, another co-senior author, said in a statement.

The scientists have proposed several possible explanations for the increases in brightness. These include gas being sucked into the black hole from the star S0-2 or the binary star G2 as they made close approaches to Sagittarius A*. Another possibility is large asteroids being dragged into the black hole.

The team say that more research is needed to determine what exactly is happening at Sagittarius A*—which is located around 26,000 light-years from Earth.

"We have a great opportunity right now to learn about how supermassive black holes are fed and how they might grow," Do said. "Supermassive black holes are not easy to study because most of them are so far away. Our galaxy's supermassive black hole is the closest one to Earth so we can see in great detail both the stars and the gas near the black hole."

"That said, whatever is going on at the Galactic Center will not affect the Earth. It is still 26,000 light-years away and its brightness is much too faint to affect us. I like to think of this as fireworks," he added. "I find Galactic center science really exciting because we can watch black hole astrophysics happening in real-time, which we are rarely able to do!"

Sagittarius A*—which has a mass of around four million suns—is the closest supermassive black hole to Earth. It was discovered by two astronomers, Bruce Balick and Robert Brown, who published a paper in 1974 describing a radio source coming from an area right in the middle of our galaxy, according to the National Radio Astronomy Observatory.

The reason we know its mass is that we can measure the motion of stars that orbit the black hole.

"Using the orbits of the stars, we can measure the mass of the black hole," Do said. "These stars move really quickly around the black hole. The only way they can do that is if they are orbiting a very massive object."

It is now thought that almost every large galaxy in the universe has a supermassive black hole at its center.

This article was updated to include additional comments from Tuan Do.

Sagittarius A*
Rendering of a star called S0-2 orbiting the supermassive black hole at the center of the Milky Way. It did not fall in, but its close approach could be one reason for the black hole's growing appetite. Artist's rendering by Nicolle Fuller/National Science Foundation
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