Joshua Hill  Bio

Email Article

Email Us

Print friendly

Super massive Black Holes

Black Hole at Center of Milky Way’s Turbulent Past

Astronomical time is one of those things hard to get your head around. Some of us (ok, me) have enough trouble working out what day it is they are currently residing in. So when someone asks me to assume millions of years is nothing more than an astronomical blip, I get worried.

Astronomers have added to that confusion, noting that the black hole in the center of the Milky Way, is probably just taking a break.

For a long time astronomers have wondered why it was that the super-massive black hole in the center of our galaxy was relatively quiet. Known as Sagittarius A* (pronounced “A-Star”), our black hole is a massive hole, containing about 4 million times the mass of our Sun. Yet, despite its size, the energy radiated from its surroundings is billions of times weaker than other central black holes in other galaxies.

“We have wondered why the Milky Way’s black hole appears to be a slumbering giant,” says team leader Tatsuya Inui of Kyoto University in Japan. “But now we realize that the black hole was far more active in the past. Perhaps it’s just resting after a major outburst.”

Tatsuya Inui is part of a team that used results from Japan’s Suzaku and ASCA X-ray satellites, NASA’s Chandra X-ray Observatory, and the European Space Agency’s XMM-Newton X-ray Observatory, to determine the history of our black hole.

It turns out that, approximately 300 years ago, Sagittarius A* let loose, expelling a massive energy flare.

Data taken from 1994 to 2005 revealed that clouds of gas near the central black hole, known as Sagittarius B2, brightened and faded quickly in X-ray light. The X-rays were emanating from just outside the black hole, created by the buildup of matter piling up outside the black hole, which subsequently heats up and expels X-rays.

These pulses of X-ray take 300 years to traverse the distance between Sagittarius A* and Sagittarius B2, so that when we witness something happening in the cloud, it is responding to something that happened 300 years ago.

Amazingly for us, in a rare occurrence of perfect cosmic timing, a region in Sagittarius B2, only 10 light-years across varied dramatically in brightness. “By observing how this cloud lit up and faded over 10 years, we could trace back the black hole’s activity 300 years ago,” says team member Katsuji Koyama of Kyoto University. “The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare.”