Image of the Day: The Sleeping Beast -Milk Way’s Supermassive Black Hole


Astronomers have long wondered why it is that the super-massive black hole in the center of our galaxy,  nown as Sagittarius A*, containing about 4 million times the mass of our Sun is relatively quiet.. Yet, despite its size, the energy radiated from its surroundings is billions of times weaker than black holes at the core of other galaxies. In fact, it was a million times brighter three centuries ago when must have unleashed an incredibly powerful flare.

The center of our Milky Way is surprisingly difficult to observe from our location, hidden about 24000 lightyears away behind enormously thick clouds of gas and dust within the Galactic disk. But todays super observatories powerful telescopes can reveal all its secrets. Above is a color-coded overlay of an infrared image (in red) of the center of the Milky Way obtained by the Spitzer Space Telescope, an optical image (in yellow) from Hubble, and an X-ray image (in blue) from Chandra which highlights the bright white patch on the lower right where our own supermassive black hole resides.

"We have wondered why the Milky Way’s black hole appears to be a slumbering giant," saysTatsuya 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.

Image Credit: Credit: X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy


NASA's High Energy Astrophysics Science Archive Research Center


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