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Milky Way enigma: Why galaxy's central black hole is silent

The black hole at the center of the Milky Way, called Sag A*, is much less active than those at the center of other galaxies. A new study points to a simple answer: heat.

An image released by NASA Tuesday, shows Sagittarius A, the supermassive black hole at the center of the Milky Way Galaxy made from data provided by the Chandra X-ray Observatory.


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One of the Milky Way's longstanding puzzles centers on the super-massive black hole at its core, in the constellation Sagittarius: Why is that monstrous black hole, known as Sag A*, so much less energetic that its counterparts in other galaxies?

The behemoth, with some 2.6 million times the sun's mass, is a cosmic dud at the moment. Something is starving it, depriving it of material that otherwise would plummet into it.

Roman Shcherbakov, a researcher at the Harvard-Smithsonian Center for Astrophysics, says he's figured out what that "something" is likely to be: heat.

As material from surrounding stars approaches the black hole and gets compressed by the monster's gravity, it heats up. Some of that heat gets conducted away from the black hole, setting up a source of pressure that sweeps material away from the voracious object.

In other words, heating around the black hole's event horizon – essentially the boundary within which material falls into oblivion – is in effect starving the black hole.

The challenges of studying black holes

In presenting his results during the American Astronomical Society's winter meeting in Washington this week, Dr. Shcherbakov says that building the process of heat conduction into models of how black holes work began a only a couple of years ago.

The processes are so complex that, until recently, "it's been hard to include them in the models," he adds.

Black holes are objects so massive that their gravity is strong enough to prevent light from escaping. Yet scientists have developed ways of detecting them through their effects on the material around them.

In galaxies with active black holes at their cores, strong magnetic fields in the black hole's neighborhood accelerate electrons to speeds up to 99.9 percent of the speed of light and collect them into jets that erupt from the poles of a central black hole. These are visible in a range of wavelengths, from radio eaves to x-rays.


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