Black Hole Beacon
The center of the Milky Way galaxy floats low in the southern sky this evening, above the spout of the teapot formed by the stars of Sagittarius. The core spans thousands of light-years, but it’s so far away that you can cover it with your thumb.
Most of the visible light from the core is blocked by giant clouds of dust. But other forms of energy get through, including X-rays. Among other things, the X-rays reveal black holes. The black holes themselves produce no energy at all, but they can be encircled by disks of hot gas that emit lots of X-rays.
The galaxy’s core is dominated by a black hole that’s about four million times as massive as the Sun. But many smaller black holes are also sprinkled through the core.
One of those was discovered last year. An X-ray telescope in space detected a bright burst of X-rays from the direction of the galactic center. Follow-up observations classified the outburst as an X-ray nova.
Such a system consists of a black hole a few times the mass of the Sun, plus a “normal” companion star. The black hole steals gas from the surface of the companion, forming a hot, thin disk around the black hole. If the flow of gas isn’t steady, the gas may pile up in the outer regions of the disk. Eventually the hot gas bursts inward toward the black hole, producing a bright flare of X-rays. As the gas cools and spirals into the black hole the system fades — until the next outburst.
More about the Milky Way tomorrow.
Script by Damond Benningfield, Copyright 2013
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