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Black Holes Encyclopedia

Matter May Get Reprieve from Milky Way's Black Hole

WASHINGTON (January 11, 2006) -- A journey into a black hole is a one-way trip to oblivion: Matter and energy fall in, but they can’t come back out. But astronomers say that some of the material that nears the black hole at the center of the Milky Way galaxy appears to get a last-second reprieve: It blasts out into space just before it would disappear forever.

The black hole, known as Sagittarius A* (Sgr A*), is about 3.6 million times as massive as the Sun. Gas and dust that are spiraling into the black hole form a superhot disk around it, with temperatures reaching millions of degrees.

A team of astronomers headed by Farhad Yusef-Zadeh of Northwestern University watched the center of the Milky Way with six telescopes on the ground and three more in space. The list included ground-based optical and radio telescopes plus Hubble Space Telescope, the European XMM-Newton X-Ray Observatory, and a gamma-ray satellite called Integral. The observations were conducted in March and September of 2004, with the results reported this month at the American Astronomical Society conference.

Observations with these instruments revealed that Sgr A* flares up every half hour. The flares, which typically last a few minutes, produce every form of energy, from radio waves through X-rays. The flares appear to be a “way of life” near the black hole, Yusef-Zadeh said, occurring about a third of the time. Such flares have not been detected around the supermassive black holes at the centers of other galaxies, he added.

Additional support for the flares from the accretion disk around Sgr A* was reported by astronomers from UCLA and Caltech, who observed the Milky Way’s core with one of the giant Keck telescopes in Hawaii. Their observations revealed brief bursts of red light coming from around Sgr A*. The light probably is emitted by electrons -- the negatively charged particles in atoms -- that are spiraling in a magnetic field around the black hole.

The astronomers snapped some of the sharpest images of the center of the galaxy ever obtained from the ground by using a combination of laser light and a small mirror attached to the Keck II telescope to compensate for distortions caused by Earth’s atmosphere.

The UCLA astronomers, led by Andrea Ghez, are using this technique to track the motions of stars quite close to the black hole. Calculating the orbits of these stars helps astronomers refine their estimates of the black hole’s mass.

The flares seen around Sgr A* may be produced by a powerful magnetic field that permeates the disk of material around the black hole. The magnetic field lines get tangled then suddenly “snap,” blasting hot gas back into space. This is similar to the mechanism that produces solar flares -- big explosions of particles and energy from the surface of the Sun.

-- Damond Benningfield