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News 10-04-12 06:10 PM

Fast-moving star is the closest yet to the Milky Way's black hole
http://cdn.arstechnica.net/wp-conten...02-640x640.jpg Reconstruction of the orbits of two stars'S0-2 and S0-102'near the black hole at the Milky Way's center. (Other stars' orbits are also depicted by fainter lines.) The background is a real high-resolution infrared image of the region.
Andrea Ghez et al./UCLA/Keck

The center of the Milky Way is a messy place, full of stars and star remnants. There's so much going on that it's hard to observe in visible light, so precision measurements rely on radio, X-ray, or infrared (IR) wavelengths. Astronomers using the Keck telescope on Mauna Kea in Hawaii have been watching stars near the galactic center in IR for 17 years, providing a detailed view of their dynamics. The fruits of these observations include the measurement of the mass of the Milky Way's central black hole: approximately 4 million times the mass of the Sun.

The Keck team, led by UCLA astronomer Andrea Ghez, has now identified a star that orbits the black hole in 11.5 years, the shortest orbital period for a star yet discovered. Thanks to this relatively quick motion, the astronomers have completely mapped the orbit of this star, known as S0-102, making it the second such star observed. Knowledge of complete orbits helps with modeling the black hole itself, which is invisible in IR light. While the data so far doesn't deviate from Newtonian gravity, observations over multiple orbits should allow for tests of general relativity in the region close to a black hole'something that hasn't been done yet using stellar orbits.

Since 1995, researchers have mapped the motions of stars around the central black hole, using high-resolution IR images. At Keck, such imaging was facilitated by adaptive optics, wherein the shape of the 10-meter-diameter mirror was adjusted dynamically to correct for atmospheric distortion. With such techniques, astronomers were able to pinpoint the location and paths of a significant number of stars near the black hole, known as the S-stars.

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