Milky Way black hole gets real

By tracking a star near the center of our galaxy, astronomers have found the best evidence yet that a supermassive black hole lies at the Milky Way’s core. Although observations have long suggested that the galaxy harbors a monster black hole, they didn’t conclusively rule out less exotic concentrations of mass (SN: 9/8/01, p. 148: Galaxy’s Black Hole: X Rays Mark Spot).

HOLEY PATH. Orbit of the star S2 indicates that the Milky Way has a black hole at its core. European Southern Observatory

The closest that the star observed by the astronomers ventures to the galaxy’s center is a distance three times that between Pluto and the sun. Traveling 5,000 kilometers per second, the star–known as S2–takes a mere 15 years to complete one orbit of the galaxy’s core. Researchers now have tracked S2 for 10 years.

The star’s elliptical path and high speed require the mass at the heart of the galaxy to weigh 3.7 million times as much as the sun, the researchers report in the Oct. 17 Nature. “The data no longer allow for a central mass composed of dark stellar objects or a ball of massive [elementary particles],” Rainer Schödel of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues assert.

To distinguish S2 from all the other stars near the Milky Way’s crowded core, the team used a powerful technique called adaptive optics. The method measures the amount by which starlight is blurred by Earth’s turbulent atmosphere and removes that blurring by commanding a bendable telescope mirror to rapidly change its shape.

“These results are the best evidence yet that supermassive black holes are not just theory but fact,” comments Karl Gebhardt of the University of Texas at Austin.


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