Author: Lisa Grossman / Source: Science News

The measure of a black hole is what it does with its stars.
That’s one lesson astronomers are taking from the first-ever picture of a black hole, released on April 10 by an international telescope team (SN Online: 4/10/19). That image confirmed that the mass of the supermassive black hole in the center of galaxy M87 is close to what astronomers expected from how nearby stars orbit — solving a long-standing debate over how best to measure a black hole’s mass.
The black hole in M87, which is located about 55 million light-years from Earth, is the first black hole whose mass has been calculated by three precise methods: measuring the motion of stars, the swirl of surrounding gases and now, thanks to the Event Horizon Telescope imaging project, the diameter of the black hole’s shadow.
In 1978, the first mass estimates to track the motions of stars whipping around the great gravitational center found that the stars must be orbiting something containing about 5 billion times the mass of the sun. A more precise estimate in 2011 using a similar stellar technique bumped its heft up to 6.6 billion times the mass of the sun.

SHADOW SIZE
Meanwhile, astronomers in 1994 made another estimate by tracing how gases closer to the black hole than the stars swirl around the behemoth. That technique suggested that the black hole was 2.4 billion…
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