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The M87 black hole image showed the best way to measure black hole masses

Its diameter suggests the black hole is 6.5 billion times the mass of the sun

SUPERMASSIVE SOURCE The gases and stars in galaxy M87, shown in this composite image from the Chandra X-ray telescope and the Very Large Array, gave different numbers for the mass of the galaxy’s supermassive black hole.


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.

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 solar masses, which was revised in 2013 to 3.5 billion solar masses. For years, it wasn’t clear which technique got closer to the truth. Now the EHT picture showing a glowing orange ring of gases and dust around the black hole has solved the conflict. According to Einstein’s general theory of relativity, the diameter of the dark space in the center of the image — the black hole’s shadow — is directly related to its mass.

“Bigger black holes cast bigger shadows,” EHT team member Michael Johnson, an astrophysicist at the Harvard Smithsonian Center for Astrophysics, said April 12 at a talk at MIT. “Easy check, we can see whether one or the other of these [mass measuring methods] is correct.” The shadow of M87’s black hole yielded a diameter of 38 billion kilometers, which let astronomers calculate a mass of 6.5 billion suns — very close to the mass suggested by the motion of stars. The size of the shadow also negated the idea that the black hole is a wormhole, a theoretical bridge between distant points in spacetime (SN: 5/31/14, p. 16). If M87’s black hole had been a wormhole, theory predicts it should look smaller than it does. “It’s a stunning confirmation” of general relativity, Johnson said. “We instantly rule out all these exotic possibilities.”

The mass confirmation may boost confidence in current simulations for how black holes develop, says Priyamvada Natarajan, a Yale University astrophysicist who was not involved with the EHT project. Most black hole mass estimates already use the stellar technique, in part because it’s easier to track a galaxy’s stars from farther away.

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