Astronomers weigh monster black hole

Scientists have weighed a nearby giant black hole and found that it has swallowed the equivalent of 6.6 billion suns. The cosmic cannibal is the most massive known in our local patch of the universe and lies at the centre of a galaxy called M87.

Artist’s impression of how the black hole might look. Credit: Gemini Observatory/Lynette Cook

Astronomers believe the black hole is so gorged and close that it might be the first to be seen. That sounds impossible considering such exotic objects are so dense that light cannot escape them.

But scientists think they will one day be able to detect its visible edge, termed the “event horizon”. That is because it is so big it stretches about three times as far from the black hole’s heart as the orbit of Pluto from the Sun.

There is no threat to us from the supermassive black hole in M87 because while on our cosmic doorstep is still lies 50 million light-years away.

In any case, there is a much closer monster black hole hiding at the centre of our own Milky Way galaxy, though that presents no danger either.

The black hole in M87, which was identified as the biggest known last year, was weighed by astronomers using two telescopes – the 8-meter Gemini North Telescope on Hawaii and the Harlan J. Smith Telescope at the McDonald Observatory in Texas.

Astronomers on Hawaii contributed by tracking the motions of stars in the centre of the galaxy as they orbited the black hole.

Those results were combined with observations of the effects on stars on the galaxy’s fringes of invisible dark matter surrounding M87 that has been termed a “dark halo”. The results were presented at a space conference in Seattle.

Karl Gebhardt of The University of Texas led the project to weigh the black hole. But it is just part of his effort to understand the whole structure of M87. He said: “My ultimate goal is to understand how the stars assembled themselves in a galaxy over time.

“How do you make a galaxy? These two datasets probe such an enormous range, in terms of what the mass is in the galaxy. That’s the first step to answering this question. It’s very hard to understand how the mass accumulates unless you know exactly what’s the distribution of mass: how much is in the black hole, how much is in the stars, how much is in the dark halo.”

Astronomer Tod Lauer of the discovery team said: “Our ability to obtain such a robust black hole mass for M87 bodes well for our ongoing efforts to hunt for even larger black holes in galaxies more distant than M87.”

The youngest known black hole, in a galaxy called M100, was reported last year.

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