Saturday science subject: the biggest black hole

Astronomers have uncovered the most massive black hole yet, according to a report by the New Scientist. The black hole, which sits at the heart of quasar OJ287 and lies a comfortable 3.5 billion light years away, has an estimated mass 18 billion times that of our Sun—that's about 3.58×1037 metric tons. In fact, the black hole is so gigantic that its mass is equivalent to that of a small galaxy. It is also orbited by another, smaller black hole, which has a mass of "only" 100 million Suns.

A diagram of the two black holes. Researchers now peg the mass of the
larger black hole at 18 billion Suns. Source: The New Scientist.

The smaller black hole orbits the larger one every 12 years, causing bright outbursts each time it punches through the disc of matter that surrounds its larger sibling. Because of the incredible gravitational forces at stake, a team of astronomers led by Mauri Valtonen of Tuorla Observatory in Finland have used the quasar to test Einstein's theory of general relativity.

General relativity predicts that the smaller hole's orbit itself should rotate, or precess, over time, so that the point at which it comes nearest its neighbour moves around in space – an effect seen in Mercury's orbit around the Sun, albeit on a smaller scale. . . . In the case of OJ287, the tremendous gravitational field of the larger black hole causes the smaller black hole's orbit to precess at an incredible 39° each orbit. The precession changes where and when the smaller hole crashes through the disc surrounding its larger sibling.

The New Scientist says general relativity also predicts that the black holes "should spiral towards each other as they radiate energy away in the form of gravitational waves, or ripples in space." According to Craig Wheeler from the University of Texas in Austin, observations of the outbursts from the quasar match expectations from general relativity closely. "The fact that you can fit Einstein's theory [so well] ... is telling you that that's working," he told the New Scientist.

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