New Secrets Of The Supermassive Blackhole

Last month, scientists unveiled a photograph of a cosmic phenomenon that defies the laws of physics, making headlines worldwide. So how did they do it, and what does this landmark achievement actually teach us?

- Marcus Chown

Wednesday 10 April was an epochmaking moment in the history of science. At six simultaneous press conferences worldwide, an international team of astronomers unveiled the first ever image of a black hole. “It was one of the most exciting days of my life,” says Feryal Özel of the University of Arizona in Tucson, who heads the modelling team. “For me, it’s the culmination of nearly two decades of work.”

In fact, the team observed two black holes: Sagittarius A*, a supermassive black hole in our own Milky Way weighing 4.3 million times the mass of the Sun, and a cousin in the galaxy M87, which is about 1,000 times bigger. The first image revealed is of the supermassive black hole at the heart of M87. Sagittarius A*, because it’s smaller, was circled by matter many times while being observed, yielding a blurrier picture.

The image of the black hole in M87, since named Powehi, shows detail smaller than the extent of its event horizon, the point of no return for in-falling light and matter. It is only possible to see such exquisite detail because the intense gravity of each black hole acts like a lens, which makes the image appear five times larger than its horizon.

The horizon in M87 shows up as a dark shadow backlit by intense radio waves, emitted by matter heated to incandescence as it swirls down through an accretion disk (gas and dust that is orbiting the object) onto the black hole. The halo around it is brighter on one side than on the other. “This is because the accretion disk is spinning, causing the light from the part coming towards us to be boosted relative to that from the part that’s receding,” says Özel.