Astronomers Spot A Star Being Ripped Apart By A Black Hole

Illustration of a black hole after it tore a star apart. NASA/CXC/U. Michigan/J. Miller et al. Illustration: NASA/CXC/M. Weiss

A team of astronomers has observed stellar debris being flung around a black hole in a (relatively) nearby galaxy. These types of events are called tidal disruptions, and the one reported in this study is the closest detected in the last 10 years.

The galaxy, named PGC 043234, is a small elliptical galaxy about 295 million light-years from the Milky Way. The scientists witnessed the consequences of a star passing too close to a central black hole within PGC 043234; the strong gravitational forces around the black hole shredded the star apart, sucking most of the material beyond the event horizon (the point of no return around a black hole) while flinging some of it outwards into a wider orbit around the black hole.

The tidal disruption was first observed in optical light by the All-Sky Automated Survey for Supernovae (ASAS-SN), then quickly followed with NASA's Chandra X-ray Observatory, NASA's Swift Gamma-ray Burst Explorer and the ESA's XMM-Newton satellite.

Materials around black holes tend to have a strong X-ray signature. As the stellar material falls towards the black hole, it forms an “accretion disk”; the acceleration and the friction within the disk, due to the immense gravitational pull, is enough to generate a substantial amount of X-rays. The accretion disk can reach temperatures of 10 million degrees, according to some studies.

"We have seen evidence for a handful of tidal disruptions over the years and have developed a lot of ideas of what goes on," said lead author Jon M. Miller, a professor of astronomy at the University of Michigan, in a statement. "This one is the best chance we have had so far to really understand what happens when a black hole shreds a star."

The formation of accretion disks has remained a mystery, but the observation of this event allowed the team to witness how such disks come to be. Scientists have been studying tidal disruptions for three decades, but this is the first detailed close-up of one.

The black hole, called ASASSN-14li, is believed to have the mass of 2 million Suns, making it about half the size of the Milky Way’s own supermassive black hole, Sagittarius A*.

The study was published in the October 22, 2015, issue of the journal Nature.


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