What actually happens when a star is sucked in by a black hole

A new research has tried to show what exactly happens during the destruction of a star as it falls into a black hole.

Enrico Ramirez-Ruiz used computer simulations to explore the universe's most violent events, so when the first detailed observations of a star being ripped apart by a black hole were reported in 2012 (Gezari et al., Nature), he was eager to compare the data with his simulations. He was also highly skeptical of one of the published conclusions: that the disrupted star was a rare helium star.

"I was sure it was a normal hydrogen star, and we were just not understanding what's going on," Ramirez-Ruiz, a professor of astronomy and astrophysics at the University of California, Santa Cruz, said.

Ramirez-Ruiz and his students explain what happens during the disruption of a normal sun-like star by a supermassive black hole, and they showed why observers might fail to see evidence of the hydrogen in the star.

First author and UCSC graduate student James Guillochon (now an Einstein Fellow at Harvard University) and undergraduate Haik Manukian worked with Ramirez-Ruiz to run a series of detailed computer simulations of encounters between stars and black holes.

Supermassive black holes are thought to lurk at the centers of most galaxies. Some (known as active galactic nuclei) are very bright, emitting intense radiation from superheated gas falling into the black hole. But the central black holes of most galaxies in the local universe have run out of gas and are quiescent.

Only when an unlucky star approaches too close and gets shredded by the black hole's powerful tidal forces does the galactic center emit a bright flare of light. Astronomers call this a "tidal disruption event" (TDE), and in a typical galaxy it happens about once every 10,000 years.

When a star gets disrupted by a supermassive black hole, the tidal forces first stretch the star into an elongated blob before shredding it.

In a full disruption, about half of the star's mass gets ejected and the other half remains bound in elliptical trajectories, eventually forming an "accretion disk" of material spiraling into the black hole.

The research is set to be published in the Astrophysical Journal.