When galaxies crash, black holes

Supermassive black holes rip up and devour hapless stars a hundred times more frequently than thought, according to research released on Monday.

Scientists had previously calculated that such cosmic cannibalism was extremely rare, happening once every 10,000 to 100,000 years per galaxy.

These so-called "tidal disruption events" had only been witnessed in astronomical surveys canvassing tens of thousands of galaxies.

But the authors of a new study said they spotted a star being destroyed by a black hole in a survey of only 15 galaxy collisions -- an extremely small sample size by astronomy standards.

The chances of a star disappearing into the maw of a black hole go up a hundredfold when galaxies crash, they concluded.

"Our surprising findings show that when two galaxies collide, it dramatically increases how often stars get ripped apart and swallowed," co-author James Mullaney, an astronomer at the University of Sheffield, told AFP. 

Supermassive black holes are millions to billions times more massive than our Sun.

Astronomers agree they reside at the centre of almost all known major galaxies. The Milky Way's own black hole -- currently dormant -- is tucked inside the constellation Sagittarius.

But exactly how these monsters form is still hotly debated among scientists.

For the new study, a team led by astronomer Clive Tadhunter, also from Sheffield, analysed 15 colliding galaxies, each containing billions of stars.

In 2015, they noticed a startling change in one of them -- galaxy F01004-2237, some 1.7 billion light years from Earth -- compared to a decade earlier. 

Sifting through historical data collected by the Catalina Sky Survey, the researchers traced the transformation to 2010. 

The brightness flared that year in a way consistent with the death throes of a star being ripped apart.

"Our study shows that galaxy collisions play an important role in causing stars to fall toward black holes," Mullaney said.

Once in the danger zone, the doomed star is sucked in by a black hole's gravitational pull. 

The stellar debris starts to travel faster and faster, heating up and emitting a flash of light as the star is obliterated.

When our own Milky Way merges with nearby Andromeda, in about 4.5 million years, such star-wrecking events will occur up to every ten years, the scientists said.

These light shows would be brighter than any stars or planets in the night sky.

"From Earth we'd see these events as a flare of light lasting a few months, or even years," said Mullaney.

Assuming, of course, there was anyone around to notice.

The findings, published in the journal Nature Astronomy, were based on observations made with the William Herschel Telescope, operated on the island of La Palma in the Canary Islands.