According to a team of astronomers at the University of Sheffield, UK, stars are ripped apart by supermassive black holes 100 times more often than previously thought.

This artists illustration depicts what astronomers call a tidal disruption event. Image credit: NASA / CXC / M. Weiss.

When an unfortunate object, such as a star, wanders too close to a dormant supermassive black hole, the intense gravity of the black hole can destroy the object in whats called a tidal disruption event.

During such an event, some of the stellar debris is flung outward at high speeds, while the rest becomes hotter as it falls toward the black hole, generating a distinct flare.

Until now, such stellar cannibalism had only been found in surveys which observed many thousands of galaxies, leading astronomers to believe they were exceptionally rare: only one event every 10,000 to 100,000 years per galaxy.

However, University of Sheffield Professor Clive Tadhunter and co-authors spotted a star being destroyed by a supermassive black hole in a survey of just 15 galaxies.

Each of these 15 galaxies is undergoing a cosmic collision with a neighboring galaxy, said team member Dr. James Mullaney.

Our surprising findings show that the rate of tidal disruption events dramatically increases when galaxies collide.

This is likely due to the fact that the collisions lead to large numbers of stars being formed close to the central supermassive black holes in the two galaxies as they merge together.

The astronomers first observed their sample of 15 colliding galaxies in 2005, during a previous project.

However, when they observed the same sample again in 2015, they noticed that one galaxy the ultra-luminous infrared galaxy (ULIRG) F01004-2237 appeared strikingly different.

They found that in 2010, the brightness of this galaxy flared dramatically.

The particular combination of variability and post-flare spectrum observed in ULIRG F01004-2237 which is 1.7 billion light years from Earth was unlike any known supernova or active galactic nucleus, but characteristic of tidal disruption events.

Based on our results for ULIRG F01004-2237, we expect that tidal disruption events will become common in our own Milky Way Galaxy when it eventually merges with the neighboring Andromeda galaxy in about 5 billion years, Prof. Tadhunter said.

Looking towards the center of the Milky Way at the time of the merger wed see a flare approximately every 10 to 100 years.

The flares would be visible to the naked eye and appear much brighter than any other star or planet in the night sky.

The teams findings were published this week in the journal Nature Astronomy (arXiv.org preprint).

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C. Tadhunter et al. A tidal disruption event in the nearby ultra-luminous infrared galaxy F01004-2237. Nature Astronomy, published online February 27, 2017; doi: 10.1038/s41550-017-0061

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