Black holes devour one star a year

Supermassive black holes could destroy around one star per year that strays to close. The effect has been studied around the cores of recently merged galaxies but could take place in other kinds as well.

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In many galaxies the stars around the central black hole are pulled into a spherical shape, but this is not always the case.
Credit: ESA–C. Carreau

Newly merged black holes could devour as much as one star a year, according to the latest models from astronomers at the University of Colorado, Boulder.

The find could explain why some galaxies appear to have lopsided centres, and also highlights how important black holes can be in the life cycles of the stars near them.

The supermassive black holes that lie at the hearts of some galaxies are usually surrounded by a cluster of stars that have been drawn there by the strong gravitational forces.

In most galaxies, these stars are pulled into a spherical shape, but in some cases – such as the Andromeda Galaxy – they form a disc instead.

These discs are usually seen in galaxies that are thought to have undergone a recent merger which could be responsible for the unusual shape.


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Stars that are in discs tend to follow elliptical orbits, meaning that occasionally the stars pass close enough to each other to gravitationally interact which can shift their orbits slightly.

“The force builds up in these stellar orbits and changes their shape,” says Ann-Marie Madigan, from UC Boulder who led the study. “Eventually, a star reaches its nearest approach to the black hole and it gets shredded.”

The team predicts the supermassive black holes of galaxies which have recently undergone a merger will swallow a star around once a year, which is 10,000 times more often than currently expected at other times.

“Andromeda is likely past the peak of this process, having undergone a merger long ago, but with higher resolution data, we may be able to find younger eccentric discs in more distant galactic nuclei,” says Madigan.


 

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