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Black Holes Can Over-Eat, Too.

Black Holes Can Over-Eat, Too.

Black holes cannot accrete more than 50 billion suns-worth of mass, according to new find.

Once upon a time, it was thought that black holes could ‘eat-up’ accreted matter. That is: gas, dust, and other such things that have been pulled into the black hole’s gravity field, usually forming a disk of rotating material. The black holes would pull the matter into infinity, with no limit to how large a black hole could become in this manner.

However, Andrew King, from Leicester University, has recently determined with rock-solid mathematics that black holes cannot grow beyond the 50 billion-sun mark before the very accretion disk that they rely on for ‘food’ becomes unstable and collapses away – leaving the affected black hole with no more matter from which to grow.

Now, this doesn’t mean that black holes can’t actually grow to mass-sizes greater than 50 billion suns – it can always do so by coming upon a star and consuming that, or by merging with another black hole.  However, mathematically, it cannot accrete (pull into a disk about itself) material beyond this particular limit.

The black hole at the center of our own Milky Way galaxy is a mere 3.3 million suns – tiny compared to some of the very largest ones ever found, inhabiting the centers of the largest, elliptical galaxies and possessing around 40 billion suns-worth of mass.

But these are just the ‘supermassive’ black holes that reside at galactic centers; most black holes are ‘stellar’-sized, only a few dozen times more massive than our Sun, and will most likely not grow much larger.  However, somewhere out in the Universe, there is likely to be a black hole, or two or more, whose accretion limit has been reached and so at this time has been ‘starved’ of matter to become any larger.

Andrew King (2015). How Big Can a Black Hole Grow? MNRAS Letters arXiv: 1511.08502v2

Jeffrey Daniels

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