How Big Can Black Holes Get?

There may be an upper limit to their growth

- BY PHIL PLAIT

Black holes like the one shown * in this illustration can grow astonishingly massive by feeding on immense amounts of matter.

IN THE EARLY 1960S ASTRONOMERS DISCOVERED a monster. Something in the constellation of Virgo was pouring out radio waves, but no counterpart in visible light was seen initially. After a lengthy effort, that changed when observers glimpsed a faint blue “star” sitting at the radio source’s exact position. Eventually they were able to determine that this object, called 3C 273, was not a star at all but rather something much stranger located a staggering two billion light-years from Earth.

To be visible across such a vast stretch of space, the “quasi-stellar object” (quasar for short) 3C 273 had to be overwhelmingly bright. Scientists ultimately settled on a feeding black hole at the heart of a far-distant galaxy as the most likely engine for 3C 273’s ridiculous luminosity. And it wasn’t just any black hole but a positively Brobdingnagian one, with a mass that was probably 900 million times that of our sun.

Since that time, we’ve found many more such supermassive black holes. In fact, by the 1980s astronomers were starting to suspect that every big galaxy had a supermassive black hole in its center. Thanks to observations from the Hubble Space Telescope and other facilities, we now know that is true—which means there could be as many as a trillion such giants in the observable universe.

And “supermassive” is definitely the right name for them. Many have been found with a billion times the sun’s mass, and the beefiest can be even heavier than that. This fact naturally raises the question: Just how hefty can one get?