Credits: NASA and K. Gebhardt (Lick Observatory)
From: B.C.
As long as the black hole has matter and energy to feed on there is no limit to the size to which it can grow. The black hole that resides at the centre of our galaxy is around 2.5 million solar masses. Other galaxies have black hole's in the billions of solar masses.
From: Chris (Avatar)
Theoretically there are no limits on a black hole's mass at all. Black holes are about mass density, meaning that for any mass M all you need to do to make a black hole is squeeze it into a small enough radius R. R depends only on M (for a non-spinning hole, R = 2GM/c2 where G and c are constants).
Because black holes are the constructs of Einstein's general theory of relativity (GR) and GR is a classical (ie non-quantum) theory, there are no theoretical bounds to how big or small M or R can be. But if we look at what else we know about the universe we may be able to derive some limits:
* Lower limit: We can use quantum theory to show that the smallest sensible radius for a black hole (R) is on a distance scale of the Planck length (about one ten-thousand-billion-billion-billion-billionth of a millimetre).
* Upper limit: This scale comes from estimating the mass of the (visible) universe compressed into one gigantic black hole. We estimate the universe contains about 1080 protons and about a billion times as many photons. A rough calculation shows that a black hole which encompasses all this mass would have a radius of about 1 billion light years (nearly one tenth the radius of the visible universe).
Having said this, there are practical limits on how large a black hole might grow. A stellar sized black hole is limited by the size of the star from which it comes. It can then increase in size by swallowing anything nearby... but it will quickly run out of local fuel. Black holes are not vacuum cleaners - their gravity is only strong when you are very close. For example if the sun was to magically turn into a black hole of equal mass, it wouldn't have the strength to swallow earth - we'd just go on orbiting as usual (although it would be a bit dark).
There may be much bigger black holes at the centres of some galaxies. These supermassive black holes may have masses of up to billions of suns. In active galactic nuclei they would still be swallowing nearby masses and churning out unthinkable energy. In quiescent galaxies they've cleared out the nearby masses.