I think you're asking about how large in theory a black hole can be, not just how large observed black holes are. The short answer is that astrophysicists aren't quite sure. There isn't believed to be an upper limit for the size of a black hole: as long as galaxies collide, these black holes can get bigger and bigger. Physicists do believe that every galaxy with a central bulge (see this picture
for a representation of what that means) has a very large (called "supermassive") black hole in its center, which keeps the stars rotating around the center of the galaxy. These supermassive black holes get larger when galaxies collide. Sometimes the central black holes merge, and other times the collision brings lots of gas and dust into the center of the galaxy where the black hole can swallow it up.
The size of black holes are directly related to their masses. A black hole is an object so dense that the escape velocity is larger than the speed of light, so light attempting to leave the center gets pulled back to the black hole, gravitationally! The radius of the black hole then is related to this speed of light. This is called the "Schwarzschild radius" after the astronomer who first solved Einstein's equations of general relativity for a black hole, and it is equal to GM/c
^2, where G
is the universal gravitational constant
the mass of the black hole, and c
the speed of light. Therefore, as long as more mass enters the black hole, the radius increases. There is no theoretical limit on the amount of mass that can fall into a black hole, so there is no limit on the radius of our hypothetical black hole.
You picked an excellent time to ask this question, since the largest black hole we know about
was just announced recently! It is 3.5 billion light years away, and has a mass of 4 x 1040
kilos --- that's an 4 followed by 40 zeros! For comparison, that is 18 billion times the mass of the sun.
Thanks for your question,
(published on 01/08/2011)