How big and hot can black holes get?
According to theoretical expectations, the biggest black holes are actually the coldest, because their so-called Hawking temperature is measured in billionths of a degree above absolute zero (zero degrees Kelvin). The hottest black holes are microscopic in size, and have fewer than 1 trillion grams of mass. Their temperatures increase from a few thousand degrees to trillions of degrees Kelvin as they evaporate into still-lower-mass objects. As for size, the biggest known black holes contain several billion times the Sun’s mass and are about 3 billion kilometres in radius. These supermassive black holes lurk in the cores of quasars and other “active” galaxies. We know of no black holes with more than 10 billion times the mass of the Sun. The universe is probably not old enough to grow such unspeakable monsters, but in the distant future, there may indeed be many such hypermassive black holes to reckon with.
