First Previous Next Last         Home


Quantum Gravity - Temperature of a Black Hole


Slide 30


Note
Black holes lose mass through radiation and eventually evaporate. This process is very slow. For example, a black hole with the mass of our sun would take approximately 1057 times the current age of the universe to evaporate. As black holes evaporate, their horizons shrink and their entropy decreases. In keeping with the 2nd law of thermodynamics, the overall entropy of the universe increases in the process since the radiation carries away a larger amount of entropy than a black hole loses.

Black holes also grow as material falls into their horizons. This process competes with the process of evaporation. As material falls in, the mass and entropy of a black hole increase while the outside entropy decreases. As a result, the horizon must grow. Since the temperatures of large black holes are so low, black holes generally grow overall under current conditions rather than shrinking.

The temperature of a black hole is inversely proportional to its mass. The temperatures of astrophysical black holes is generally very small. For example, a 3 solar mass black hole has a mass of around 6 × 1030 kg, which gives it a temperature of approximately 2.1 × 10-8 K. The supermassive black hole at the center of our galaxy has a mass of about 5 × 1036 kg, which means that it's temperature is about 2.4 × 10-14 K. However, a black hole with the mass of Mt Everest would have a horizon the size of an atomic nucleus and a temperature hotter than the temperature at the center of a star.

I am the author of the images and text except where otherwise indicated. Please contact me for permission if you wish to use any of my images or text.

Created on Wednesday 03 May 2006 by Mark A. Martin with KPresenter