# How does a black hole change over time?

Jan 5, 2017

Black holes can grow or even evaporate over time.

#### Explanation:

First of all the radius of a black hole is defined by its Schwarzschild radius ${r}_{s}$:

${r}_{s} = \frac{2 G M}{c} ^ 2$

Where G is the gravitational constant, M is the mass of the black hole and c is the speed of light.

Curiously the radius of a block hole is directly proportional to its mass.

The first way a black hole can change over time involves Hawking radiation. In space, even in a vacuum, particle - anti particle pairs are constantly being produced. They usually quickly annihilate each other. Stephen Hawking predicted that if a particle - anti particle pair forms close to a black hole one particle can fall into the black hole and the other can escape. This process, known as Hawking radiation, causes the black hole to lose mass and it can ultimately evaporate.

If material falls into a black hole it will add to the black hole's mass and hence make it bigger.

Also, black holes can collide to form bigger black holes.

There are three types of black hole, each of which is likely to change differently over time.

Very small black hole were thought to have been created soon after the Big Bang. These are most likely to evaporate due to Hawking radiation.

Stellar black holes are formed by the death of a large star. It would take a very long time for them to evaporate by Hawking radiation and they are likely to gain material from their surroundings.

Supermassive black holes exist at the centres of galaxies. They are so massive that they are more likely to grow by consuming material or colliding with other black holes.