Freda Moonbeam, of Tui community, Takaka, asks :-

How do black holes form?

Frank Andrews, an astronomer at Wellington's Carter Observatory, responded.

The most common type of black hole forms when an old star, which is more than six times the mass of our sun, dies.

As a star runs out of hydrogen fuel it starts to collapse under gravity and, if massive enough, undergoes a huge explosion. This is a supernova, a `new' star, actually an old one that suddenly becomes very bright. In this explosion the outer layers of the star are blasted into space while the central core is compressed under its own gravity.

If the resulting core of the star is between 1.4 and 3.2 times the mass of our sun the electrons around the atom are compressed into the atomic nucleus to form a neutron star which is only 10 to 15km in diameter. This is the heavest form of matter known. These stars rotate rapidly, giving off light and radio waves that we observe as short pulses. For this reason they are called Pulsars.

If the mass of the star is more than six times our sun's mass, its core of more than 3.3 times our sun's mass is compressed under gravity to form a black hole. For these objects gravitation is so high that even light cannot escape from it, hence its name.

Astronomers now believe that black holes of vast size can exist at the centres of many galaxies. They have a mass ranging from a few tens of thousands to billions of times the mass of our sun and are known as super massive black holes. They are thought to be the engines that drive the brilliant centres of special, distant galaxies called quasars. No one knows how or why these super massive black holes formed, but they do not seem to form in the universe today. Remember that the light from distant quasars left there some ten billion years ago.