Galaxies and Stars

Galaxies and Stars

The Universe contains hundreds of billions of galaxies, each one a massive group of stars. A star is a huge spinning ball of hot gases, mostly hydrogen and helium. These gases cause chemical reactions, which produce a star's heat and light.

Galaxies are made up of millions to trillions of stars. Each galaxy spins around. At the centre of most galaxies is a black hole - a region of pressed-together matter and energy. Black holes have immense pulling power called gravity. This gravity 'swallows' nearby stars, so they seem to disappear.

A Star is Born

A star begins when clouds of dust in space are pulled together by gravity. Gradually these clouds form a clump, called a protostar. As time goes on, the temperature and pressure rises in a protostar, causing its hydrogen atoms to join. This gives off vast amounts of light heat and other rays. However, a star's energy gradually runs out and it dies.

A Star dies

Stars only shine for a certain time, depending on their size. Small stars called red dwarfs use up their hydrogen, and then cool and shrink into white dwarfs. Medium stars such as our Sun last around 10 billion years, grow into red giants and then fade away. Massive supergiant stars burn briefly and explode in a colossal fireball called a supernova. This leaves a tiny but immensely heavy neutron star or, for the biggest supergiants, a black hole.

Fascinating Facts
• The Universe probably has more than 120 billion galaxies. The actual number may be as high as 3,000 billion.
• Small galaxies have about 10 million stars, while big ones have more than a trillion (a million million).
• Our own galaxy, containing the Sun and planets, is called the Milky Way.
• The Milky Way is a spiral galaxy spread across almost one billion billion billion kilometres and containing around 100 million billion stars.