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• The Sun is our nearest star. It is a yellow dwarf star and is found at the centre of our solar system.
• The Sun is composed mostly of hydrogen and helium and it releases energy because of nuclear fusion.
• The Sun will eventually run out of hydrogen and stop releasing energy.
• The changes that a star goes through during its ‘lifetime’ depend on the mass of the star.

Looking at the night sky, it is possible to see many thousands of stars with the naked eye alone. You can see many more using a pair of binoculars or a small telescope.

The actual number of stars in the night sky is more than you could ever see. The European Space Agency (ESA) estimate that there are about 100 thousand million stars in the Milky Way alone.According to Brian Greene (theoretical physicist), if you were to hold your thumb up to the night sky at arms length, you are estimated to be covering around 10 million galaxies.

Different types of star vary enormously in brightness, size, mass and colour. Each star goes through numerous stages that change their properties, as they move through their lifecycle.

Our nearest star is the Sun, a yellow dwarf star which is found at the centre of our solar system. The Sun appears larger and brighter than other stars because it is so much closer to us, even though it is only an average sized star. The biggest stars are more than 100 times as massive as the Sun, and the smallest stars are less than one tenth as massive as the Sun.

Our star is just one of around 100 billion stars in our galaxy, the Milky Way. The Milky Way is a spiral galaxy, like the one shown in the image below.

Looking at the night sky from a location with little light pollution, you can see a light-coloured strip across the night sky. When you are looking at this, you are looking through the disc of the Milky Way, and at thousands of millions of stars.

Stars form from massive clouds of dust and gas, known as nebulae. Gravity pulls the dust and gas together and this compresses the gas, causing the temperature to rise.

When the gas gets hot enough, nuclear fusion reactions start, which produce light and heat.

The star will continue to release energy for many millions of years. This stage of the lifecycle of the star is called the main sequence stage.

Our Sun is currently in the main sequence stage.

When a star has used up most of the hydrogen in its core, it cannot carry on releasing energy by nuclear fusion. Small stars like the Sun will change in size and reduce in temperature until they eventually stop releasing energy and become a black dwarf.

Stars which have a much bigger mass than the Sun go through a different sequence of changes at the end of their lives, and this can result in a huge explosion called a supernova. Afterwards, they can become a neutron star or a black hole.

High mass stars go through a different series of changes to low mass stars, as they progress through their lifecycle, eventually forming a black hole or a neutron star.

Just like a low-mass star, a high-mass star expands and cools at the end of its main sequence (stable) phase, becoming a red supergiant star. After this, a powerful and highly luminous explosion occurs, known as a supernova.