When we look up on a clear night we see a host of stars above us. They look like tiny points of light but stars are actually giant balls of glowing gas.
The Sagittarius Star Cloud: A Sky Full of Glittering Jewels
The Hubble Heritage Team (AURA/STScI/NASA)
The star that we are most familiar with is the Sun. In all aspects the Sun is a fairly average star, except that it is the one star we can see close-up.
The Sun is a mere 8 light minutes away, while most stars are hundreds and thousands of light years away, which is why they look so faint and small.
The closest star to our Sun is Alpha Centauri, which lies about 4.3 light years away. Alpha Centauri is actually a group of three stars, including the faint star Proxima Centauri, that are linked together by gravity. Over half the stars in the sky are double or triple star systems.
Stars vary in size, mass, brightness, age and colour. The largest stars are hundreds of times bigger than the Sun. The smallest stars (neutron stars) are some 10km across. Massive stars, more than 50 times the mass of the Sun, shine more brightly and have shorter lives than less massive stars. The temperature of a star is revealed by its colour. Hot stars, which are generally young, are blue while cool stars, which are generally old, are red.
Stars shine because of nuclear fusion reactions. The centre of a star is so hot that elements can fuse together to form new elements. The majority of a star's life is spent converting hydrogen into helium. This process produces lots of energy which is released as light to make the star shine and heat to stop the star from collapsing under gravity. In the core of our Sun millions of tonnes of hydrogen are fused together every second to form helium. All the elements around us, including those that are part of us, were originally formed in stars.