What are stars?

The Sun
Credit: NASA

Our Sun is a star and it is the only star in our Solar System – but it lives in a galaxy called the Milky Way, which is full of stars. The Sun is so big that 64 million Earth Moon’s would fit inside it! It orbit around the middle of our galaxy, just like the planets orbit around the Sun.  

A star is born when a cloud of gas and dust collapses into a big ball. This ball must be massive – big enough that the temperatures inside are hot enough to start nuclear fusion. The temperatures need to be at least 100 million degrees Celsius. Smaller clouds don’t get this hot when they collapse. This means that starts can never turn into planets or the other way around. Planets will never be hot enough to burn atoms because they are too small. When a star is born there is still some dust around it which eventually collapse down to become planets. 

These stars are all made of mainly hydrogen and helium gas (or plasma) which is being fused, or glued together, into bigger atoms like carbon and oxygen. This process is called nuclear fusion and it produces energy to keep the star burning – and all of the light we see. The stars are also different colours and this depends on how hot they are. The ‘cooler’ stars are red and the hottest stars are blue. We measure the temperature of a star by looking at its colour – and we get its colour by seeing how bright the star is in 2 different colours or wavelengths of light – like yellow and blue.  

When stars are burning fuel they produce a lot of light and heat. People could only get to within 2.1 million km of the Sun before they would burn up form the heat. The heat from stars reduces quickly as we move away from a star though, and even though there are so many stars in the Universe there is so much space between them that the average temperature of space is -270 degrees Celsius! Space is mainly empty and cold! 

When stars start to run out of fuel to burn (or fuse) they start to cool down. Our Sun is an average sized star – you can get some which are much bigger and some are smaller – but these stars are much more rare. An average star, like the Sun, will run out of fuel to burn in about 9 billion years (the Sun is half way through this lifetime). When it runs out the stars slowly collapse until they are very small and dense. They slowly fade and cool over time. 

The next closest star to Earth (other than the Sun) is called Proxima Centuri. It takes the light from that star over 4 years to reach us here on Earth – the distance between us is over 40 million million km! 

When we look at the night sky we make patterns in the sky with the stars called constellations. There are 88 constellations which cover the whole night sky around the world. These constellations are not the only stars though – they are just the brightest stars we can see from Earth. There are actually around 100 billion stars just in our galaxy, the Milky Way. There are just as many galaxies in the Universe – meaning that in the whole Universe there is an amazing 1000 billion billion stars! More than grains of sand on the whole of planet Earth! Every single one of these stars could have it’s own Solar System of planets. We have currently discovered 2,500 other ‘Solar Systems’ in our galaxy alone – and we find new ones all the time!