The place where a star is born is known as ‘nebula’. We can see a nebula that is ‘nearby’, called the Orion nebula. Millions of years after the big bang, the clouds of hydrogen which had formed started to collapse under their own gravity. It’s a bit like us after a really long and tiring day, we just can’t hold ourselves up and flop on the sofa or in bed. Well, when a cloud collapses, it starts to get super hot as all of the different molecules start to move faster and faster. It starts to break into lots of little clumps of molecules, which get so hot and so dense that molecules can start sticking together. This is called a ‘nuclear reaction’. When the temperature reachers 10,000,0000 C (yikes!) it is hot enough to become a star! There’s a disc of hydrogen and dust surrounding a burning star. When the different molecules start hitting together, they can stick together and start forming heavier and heavier elements. The nuclear reactions release their
own energies which keeps to stars hot, and push outwards to stop the star contracting due its own gravity.
The stars go through a crazy life cycle of nuclear explosions and what happens to them depends on how much they weigh! Red stars are the coolest, and they are only around 2500 C. Blue stars are the hottest, and they are a scorching 40,000 C!
Yellow stars, like our sun, burn at around 5,500 C. The sun is halfway through his 10 billion year life! Our sun is a medium sized star, so he’ll become something called a red giant, then a planetary nebula, then a white dwarf. A red giant is huge- around 1,000 times the size of the sun. Because it is so big, it isn’t very hot (the heat is spread over a big volume), which is why they glow
red. If there was a red giant at the centre of the solar system, it would swallow up all of the inter planets. They are like big American cars, and use up their fuel of hydrogen really quickly. A white dwarf is a small dense star, about the size of a planet. All of the outer layers of the stars fly away, leaving behind a very hot, small, dense dwarf.
If or sun was a bit heavier his life would be a bit different after the red-giant phase. A huge explosion would happen called a super nova, which is brighter than anything we know and outshines everything in the galaxy whilst it’s burning.
Supernovae happen when white dwarfs become so hot that they explode. The last supernova to take place in the milky way was 340 years ago and happened ten-thousand light years away from earth. That means it takes light 10,000 years to travel the distance! When a supernova happens, they spread their star dust all across the galaxy. Everything in Earth is made from the start dust!
The supernova would collapse and leave behind a super dense star called a ‘neutron star’ or a black hole, if it is heavy enough. A black hole is another story!