The life-cycle of a star

Galaxies are littered with clouds of dust and gas.

A star is formed when a cloud made of hydrogen gas gathers a knot of dust inside it that is so large it forms its own stronger gravitational field. The dust around it is all pulled in towards the knot, making the gravity even stronger and squashing the dust and gas in the middle cloud closer and closer together.

The centre of the cloud becomes so crushed by the immense gravitational pull that the individual hydrogen atoms are actually mashed together.

When two hydrogen atoms are mashed together in this way they are changed into a single helium atom. But a single helium atom actually weighs a little bit less than the sum of two hydrogen atoms. The leftover weight is expelled by the new atom in the form of energy. This process is called "nuclear fusion" and is the same process that is used to make the explosion in a nuclear bomb.

So now, instead of a cloud of hydrogen, we have a core made of helium and a big explosion of extra energy swirling all around it in the form of heat. This is called a "protostar".

The rest of the dust in the cloud will either fall into the new protostar, form into planets, asteroids, or comets or stay as space dust. It forms a solar system. This process takes millions and millons of years.

Our Sun is a mature, grown up, star. It took our Sun about 50 million years to become what it is today - big and yellow and hot with a steady gravitational pull that can hold all the planets of our Solar System in a steady orbit.

The Sun is about 4.6 billion years old today.

A star like the Sun will have a lifespan of about 10 billion years and over that time it will squash about 1/10th of its hydrogen fuel into helium to keep itself burning. The helium core keeps squishing and squashing together and getting denser and denser until it stops behaving like a gas and starts to behave like a liquid. It can't get any closer together and so it just gets hotter and hotter but stays the same pressure. This super-heated centre triggers a chemical reaction where the helium is converted into carbon - kind of like how firewood is converted into charcoal when it burns.

At the same time, the swirling energy bit of the star starts to spread out, looking for more fuel to keep itself going. The problem is, as it spreads out, this part loses some of its heat. The outside part of the star starts to cool down.

Instead of burning white or yellow, now the star colour changes to red. It gets bigger and bigger and redder and redder as it spreads itself out, trying to find more hydrogen. We call this kind of star a "red giant".

After about 100 million years as a red giant, the star has burned most of its helium into carbon. The explosions caused by the hydrogen turning into helium and releasing energy aren't happening as much any more, so the force that was pushing out from the core starts to drop. This change allows gravity to crush the carbon core. Then the middle of the atoms don't want to be crushed so they push back out and these two forces - gravity and the atom nuclei - keep bouncing in and then out again like a rubber ball bouncing off two walls. They bounce out and then in and then out and then in, sending out purple, glowing waves of gas. These gas waves are called a "nebula".

The middle of the star is now called a "white dwarf". It becomes very, very hot as it burns up all the leftover bits of itself. It is believed that the white dwarf will do this until it is all gone and will then cool to the same temperature as the rest of space around it, becoming a "black dwarf". No black dwarfs are thought to currently exist because the universe is calculated to be 13.8 billion years old and it is believed that the process of a white dwarf cooling to a black dwarf takes longer than this to happen.

But there is another way for a star to end - one we have seen and is spectacular. When the star that is dying is 10 or more times bigger than our Sun it is called a "supergiant". When a supergiant has burned its helium into carbon it is big enough that the pressure inside its core can make that carbon into iron. The process of making carbon into iron takes more energy than it produces, dramatically reducing the cores pushing out pressure. This throws off the balance of pushing in and pushing out forces so quickly that it causes a massive explosion known as a "supanova".

This also causes a nebula to form.

If the core of the star that has collapsed is between 1 and 3 times the size of our Sun then the result is a "neutron star" - a small new star about the size of an Earth city. A teaspoon of this new star would weigh as much as a mountain.

If the core of the star was more than 3 times the size of our Sun it forms a point as small as the full-stop at the end of this sentence. It is a full-stop so heavy, it has gravity that is so strong, that nothing, not even light can escape its pull. It has become what we call a "black hole".

Whether the star has died by supanova or in expanding nebula shells, the atoms that formed that star to begin with are afterwards left all floating around in a big dust cloud again. They will one day again form new knots and those knots will eventually become new stars and planets and comets and asteroids. Every element, every part of the dead star is recycled into a brand new solar system.

So everything we have on our planet: every rock, every tree, every animal, every molecule of water, every breath of air, even you, are made from star dust. And every part of us and our planet will one day make new stars and new planets.

Sources: Cosmotography.com, NASA

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