Exploding stars and why everyone has a little star quality

“Albert, quick look at that amazing star. I can't believe it's shining so brightly.”

That star isn’t shining, it’s exploding with style. That’s how some stars die, in a blaze of glory where they can outshine a whole galaxy for a few days or weeks That’s called a Supernova. You are very lucky to see one as they don’t happen everyday, you are also lucky it was a long way away.

"How often do these supernovas happen?”

In our galaxy, the milky way, only a few stars have exploded that violently while we’ve been travelling for the last couple of thousand years.

“Couple of thousand years? Rubbish we’ve been gone hardly anytime.”

Strange isn’t it, that time doesn’t do what we expect. That’s where I disagree with Newton, I don’t think the universe has a ticking clock that tells the actual time. Time only exists to stop everything happening at once.

“What a strange way of looking at things.”

One day I’ll explain my theory of relativity to you. Then you’ll really know how strange a place the universe is. Anyway do you want to know about exploding stars or not?

“Oh, yeah, sorry.”

Well the earliest one the humans saw and wrote about was in the year 185 AD when Chinese astronomers discovered a new star that slowly faded. The brightest one ever seen was seen on Earth in May 1006. After that there others in 1054, 1181, 1572 and the last one seen from earth from this galaxy was 1604 which is probably the one you are looking at now.

“There hasn’t been one since?”

Oh there probably has been quite a few, but the light just hasn’t reached us. Don’t forget the Milky Way is almost 100,000 light years across. The light from a supernova happening right now half way across the galaxy won’t reach here for 50,000 years.

“Will the Sun blow up like that?”

The Sun is too small to explode. Only very big stars explode like that.

“So what do stars like the Sun do?”

Smaller stars first swell up as they start to run out of hydrogen and then start cooling and shrinking. To know why that happens you need to understand how a star normally stays the same size. When a star is shining normally, the nuclear fusion reactions in the centre make a huge amount of energy and particles that are continually rushing out of the centre of the star. This balances the force of gravity so the star stops shrinking until it finds a balance and just shines away and stays the same size for most of its life. Think of a balloon, the stretchy rubber of the balloon is trying to shrink the star like gravity and the pressure of the air in side is pushing against the balloon.

“Until the air in the balloon escapes.”

That’s right the air escaping would be like nuclear fusion stopping so the star would star to contract. In a star the nuclear fusion reactions happen not just in the centre but also in layers further out. So it’s like having lots of balloons inside each other.

“Or like an onion?”

Yes you could think of it like the layers of an onion, either way when the centre runs out of hydrogen and the nuclear reactions start to slow down, this upsets the balance with gravity and the star starts to contract again. This heats up the next layer up which is still full of hydrogen. The outer parts get hotter as the fusion reactions move further from the centre and so they expand. The bits in the centre start combining the helium made by nuclear fusion into bigger and bigger atoms. This keeps going until the star gets bigger and bigger and ends up as a something called a red giant. By the time this happens to the Sun in about 5 billion years humans, or whatever the dominant species on the planet is by then, will have to find somewhere new to live.

"So what happens to stars after they swell up and become red giants?"

That’s where the size of the star comes in. Once all the atoms that can be used in nuclear reactions are used up, a normal sort of star starts to contract. There is no more nuclear fusion energy from the centre counteracting the pull of gravity. So they get smaller and smaller because of gravity. These star remnants are called white dwarfs, because even though they are still shining they may be no larger than the Earth which is tiny compared to your Sun. A piece of a white dwarf star the size of a sugar lump could weigh more than a ton.

“So what stops them shrinking to nothing?”

Despite all their gravity they can't keep on shrinking forever because all the charged particles in the atoms like electrons and protons start pushing away from each other if they get too close, a bit like two north poles of a magnet. This helps to balance the force of gravity but doesn't help the star to shine so it slowly cools down. White dwarfs cool down, shine less brightly and end up first as red dwarfs and then finally even colder brown dwarfs and finally cold lumps of dead stars.

“You said that large stars explode, what makes them so different?”

The biggest stars have a very different way of living and dying. A star ten times the size of the sun can be born from a cloud of gas and dust in only a few hundred thousand years because the greater the mass the greater the gravity pulling everything together. So it all happens faster. At ten times the size it will shine 10,000 times brighter than the Sun but last only 2 million years until they do what that Supernova just did. They live fast and die young.

“But why do they explode when smaller stars don’t?”

The short answer is that big stars have so much gravity that when the balance between gravity and nuclear fusion goes wrong the star can suddenly collapse crushing the centre which gets massively hot and explodes. When really large stars run out of hydrogen fuel for fusion reactions they can move on to fusion reactions with bigger and bigger atoms because they have much more gravity which makes the centre much hotter than a normal star. All these fusion reactions build up bigger and bigger atoms ending up with the formation of iron.

“Iron doesn’t explode does it?”

Not by itself. It’s when these big stars try to use iron atoms in fusion reactions that everything starts going very wrong. Rather than release energy, nuclear fusion reactions with iron, absorb energy. So rather than making the centre of the star expands to counterbalance the effects of gravity the centre of the star suddenly starts to collapse. The matter in the centre then gets squashed into a super-concentrated form and the gas around the centre gets blasted out into space. This cosmic explosion is a supernova, and for a brief time a single supernova can outshine an entire galaxy. In the process of exploding a huge variety of nuclear reactions happen which result in the formation of all the complicated heavy atoms that are even bigger than Iron.

Remember I told you that humans are mostly made up of just six types of atoms; carbon, nitrogen, oxygen, hydrogen, calcium and phosphorus. Well all these atoms, except the hydrogen, were made inside a star before it died. There are some even bigger atoms in humans and these would have made in a supernova explosion millions or billions of years before the sun formed

"So this huge explosion blasts into space all the new types of atoms the star's been making from nuclear reactions, but how does it get into humans?"

Well the remnants of these explosions become the gas and dust that makes up the next generation of stars and planets.

"So the planet Earth and all the people on it are recycled space junk?"

Every one of them is made up of little bits of recycled stars. It may be well hidden but there’s star quality in every one of them.

"Do they know that?"

Well the smart ones, like you, do. But remember exactly the same star quality is found in mosquitoes, earthworms and the ink in a Bic biro. So don’t be getting any grand notions about yourself.