"Weren't you trying to explain how stars shine?"
Oh yes, back to nuclear energy. The nuclear energy industry, nuclear weapons and stars have one thing in common: mess around with atomic nuclei and big things happen. There are two types of nuclear reactions; nuclear fusion and nuclear fission. Nuclear fission is the splitting of the nucleus of big atoms like uranium or plutonium into smaller atoms. This is what happens in a typical nuclear reactor and a typical nuclear bomb. Nuclear fusion is knocking two small atoms together to make a bigger atom and happens in stars and hydrogen bombs. This also produces a totally new element.
"So big atoms split and little ones stick."
Precisely. In stars four hydrogen atoms start sticking together to make one atom of helium. When four hydrogen atoms fuse to make one atom of helium, something is lost in the process. Helium is a little less heavy (or has less mass as a scientist would say) than four hydrogen atoms. That lost bit of matter is converted in the collision into energy like us.
"So why are nuclear explosions different from normal explosions?"
A normal bomb or explosion is just a chemical reaction. At the end there are the same number and same type of atoms as there were at the start, they're just rearranged. In the process lots of heat and gas are released that makes things blow apart. All the atoms stay the same but just end up joined up in different ways to make a new set of molecules. In an atomic bomb or the Sun it's the atoms themselves that are changed or destroyed by nuclear reactions. So you end up with different types of atoms.
"So where does the energy come from in nuclear bombs?"
"That was Einstein. Wait a minute Albert that’s you!”
Everyone seems to remember that equation. Anyway this is the key to understanding what happens when matter is converted into energy in a star.
"I'm not that good with equations I’m afraid."
Don't worry, this is the only one I'll throw at you during our journey and it's easy enough to explain how it works as an equation. Remember, equations are only a type of short code that captures an idea, so every equation has an English description. In 'E = Mc2' M is mass (the amount of material or matter), c is the speed of light (300,000,000 metres per second) and c2 is a sort of short hand to mean the speed of light multiplied by itself (a really big number).
"So that's the same as 'E = M multiplied by c multiplied by c' "
Exactly, so you see, it's not as complicated as it looks.
"Hold on, what about 'E', what's that?"
Sorry, 'E' stands for energy. If you want to know how much energy, 'E', is made when you loose that little bit of mass in a fusion reaction to make helium you multiply that amount of mass that is lost, 'M', by the speed of light, 'c', and multiply the answer by the speed of light again. The final answer is the amount of energy. Because 'c' or the speed of light is such a large number, even a tiny amount of mass lost creates a huge amount of energy. This wonderfully simple equation says that a little matter turned into pure energy releases a lot of energy. If all the mass in a normal 60 Watt light bulb could be converted into electrical energy, there'd be enough energy to power an identical light bulb for 80 million years. The reason you still have to pay for electricity to keep your electric lights working is that all this energy is trapped. Even in fusion reactions only a tiny percentage of the mass of the starting materials ends up as energy. Luckily stars are so huge that they can get energy from nuclear fusion for billions of years. Sadly that little equation can also measure the misery humans can inflict on each other.
“I don’t understand?”
Wars and bombs. Remember I told you that stars are like continuous nuclear explosions? Well that same equation explains why atomic bombs are so destructive. In sensible hands atomic energy will be a great benefit for humanity, but powerful hands are rarely sensible in my experience. Remember my friend Leo Szilard that I invented those new types of refrigerator with? Well he turned into a brilliant physicist who worked out how an atom bomb might work. Leo helped me write a letter I sent to the
“Do you wish you’d never discovered that equation then?”
The equation? No. I regretted writing the letter to Roosevelt but who knows what would have happened if I hadn’t. I think
"So are all those scientists that worried about the age of the Earth happy now?"
Well they have enough time for all the theories of geology and evolution to make sense, but they're just worrying about different things now. Global warming, something no-one worried about in my first lifetime, seems to be worrying a lot of people now. As soon as scientists solve one problem there is always another one to solve and we’ve barely scratched the surface of science yet. We'll get to global warming much later in our trip but there's a lot of science to talk about before we get there.
"So we're only starting the journey now?"
This is still just the beginning. It will take us three thousand years to reach Earth but it would have taken us just as long to get from the centre of Deneb to here. There is so much going on in the centre of a star that light particles can't travel more than about a centimetre without colliding with some atom or other. A tiny fraction of a second later the collision produces another light particle or photon which makes a dash for it until colliding with something else. Finally photons get released from the surface and can make it into open space. But with all the random bouncing backwards and forwards it takes thousands of years for the energy of a single photon to reach the surface and escape. Even in our super giant star Deneb, light could travel from the centre to the surface in only a few minutes if there was nothing in the way. Like in a long car journey, getting out of the city is often the slowest part.
"So there’s no point in asking are we there yet?"
Not for a few quadrillion miles.
"Where are we actually going?"
Our destination? A rather pretty, bluey-green circle surrounding a pool of blackness that is situated in the back garden of a house on a planet called Earth. But there is a lot of space and time between here and there.