How Stars Shine

Let’s imagine what was going on back on Earth as we are leaving Deneb. For the first few thousand years of our journey the human race had little inkling that stars and the sun were even related. As to how the sun managed to shine so brightly they were equally misguided. As is often the case, this didn't seem to stop some people having strong views about what it wasn't. In 500 B.C. a Greek gentleman called Anaxagoras of Clazomenae almost got sentenced to death for the heresy of saying that he thought the sun was a very large, very hot lump of rock. He escaped punishment with the help of a powerful friend Pericles, but was forced to leave Athens in disgrace. Anaxagoras also calculated that the sun was 35 miles across and about the same distance from Greece as New York would be when someone finally got around to stealing it from the American Indians. His maths was fine but he thought the Earth was flat, which was why his calculations were a bit off.

"A bit tough almost being stoned to death just for getting the answer wrong, isn’t it?"

It wasn't being wrong that was the problem. It would have been just as bad for him if he'd got the right answer.

"How come?"

By suggesting his own ideas he was saying that everyone else was wrong. The real problem was that back in 400 BC the Sun was thought of as a God. So it's no wonder the rulers of ancient Athens didn't like the hot rock idea.

"So when did they work out what stars really were?"

Well they didn't really make any more progress in next two thousand years either. In the middle of 1800's a German gentleman called Julius Robert von Mayer tried to work out how long the Sun had been shining for. He worked out that without some source of energy it could only shine for around 5,000 years. This was based on getting a rock as hot as a rock can get and seeing how long it would take to cool to its temperature today. This time scale would at least have pleased an Irishman, a Mr James Ussher who ended up becoming Archbishop of Armagh, Primate of All Ireland, and Vice-Chancellor of Trinity College in Dublin. Despite all these jobs he spent a lot of time working out how long all the events and stories in the Bible would have taken and worked out the family tree of all the people in the Bible, back to Adam and Eve. He calculated from this that the world began on October 23rd 4004 B.C. He also worked out that Adam and Eve were driven from Paradise on Monday 10 November 4004 BC so it was a short two week holiday in the Garden of Eden.

For scientists back in the 1800's, a 5,000 year old sun was a rather alarming concept because it didn't give them enough time for their theories on geology and evolution to work. The new sciences of geology and evolution assumed that the shaping of the Earth and the creatures that walked on it was a very slow process. Charles Darwin, the man who invented the theory of evolution, was saying that life must have been for around for much longer for humans to have evolved from fish and tadpoles. In Darwin's first edition of On the Origin of the Species by Natural Selection, published in 1859, he made a calculation that the Earth and therefore the sun must be at least 300 million years old. It was either abandon the hot stone theory, or invent a whole new theory to explain where dinosaur fossils came from. In 1857, just before Darwin published his famous Origin of Species, Philip Gosse had another suggestion for dinosaurs. In his book Omphalos, Gosse claimed that God had planted fossils and other evidence of a non-existent past during the six days of Creation to test our faith. After all if the world was created yesterday complete with your memories, photo albums and tree rings how could you tell?

“You couldn’t I suppose.”

Exactly, but believing something that is unprovable isn’t science it’s faith. As scientists were becoming quite attached to Darwin's theory, they kept looking for ways to explain how the sun could have been burning for millions rather than thousands of years.

"So why aren't stars just like big burning fires?"

Well stars are hot and yellow like fire so thinking that the sun is a large burning lump would make sense. But...if the sun were burning like a coal fire it would last barely a thousand years before ending up as cooling embers. Never mind the fact that fires need air to burn and there is no air in space. A thousand years isn't long enough for history books let alone evolution. There was another theory at this time that was a bit more promising. This new idea was that the gravitational contraction of the sun would generate a lot of heat. This new idea came from Hermann von Helmholtz in 1854 and developed by Sir William Thomson. He became Lord Kelvin after this and I’ve a few entertaining stories about him I’ll tell you one day. Anyway Sir William Thomson as he then was calculated this was enough to give the Earth about 30 million years of free lighting.

"But how does gravity make things hot?"

By squeezing things closer together. When you put your finger over the end of a bicycle pump and pump away. The pump gets hot because when air or anything else for that matter is squeezed together or compressed it gets hotter. So the gravity of the sun squeezing together the inner layers would heat up the sun from the inside. This was a nice idea but it still wasn't a long enough time. Evolution needed hundreds of millions of years not just 30 million years, after all the dinosaurs died out 60 million years ago. But this didn't worry Sir William Thomson at all as he didn't believe in Darwin's theory of evolution anyway.