A Pint Of The Sun


Descriptions of nuclear `burning' in stars sometimes give the impression that the central furnace of a star is a place of violent activity. In fact, the inside of a star is rather peaceful, and hydrogen burning goes on very slowly.

To appreciate this, imagine we had a magic transporter which could beam one pint of gas from the center of the Sun right into this room. One pint of water weighs one pound, but the center of the Sun has a density about 150 times the density of water, so a pint of sun-stuff weighs almost as much as I do.

Now the first thing that would happen is that this building would vanish in a huge explosion. When it was down these in the center of the Sun, the gas was compressed by the vast weight of all the thick layers of dense material above it, so it was under enormous pressure. When it's suddenly transported to Earth, the confining pressure is removed, and the gas expands -- very rapidly. The explosion would have the force of a small nuclear bomb.

So if we want to get this experiment approved by the University administration, we need to make a container which can hold our pint of sun-stuff under pressure without bursting apart. That's not easy to do, but we've already assumed we have a transporter right out of Star Trek, so a little more magic won't be noticed. But our troubles are not over, because this gas from the center of the sun is incredibly hot, and the heat would escape in the form of X-rays, cooking everyone in the vicinity.

OK, let's assume we can make the walls of our container perfectly reflective, so that all the escaping heat energy is reflected right back in. I said we were using magic, didn't I? So we have a pint of sun-stuff sitting right there in front of us, safe as can be. Now let's allow a little energy to escape -- just exactly the amount of energy being generated by nuclear reactions, so the gas stays at a constant temperature. We can use the escaping energy to run a generator and produce electricity. Thermonuclear power!

But before we call a press conference or make any big deals with HECO, we better figure out how much energy those bottled nuclear reactions are generating. And the answer is...

About a thousand times less energy than I'm generating by breathing. That's all! Per unit mass, the Sun produces much less energy than a living human body. In total, the Sun generates a lot of energy, but that's only because it's so massive. If I was as massive as Jupiter -- perish the thought -- I'd produce more energy than the Sun, just from my body heat.

Of course, the Sun produces energy by nuclear reactions, while I produce energy by chemical reactions. That's how the Sun can go on shining for ten billion years, whereas I get hungry every few hours.

The enormous lifetime of the Sun gives us another perspective on the same basic point, which is that nuclear reactions in stars are, for the most part, very slow and gentle. It takes about ten billion years for all the hydrogen in the center of the Sun to be burned to helium. That means that per year, a hydrogen nucleus has about one chance in ten billion of being involved in a nuclear reaction. The center of the Sun is an incredibly safe place for hydrogen nuclei! A hydrogen nucleus in the Sun runs much less risk of undergoing a nuclear reaction than I do of being hit by lightning.


Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: August 15, 2001
http://www.ifa.hawaii.edu/~barnes/ast110/sunfire.html