Re: Why doesn't the sun explode all at once?

NOTE TO THE READER: I use metric units below; the "tonne" is 1,000 kg, which is about 1.1 American "tons."

Stars are a huge fusion explosion. So why is the fuel not consumed all at once as in a fusion bomb? Surely the neutron flux is high enough for a chain reaction to occur.

Actually, the portion of the Sun actually undergoing fusion more-or-less does explode all at once. Every second some 7x10⁸ tonnes of hydrogen are converted into helium and energy by the Sun.

By comparison, a 1-megaton thermonuclear bomb might weigh one or two hundred kilos, of which about 15 kilos would be ⁶Li²H (lithium deuteride) fusion fuel. To burn at the same rate as the Sun the bomb would have to consume its fuel in about 20 picoseconds -- 20x10^-12 seconds! In fact, that number is about right, so fusion in a nuclear bomb and in the Sun do proceed at about the same pace.

So why doesn't the Sun explode all at once? Well, the mass of the Sun is about 2x10²⁷ tonnes! Of that, about 75% is hydrogen, or 1.5x10²⁷ tonnes. If all that could be burned, the Sun would continue its thermonuclear "explosion" for some 68 thousand million (that's "68 billion" to us Yanks) years. Of course, only a fraction of solar hydrogen is accessible for fusion, so the Sun can only live a paltry 10 thousand million or so years (of which 5 thousand million are past) before going into its death throes.

Useful references:

• The Sun page from The Nine Planets.
• J.V. Hofmann's Intr oduction to Fusion.
• The Fusion Energy Education Site, sponsored by the Princeton Plasma Physics Laboratory.
• MilNet's Nuclear Weapons Page.