MadSci Network: Astronomy

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

Date: Thu Mar 12 08:38:01 1998
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Astronomy
ID: 889219806.As

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

Why doesn't the sun explode all at once?

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.

First, I think you have fission and fusion confused. Neutron flux doesn't do much for your standard fusion reaction (though it does do something for thermonuclear bombs: neutron flux from a fission explosion is used to change 6Li into 3H + 4He; the 3H then fuses with 2H already present in the device).

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:

  Dan Berger
  Bluffton College

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