03 Where does fusion occur in nature?
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This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
03 Where does fusion occur in nature?
The conditions needed to induce fusion reactions are extreme;
so extreme that virtually all natural fusion occurs in stars,
where gravity compresses the gas, until temperature and pressure
forces balance the gravitational compression. If there is enough
material in the star, pressures and temperatures will grow
large enough as the star contracts that fusion will begin to occur
(see below for the explanation why); the energy released will then
sustain the star's temperature against losses from sunlight being
radiated away. The minimum mass needed to induce fusion is roughly
one-tenth the sun's mass; this is why the sun is a star, but
Jupiter is merely a (large) planet. (Jupiter is about 1/1000th
the sun's mass, so if it were roughly 100 times bigger, it
too would generate fusion and be a small, dim star.)
Stellar fusion reactions gradually convert hydrogen into helium.
When a star runs out of hydrogen fuel, it either stops burning
(becoming a dwarf star) or, if it is large enough (so that gravity
compresses the helium strongly) it begins burning the helium into
heavier elements. Because fusion reactions cease to release
energy once elements heavier than iron are involved, the larger
stars also eventually run out of fuel, but this time they
collapse in a supernova. Gravity, no longer opposed by the internal
pressure of fusion-heated gases, crushes the core of the star,
forming things like white dwarfs, neutron stars, and black holes
(the bigger the star, the more extreme the result). (For more
details, try the sci.astro or sci.space.science newsgroups.)
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