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Stellar Evolution
The Milky Way
Galaxy contains several hundred billion stars of all ages, sizes and masses.
A typical star, such as the Sun, radiates small amounts of X-rays continuously and larger bursts of X-rays during a
solar flare.
 The Sun and
other stars shine as a result of nuclear reactions deep in their interiors. These
reactions change light elements into heavier ones and release energy in the
process. The outflow of energy from the central regions of the star provides the
pressure necessary to keep the star from collapsing under its own weight.
A star collapses when the fuel is used up and the energy flow from the core of
the star stops. Nuclear reactions outside the core cause the dying star to expand
outward in the "red giant" phase
before it begins its inevitable collapse.
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