"Stellar
evolution is the process by which a
star
undergoes a sequence of radical changes during its lifetime.
Depending on the mass of the star, this lifetime ranges from only a
few million years for the most massive to trillions of years for the
least massive, which is considerably longer than the
age of the universe.All stars are born from collapsing clouds of
gas and dust, often called
nebulae
or
molecular clouds.
Nuclear fusion powers a star for most of its life. Stars similar
to our Sun
gradually grow in size until they reach a
red giant phase, after which the core collapses into a dense
white dwarf and the outer layers are expelled as a
planetary nebula. Larger stars can explode in a
supernova as their cores collapse into an extremely dense
neutron star or
black hole. It is not clear how red dwarfs die because of their
extremely long life spans, but they probably experience a gradual
death in which their outer layers are expelled over time.Stellar
evolution is not studied by observing the life of a single star, as
most stellar changes occur too slowly to be detected, even over many
centuries. Instead,
astrophysicists come to understand how stars evolve by observing
numerous stars at various points in their lifetime, and by
simulating
stellar structure using
computer models.
Source:
Wikipedia
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"Stellar
evolution is the process by which a