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No
one
has
ever
witnessed
the
complete
evolution
of
any
star
from
birth
to
death.
Stars
take
a
very
long
time
-
millions,
billions,
even
trillions
of
years
-
to
evolve.
Yet
in
less
than
a
century,
astronomers
have
developed
a
comprehensive
theory
of
stellar
evolution
that
is
one
of
the
best-tested
in
all
of
astronomy.
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