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      The cloud that is to become a star consists
mostly of hydrogen mixed with dust. Astronomers 
believe some of the hydrogen and dust comes from 
old stars. The cloud may include the remains of a
star that exploded, or it may be a collection of 
gases thrown from the surface of rotating stars. 
    The first step in the formation of a new star
is the contraction of part of an interstellar 
cloud into a ball. Astronomers have never watched
a new star flash into life. But they have 
discovered several dark, ball-like interstellar 
clouds that may be new stars beginning to take 
shape.
      Through millions of years, the cloud 
of gas and dust contracts as gravity pulls it 
together. As the material pulls together into a 
ball, the pressure of the gas increases.  The gas 
at the center of the ball becomes extremely hot. 
When the temperature at the center reaches about 
2,000,000ø F. (1,100,000ø C), the nuclear fusion 
reaction begins.  The hydrogen in the center 
begins to change into helium and to produce great
amounts of nuclear energy.  This energy heats the 
gas that surrounds the center. The gas begins to 
shine--and a star has come to life.
      The kind of star that takes  shape depends
on the mass of the contracting cloud.  A cloud 
with a mass about 1/20 that of the sun becomes a
red, low-luminosity main-sequence star.  A cloud 
with a mass about 50 times that of the sun 
becomes a blue, high luminosity, main-sequence 
star.

   After a star begins to shine, it starts to 
change slowly. The speed of its change depends 
on how rapidly the nuclear energy-producing 
process takes place inside it.  The speed of 
this process, in turn, depends on the mass of 
the star. The greater a star's mass, the higher
its luminosity and temperature--and the faster 
it changes.  Stars with a mass about 10 times 
that of the sun take a few million years to 
change.  Smaller stars with a mass about 1/10 
that of the sun take hundreds of billions of 
years to change.
      A star changes because  its supply of 
hydrogen decreases.  The star's center 
contracts, and the temperature and pressure 
at the center rise.  At the same time, the 
temperature of the outer part gradually 
drops.  The star expands greatly and becomes 
a red giant.
      What happens after a star's red giant
phase depends on how much mass the star 
contains.  A star with about the same mass 
as the sun throws off its outer layers, 
which can be seen as a glowing gas shell 
called a planetary nebula.  The core that is
left behind cools and becomes a white 
dwarf.  A star with more than about three 
times the mass of the sun becomes a super 
giant.  Elements as heavy as iron are formed
inside the star, which then explodes into a
supernova.
    If less than three times the mass of 
the sun remains after the supernova 
explosion, it becomes a neutron star. If 
more than three times the mass of the sun 
remains, the star collapses and forms an 
invisible object called a black hole.  A 
black hole has so much gravitational force 
that not even light can escape from it.