Neutron stars are the burned-out cores of high mass (more than 8 solar
masses) collapsed stars. All Main Sequence stars begin by burning through
their hydrogen and helium reserves, staving off the pressure of gravity
through the power of nuclear fusion. Once low mass stars have exhausted their
fuel this way, gravity forces them inwards until the pressure of the sea of
degenerate electrons is sufficient to counteract the gravitational force.
They then die a slow, lingering death as white dwarfs, slowly cooling off over
the course of millions of years. High mass stars behave differently.
Like their low mass cousins, high mass stars begin life as a diffuse
cloud of hydrogen gas. Over time, gravity collapses the gas cloud, forming a
solar nebula. (Perhaps planets form!) The star spends most of its lifetime
on the Main Sequence, existing as a star very much like the Sun. (It will be
hotter and shine more brightly because of its larger mass, but its processes
of thermonuclear fusion are the same as those within the Sun.) Once the star
has lost all of its hydrogen and helium it will briefly enter the red supergiant
phase, expanding outwards in size.
