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the following life cycle paths it will
take from there.
A low-mass star (like the Sun) does
not have the gravitational pressure to
fuse carbon, so once it runs out of its
helium core, the outer layers are
expelled. This gaseous shell is called a
Planetary Nebula and can serve as the
nidus for the birth of new stars. The
isolated core continues to contract
down, cooling and dimming. It is now
called a White Dwarf. When it finally
stops shining, it is called a Black Dwarf.
A large-mass star (10x the size of the
Sun) will evolve from a main sequence
star to a Red Super Giant (same as a red
giant with helium fusing to carbon at its core
- the “super” added to denote that it
originates from a large-mass star). The star
then undergoes a Supernova
explosion (a death explosion in which the
outer layers of the exploding star are blasted
out in a radioactive cloud).
If the remnant core of the
supernova is 1.4 to about 3x more
massive than our Sun, it will become a
Neutron Star (a celestial object of very
small radius and very high density - a
quadrillion times denser than a normal star -
composed predominantly of closely packed
neutrons). A rapidly spinning neutron
star is called a Pulsar.
If the remnant core is greater
than 3x the mass of our Sun, the core
is swallowed by its own gravity
becoming a Black Hole (a region of
spacetime exhibiting gravitational
acceleration so strong that nothing - no
particles or even light - can escape it).
A star at this stage of life is held in
balance as long as its supply of
hydrogen fuel lasts.
And how long does this stage last? It depends
on the mass of the star. Very large, massive
stars burn their fuel much faster than smaller
stars and may only last a few hundred
thousand years. Comparatively, smaller stars
can sip away at their fuel over billions to
trillions of years. Most of the stars in the
Milky Way, including the Sun, are considered
main sequence stars and are expected to sit
in the main sequence phase for 10-15 billion
years.
Protostars with masses less than
roughly 0.08 M☉ (1.6×10 kg) never
reach temperatures high enough for
nuclear fusion of hydrogen to begin.
These are known as Brown Dwarfs.
Brown dwarfs shine dimly and die away
slowly, cooling gradually over hundreds
of millions of years.
Eventually, the core exhausts its
supply of hydrogen and a main
sequence star can no longer generate
heat by nuclear fusion. Without the
outward pressure generated by the
fusion of hydrogen (to helium) to
counteract the force of gravity, the core
becomes unstable and contracts. The
outer shell of the star, which is still
mostly hydrogen, starts to expand. As
it expands, it cools and glows red. The
star has now reached the Red Giant
phase. It is red because it is cooler than
it was in the main sequence star stage
and it is a giant because the outer shell
has expanded outward. In the core of
the red giant, the remaining helium
fuses into carbon. All stars evolve the
same way up to the red giant phase.
The mass of a star determines which of