Download Star Cycle Notes

Star Cycle Notes
Stars begin their life as collections of gas and dust called stellar nebulas. These
nebulas condense and become more massive. Once gravity exerts enough pressure on
the core, nuclear fusion begins fusing hydrogen atoms together to form helium, and
releases a tremendous amount of energy in the process. All stars are powered by
nuclear fusion.
The force of nuclear fusion pushes the star outward. The force of gravity holds the
star together. These forces are in balance (equilibrium) in an adult, normal star – like
our sun.
When a star runs out of fuel, nuclear fusion stops, and the outward force on the star is
reduced. The force of gravity now pulls the star inward and the star becomes smaller
(collapses).
As the star collapses, the particles within the star (now mostly helium) heat up and get
squeezed so tightly they begin to fuse and form larger atoms like carbon and oxygen.
Nuclear fusion restarts, but much more powerfully this time.
The restart of nuclear fusion in the star’s core expands the star much larger than it’s
original size. The star becomes a red giant (or supergiant). Fusion continues to form
larger atoms and eventually the star explodes, shedding most of its mass.
Red giants expel their mass into space, which eventually condenses and form
planetary nebulas. Planetary nebulas, as their name suggests, further condense and
form planets.
Supergiants explode so violently, they form supernovas. Larger, complex atoms like
iron and silicon are formed in the tremendous pressure and heat of supernovas. The
last supernova to be seen from earth happened in 1024, and was so bright the
supernova could be seen during the day.
After the star has expelled most of its mass into space, the core once again collapses.
Normal sized stars collapse from red giants into white dwarf stars – much smaller
versions of their previous self. Larger stars collapse from supergiants into neutron
stars or black holes.
Expelled particles from the stars eventually condense into stellar and planetary
nebulas and the cycle begins again. New stars and new planets are formed from these
nebulas.