- [Narrator] Like fireflies
on a still summer night,
they gently dot and illuminate
the infinite velveteen sky.
Stars.
Be they millions or billions of years old,
are all born in nebuli, clouds of dust
and mostly hydrogen gas.
Within these stellar nurseries,
stars begin life as protostars
or hot cores formed by the collection
and collapse of dust and gas.
As the protostars become hotter,
hydrogen nuclei inside of
the cores begin to fuse
and create helium.
It is this chemical reaction,
thermonuclear fusion
that generates a star's heat and energy
and causes it to shine.
Stars are categorized
by a number of characteristics.
One of these classifications
is by surface temperature
called spectral classes.
These seven major groups range
from the coolest stars
which are designated
as M and up to the hottest stars
which are designated as O.
Stars are also classified by the amount
of light they emit or luminosity.
Called luminosity classes,
these nine major groups range
from the small, less bright white dwarfs
to the large and extremely
bright hypergiants.
But no matter their luminosity
or surface temperature,
all stars eventually burn
through their hydrogen fuel
and die out.
Less massive stars
such as our sun release their
stellar material into space
leaving behind a white dwarf surrounded
by a planetary nebula.
More massive stars instead blast matter
into space in a bright supernova
leaving behind an extremely dense body
called a neutron star.
But the most massive stars,
stars that are at least
three times our sun's mass
collapse into themselves
and create a bottomless well
of gravity, a black hole.
But from the remnants of stars,
heavier elements are
cast into the universe
and it is this star dust
that form the seedlings of life itself.
(dramatic music)
