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Stellar Evolution
It all depends
On Mass
The Role of Mass and Gravity
The Mass of a Star determines how long it ‘lives” and
its fate, because the mass determines its Gravity.
Gravity determines the temperature and pressure in the
core
which in turn determines what elements can fuse in the
core and the rate at which fusion occurs.
The amount of gravity also determines the force that
balances the nuclear forces making Hydrostatic
Equilibrium
Main Sequence Stars
Main Sequence stars fuse Hydrogen into
helium to make energy
The “main-sequence life-span” of a star is
how long a star is a main-sequence star
Higher mass stars use hydrogen faster and
thus have shorter main-sequence life-spans
Brown Dwarfs
A Brown Dwarf is a ‘star’ with a mass of less
than .1 (1/10) of the Sun.
so little mass that no fusion can occur.
Therefore they are never main-sequence stars.
It glows with infrared light generated from its
gravitational contraction like Jupiter does.
They don’t “evolve” but stay brown dwarfs and
slowly fade over 100s of billions of years
Red Dwarfs
Stars with less than half the mass of the Sun have enough
mass to fuse Hydrogen into Helium at a slow rate,
generating enough energy to glow red.
They are the coolest main-sequence stars
… Red Dwarfs
They take about 100 billion years or more to use up their
hydrogen.
As their gravity is too weak to create the heat and pressure
needed to fuse helium into carbon
When they run out of hydrogen in their core their nuclear
fusion stops.
Lacking nuclear forces, gravity makes
them shrink
and heat up becoming “white dwarfs”
Slowly their energy is lost as they cool
going yellow,
orange,
red
and finally stop shining…a black dwarf.