The Evolution of Low-mass Stars
Stellar Birth and Reaching the Main Sequence
After birth, newborn stars
are very large, so they are
very bright. Gravity causes
them to contract, and they
become fainter because of
their smaller sizes.
After contracting for millions
of years, stars eventually
become hot enough to fuse
hydrogen, which halts their
contraction. They now
maintain a stable size and
luminosity until they run out
of hydrogen fuel. This is the
main sequence.
contracting
The Main Sequence Lifetimes of Stars
All stars spend most of their lives on the main sequence, but the
length of time on the main sequence (i.e., the time spent fusing
hydrogen) depends a lot on a star’s mass.
A star with a higher mass has a core with:
 higher gravity
 higher pressure
 higher temperature
 fuses hydrogen faster
Although more massive stars have more hydrogen fuel, they
consume that fuel at a much higher rate than less massive stars.
As a result, more massive stars exhaust their fuel much faster
and have shorter main sequence lifetimes. The lifetimes for the
most massive stars are only a few million years while the least
massive stars fuse hydrogen for trillions of years. A star with the
mass of the Sun fuses hydrogen for 10 billion years.
The Main Sequence Lifetimes of Stars
few million years
10 billion years
>1 trillion years
Sun
The Main Sequence Lifetimes of Stars
Evolution Beyond the Main Sequence
After a star exhausts its
hydrogen fuel, its fate is
determined by its mass at
birth.
Low-mass stars (<8 M) end
their lives as white dwarfs
High-mass stars (>8 M )
undergo supernova
explosions, leaving behind
neutron stars or black holes
Central
Temperature
100,000 K
Gravity pulls the
star inward
The protostar collapses and
gets smaller, causing the
pressure and temperature to
increase in the center.
H
Gas pressure
resists gravity
Central
Temperature
1,000,000 K
Gravity pulls the
star inward
The protostar collapses and
gets smaller, causing the
pressure and temperature to
increase in the center.
H
Gas pressure
resists gravity
Central
Temperature
10,000,000 K
The star’s center eventually
becomes hot enough to ignite
hydrogen fusion, which stops its
collapse. The star is now on the
Main Sequence.
H
Central
Temperature
10,000,000 K
H
Lots of hydrogen out
here, but it’s not hot
enough to fuse
He
Eventually, fusion converts all of
the hydrogen in the core to
helium.
Central
Temperature
30,000,000 K
H
He
Without fusion to hold it up,
the core of the star contracts
because of gravity.
Central
Temperature
50,000,000 K
H
He
As it contracts, the core grows
hotter, and a shell of hydrogen
surrounding the core becomes
hot enough to fuse.
Central
Temperature
50,000,000 K
H
He
The radiation from the shell of fusing hydrogen
is so intense that it pushes on the outer layers
of the star, causing the star to expand to a huge
diameter. The star is now a red giant.
Red Giants
Because the surface of the star
has expanded so far from the
core, it becomes cooler, and
hence redder. This is why red
giants are red.
Although cooler objects produce
less light (for a given size), the
larger diameter and surface area
more than make up for this, and
the star’s luminosity increases a
great deal.
expanding
Central
Temperature
100,000,000 K
Expanded view of core; outer layers of star not shown
H fusion
He fusion
The contracting core eventually
becomes hot enough to begin
fusing helium. This new energy
source halts the core’s collapse.
Central
Temperature
100,000,000 K
Eventually, the He in the core is
converted to C and O. The core
resumes its contraction once again.
H fusion
He fusion
C and O
Central
Temperature
200,000,000 K
H fusion
He fusion
C and O
Central
Temperature
300,000,000 K
H fusion
He fusion
C and O
If the core’s mass is <1.4 M, the contraction is halted when the core
becomes so dense that the atoms can’t be packed more tightly. This
resistance to further compression is called electron degeneracy.
white dwarf
C and O
planetary nebula
The radiation from the core pushes the outer layers of the red giant into
space, forming a planetary nebula. After this nebula dissipates, only the
core of the star remains. This is called a white dwarf. It is not hot
enough to fuse C and O, so it will cool and fade very slowly forever.
A star with an initial mass of
<8 M will produce a core
that has a mass of <1.4 M.
In other words, stars with
masses <8 M end their lives
as white dwarfs. So the Sun
will one day become a white
dwarf.
eventual C/O core = 1.4 M
initial mass of entire star = 8 M
Evolution of Low-mass Stars after the Main Sequence
planetary
nebulae
white
dwarfs
red giants
Planetary Nebulae
Helix Nebula
Ring Nebula
When astronomers first looked at planetary nebulae through
telescopes, the colors reminded them of planets like Mars, which is
how they were given their name. We now know they they are
unrelated to planets, but the term is still used.
Planetary Nebulae
Eskimo Nebula
NGC 6751
Planetary Nebulae
Hourglass Nebula
M2-9
White Dwarfs
White dwarfs have diameters that
are similar to that of the Earth, but
they can have as much mass as the
Sun, so they are very dense.
White dwarfs have masses <1.4 M
because if the mass was higher,
gravity would be strong enough to
overcome electron degeneracy, and
it would collapse and become an
even denser and more compact
object, either called a neutron star
or a black hole.
White Dwarfs
White dwarfs are not hot enough to fuse the carbon and oxygen
within them, so they become steadily cooler and dimmer for the
rest of eternity. Eventually, they will cool enough to crystallize,
and will resemble a diamond the size of Earth!