Stages of Star's Life

At that point, the He core is supported by electron
degeneracy pressure
○ The plasma starts to behave more like a liquid or solid, it
resists being compressed any further
○ Essentially independent of temperature (but not density)
○ Becomes important at high densities in stars

Stars
★ Electron degeneracy pressure is due to a
quantum mechanical effect called the
Pauli-Exclusion Principle which forbids
certain particles from occupying the same
state
○ State meaning position, velocity, energy, etc.
★ We can only compress an electron-rich plasma
(like in the Sun) so far - as otherwise
multiple electrons would have to be in the
same state

Stars
★ At extremely high temperatures (>100 million K) and
densities, it is possible to fuse Helium
★ We call this "Helium burning" or the “triple-alpha process”
★ At high enough temperatures, it would also be possible to
fuse carbon, oxygen, etc.
★ The temperature required goes up because the electric charge
of a nucleus goes up

Stars
★ When a star begins to fuse Helium,
it happens in waves called Helium
flashes
○ Pressure does not change
significantly, so the core does not
expand and cool
○ Instead of puffing up the core, the
sudden burst of energy from Helium
burning raises the temperature,
increasing the rate of Helium fusion
still further (exponential growth in
energy output)

Stars
★ Helium begins to fuse very rapidly
★ After a few hours, the temperature
gets high enough that ordinary gas
pressure dominates, the core expands
and cools, and the star reaches a new
equilibrium: the Horizontal Branch
★ This is like a Main Sequence for red
giant stars: they are stars burning
Helium stably (instead of hydrogen),
in hydrostatic equilibrium

Stars
★ As the helium in the core gets turned to carbon, the core
compensates for the lost Helium by becoming denser and hotter
★ The star is now similar in
structure to when it left
the Main Sequence, except
now there are two shells
around its core

Stars
★ There is no more fusion
energy being generated in
the central core, which is
now only carbon+oxygen
"ash" and continues to
contract and become denser
★ Eventually fusion stops in
the shells as well

Stars
★ The series of Helium flashes
occur on the outside of the
degenerate core, blowing out
the outer layers and then
everything settles down
★ The ejected envelope expands
into interstellar space,
forming a diffuse, semi
transparent nebula

Stars
★ After Helium flashes blow off the
outer layers, the star splits into
two parts:
○ White dwarf: a small, extremely
dense carbon/oxygen core, containing
about half its original mass and
supported by electron degeneracy
pressure
○ Planetary nebula: the remainder of
the star, now a cloud of gas about
the size of our solar system

Stars
Spirograph Nebula Cat’s Eye Nebula

Stars
★ Planetary nebula are quite hot and
bright though they get cooler and
less luminous as they expand and
eventually dissipate into the ISM
★ Sun will lose ~40-50% of it’s
starting mass by the time it
becomes a white dwarf (the
material ends up in the planetary
nebula)

Stars
★ The remaining core is extremely
dense and hot, but very small -
about the size of the Earth
★ It is luminous at all only due
to high temperature as there is
no longer any fusion going on
★ It can't collapse any further
due to electron degeneracy
pressure

Stars
★ As the white dwarf cools, its
size does not change much
★ It gets cooler and dimmer and
finally ceases to glow
significantly (black dwarf)
★ This will be the ultimate fate
of the Sun, but the story can be
different for white dwarfs with
binary star companions...

★ Summarize named stages in the Sun’s life cycle:
○ Main Sequence: where a star is born and lives most of its life
○ Red Giant: stage after leaving the main sequence and expanding
○ Horizontal Branch: like the main sequence for red giants
○ Planetary Nebula: outer layers of the red giant that are
expanding away from the core
○ White Dwarf: the degenerate core of Carbon/Oxygen by products
Stars

★ Summarize fusion at each named stage:
○ Main Sequence: fusing Hydrogen in the core
○ Red Giant: fusing Hydrogen in a layer around the core
○ Horizontal Branch: Helium fusion starts in flashes in the core,
Hydrogen still burning in a layer around the core
○ Post-Horizontal Branch but pre-planetary nebula: Helium and
Hydrogen still burning in a layer around the core
○ Planetary Nebula/White Dwarf: no fusion, just degenerate core of
Carbon/Oxygen by products
Stars

Stars
★ Most stars are found in binary (or triple or quadruple)
systems
★ If they are far enough apart, they will evolve the same way
isolated stars do

Stars
★ If the stars are close enough together, then when one
becomes a red giant and expands, gas can flow onto the other
star due to its gravity
★ The white dwarf accretes,
or collects, the other
star’s mass which leads
to an explosion called a
nova

Stars
★ When enough material has accreted onto the surface of the white
dwarf, fusion can reignite suddenly, burning off the new
material quickly
★ Fusion, in this case hydrogen,
grows exponentially like a
helium flash since the white
dwarf is supported by
degeneracy pressure
★ Material falls back onto the
white dwarf and the process
repeats

Stars
★ While the white dwarf supports
fusion, it becomes extremely
bright for weeks, overwhelming the
light from its companion
★ Eventually, it returns to normal,
once hydrogen fuel is used up

Stars
★ White dwarfs are kept from collapsing
by electron degeneracy pressure, but
this gets harder as mass increases
★ More massive white dwarfs are smaller
than less massive ones, and white
dwarfs beyond ~1.4 solar masses
cannot support their own weight (this
is called the Chandrasekhar limit)

Stars
★ Sometimes a white dwarf passes the 1.4 solar mass limit due
to a companion or a merger with another white dwarf and
collapses

Stars
★ As the white dwarf collapses, energy
is released and it heats up
★ If it gets hot and dense enough,
Carbon fusion begins at the core of
the white dwarf and spreads through
it almost instantaneously
★ This results in a Type I supernova:
a huge, rapid energy release which
leads to an explosion at ~0.03x the
speed of light!

Stars
★ All of the white dwarf's carbon is fused into Magnesium &
Neon, and then other elements like Nickel, in about a second
★ Shines a billion times as bright as the Sun for weeks

Stars
★ The white dwarf is entirely
converted into an expanding
cloud of gas called a supernova
remnant, largely made up of iron
★ The gas spreads throughout the
interstellar medium and
eventually gets incorporated
into new stars

Stars
★ A nova is a surface explosion on a white dwarf, while a
supernova is a far more powerful and destructive explosion
of an entire star
★ In a nova, a white dwarf survives and the process can repeat
while in a supernova, the star is destroyed
★ The key difference lies in the scale of the event: a nova is
a burst of fusion in a surface layer, while a supernova
involves the complete detonation the entire star

Massive Stars
★ A star of more than about 8
solar masses can fuse elements
far beyond carbon in its core,
leading to a very different
fate from the Sun
★ Eventually the star will die
in a violent Type II
(core-collapse) supernova