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Deaths of Stars
Low Mass
Intermediate Mass
High Mass
Recap
• At the end of its main sequence life, the core of
any star depletes its hydrogen supply.
• Hydrogen burning continues in a shell around
the core.
– This layer burns fuel more rapidly than the core did.
– The atmosphere expands to a
huge radius and the star becomes
a Red Giant.
– The core shrinks and becomes
more dense.
RGB Stars
• During hydrogen
shell burning the
star moves up and
to the right on the
HR diagram.
– We say that the
star ascends the
Red-giant Branch
The Helium Core
• Eventually the core will get dense and hot
enough to start burning helium to carbon
and oxygen.
• This will happen differently depending on
the mass of the star
– In more massive stars (2-3 solar masses) the
helium burning begins gradually.
– For low mass stars like the Sun, the helium
turns on rapidly in a Helium Flash.
Horizontal Branch Stars
• Stars will burn helium
in their cores for a short
time (about 1% of the
time they spent on the
main sequence)
• At this time the star
moves down and to the
left in the HR diagram
– It gets smaller, a little
fainter, and hotter.
– We call these Horizontal
Branch Stars
Helium Shell Burning
• Soon core helium is used up, leaving carbon and
oxygen ashes behind.
• Now there are two burning shells, helium and
hydrogen.
• Again the atmosphere expands and the star
becomes a red giant for the second time.
AGB Stars
• The second ascent is
onto the Asymptotic
Giant Branch.
– It parallels the
previous ascent.
– But the star is larger
and more luminous.
I. Low Mass Stars
• If AGB stars have less than ~ 4 solar masses
their cores will not get hot enough to fuse
carbon or oxygen.
• The shells grow thin and eventually become
dormant. The core cools.
– The dormant helium shell compresses
and heats until it is hot enough to
ignite again in a short-lived fusion
outburst called a helium shell flash.
• Bursts of energy from the
helium shell flash are called
thermal pulses.
Mass Loss
• Thermal pulses eventually
drive off the outer envelope
in a series of ejections,
exposing the very hot core!
– UV radiation from the hot core
ionizes the gas in the
expanding atmosphere causing
them to glow
• This ionized shell of gas is
called a Planetary Nebula.
The Ring Nebula
Planetary Nebula
• About 30,000 in the
Milky Way alone.
– Expanding at 10-30
km/s.
– Can reach 1 lightyear
in diameter
– Can only exist for
about 50,000 years
before dispersing.
The Dumbbell Nebula
Helix Nebula
HST Images of Planetary Nebula
M2-9 in Ophiuchus
Cat’s Eye Nebula
The Hourglass Nebula
What about the Core?
• The exposed core is called
a White Dwarf
Dumbbell Nebula White Dwarf
– About the size of the Earth
• The star will continue to
glow for millions of years
as it cools down.
• Eventually it will fade
away, becoming a Black
Dwarf.
Electron Degeneracy
• If the core could shrink and compress
like it did at the horizontal branch
phase, it would eventually heat up and
fuse the carbon and oxygen.
• But for stars with masses less than 4
times solar a property called electron
degeneracy kicks in.
• The white dwarf acts like a giant atom
with electrons in unique energy states.
– This keeps the nuclei separated and the
core cannot shrink further.
The Chandrasekhar
Limit
• Subrahmanyan
Chandrasekhar, possibly the
greatest theoretical
astronomer ever, found an
upper limit to the mass that
could be supported by
electron degeneracy.
• That limit, known as the
Chandrasekhar Limit, is 1.4
Solar Masses.
White Dwarf Masses vs Original Star Mass
Original Mass
Ejected mass
White Dwarf
0.8
0.2
0.6
1.5
0.7
0.8
3.0
1.8
1.2
4.0
2.6
1.4
II. High Mass Stars
• If AGB stars have more than ~ 4 solar masses, their cores
will get hot enough to fuse carbon and oxygen.
– Stars with masses between 4 and ~8 times solar still end up as
planetary nebulae/white dwarfs.
• If AGB stars have more than ~ 8 solar masses, their cores
will get hot enough to fuse elements up to iron
– Stars with masses between 8 and ~15 times solar are called
intermediate mass stars. They end as neutron stars.
– Stars with masses greater than ~15 times solar are called high
mass stars. They end as black holes.
– (There is confusion in the books on these values and names…)
Fusion Stages
• Carbon burning fuses carbon into oxygen, neon,
sodium, and magnesium
• Neon burning fuses neon into oxygen and magnesium
• Oxygen burning produces sulfur and silicon
• Silicon burning produces iron which does not burn.
Elemental Abundances in Solar
System Revisited…
J
J
J J J
T
IG
T
T
T
T
IG
T
Fusion Times for a 25 Solar Mass Star
Fusion Type
Core Temp (K)
Duration (yr)
Hydrogen
40,000,000
7,000,000
Helium
200,000,000
700,000
Carbon
600,000,000
600
Neon
1,200,000,000
1
Oxygen
1,500,000,000
0.5
Silicon
2,700,000,000
1 day
During these phases the star, a supergiant, drifts back
and forth on the top of the HR diagram.
End of the Line
• The iron core is held up by electron degeneracy
pressure as noted before.
• But with denser nuclei per atom, the core mass
can exceed the Chandrasekhar limit.
– When this happens, electron degeneracy pressure
fails.
– The electrons are driven into the nuclei, combining
with protons, creating neutrons and neutrinos!
• With no support pressure, gravity suddenly pulls
all the material violently to the center.
Violent Death
• The temperature jumps to
5 billion °K in less than
1/10th of a second,
generating blackbody
photons - mostly high
energy gamma rays.
– The gamma rays
“photodisintegrate” the core
turning the iron atoms into a
soup of protons, neutrons,
alpha particles, and
electrons.
Nuclear changes
• The electrons are pushed
into the protons and alpha
particles creating neutrons.
– The neutrinos from this
process leave in a pulse,
taking support energy with
them.
• The core reassembles in ¼
second as a giant “atomic
nucleus” of neutrons.
– The neutron ball will not
compress further and
becomes rigid.
The Supernova Explosion
• The collapse of the core creates a perfect vacuum
outside of it. Material above this falls in toward the
core at 15% of the speed of light!
– When it reaches the rigid core it bounces back!
• This explosive rebound blasts the material into space as
a Supernova Explosion.
Supernova
• The explosive energy
brightens the star by a
factor of 108 - about 20
magnitudes.
– It outshines the entire
galaxy for a few weeks.
– Up to 96% of the star is
blasted into space.
• We call these Type II
Supernovae.
– (So, yes these is another
way of blowing up a star,
the Type Ia Supernovae).
Supernova 1987a, Feb 23 1987
• The only
supernova
close enough
to study the
progenitor
star occurred
in the Large
Magellanic
Cloud in
1987.
A Blue Supergiant
• The progenitor star was found in an archival study
by researchers at CWRU.
• It didn’t match theory!
– should have been a red supergiant.
– was instead a B3 I – a blue giant!
– Soon it was realized that evolved different because it
was metal poor. It also probably lost outer material
• Estimated to be 20 Solar Masses.
SN1987a Remnant
Neutrinos
• Neutrino detectors on
earth recorded a
significant neutrino burst
3 hours before the light
rise was detected.
– It takes a while for the light
to work its way out.
Neutrinos fly out
unimpeded.
• This was a great test of
supernova theory that
fortunately worked!
Other Supernovae
• Collapsing high mass stars are of types II, Ib, and Ic.
– Types II have their outer envelopes intact.
– Types Ib are missing hydrogen from the envelope but not
helium.
– Types Ic are missing both hydrogen and helium.
Type Ia
• The second type of supernova, Type Ia, is formed in
binary star systems where one star is a white dwarf
and the other is an evolving red giant.
• The red giant fills its Roche Lobe and dumps material
onto its companion.
Smaller WDs  Nova
• The pressure on the surface material builds
until hydrogen fusion ignites in a nuclear
explosion!
– The entire layer is blasted into space.
– The star brightens by 10,000 to 1,000,000 times!
• Neither the white dwarf nor
its companion are destroyed.
So this process will repeat.
• This is a Nova.
GK Persei
Type Ia Supernovae
• On larger white dwarfs, the pressure of the
incoming material combined with the internal
pressure causes the white dwarf core to suddenly
begin to fuse carbon.
– The process quickly runs away and a thermonuclear
explosion occurs blasting the white dwarf to pieces.
Supernova Remnants
SN Remnant in LMC
Cassiopeia A
Supernovae are Useful
• The universe started out as mostly hydrogen
and helium. All heavier elements come from
supernovae.
• Elements up to iron are formed in massive
star cores.
– Most of that iron was destroyed before the
supernova explosion.
– So then where do all the heavier elements
including iron come from?
Element
Production
• A supernova has
– A tremendous amount
of energy
– a lot of neutrons running
around
• The outer shells of the core contain all elements lighter
than iron. These are now targets for the neutrons.
• These elements capture neutrons until they are swollen
up to isotopes like 250Fe! Then they decay into copper,
gold, lead, etc. - all the remaining elements in the
periodic table.
From elements formed inside
massive stars!
• These elements drift
through interstellar
space creating dust!
• Some will sweep
together during star
formation to form
terrestrial-like
planets!