Low Mass Intermediate Mass High Mass 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 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. Dumbbell Nebula White 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 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 T IG

Fusion Times for a 25 Solar Mass Star 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!