9. Evolution of Massive Stars: Supernovae.

• Evolution up to supernovae: the nuclear burning sequence; the iron catastrophe.

• Supernovae: photodisintigration; electron capture; neutron degeneracy.

• Nucleosynthesis in supernovae.

9.1 Evolution up to Supernovae

Recall: a 1 solar mass star goes through the sequence:

• Core hydrogen burning (Main Sequence)

• Re-adjustment when core hydrogen is exhausted (Sub-Giant Branch)

• Shell hydrogen burning while core contracts (Red Giant Branch)

• Core helium burning and shell hydrogen burning (Horizontal Branch)

• Shell hydrogen and helium burning while core contracts (Asymptotic Giant Branch)

Final stages in evolution of a 1 solar mass star:

• Carbon-Oxygen core supported by electron degeneracy pressure

• Helium and hydrogen burning shells

• Envelope being ejected as a Planetary Nebula

• There is not enough mass to provide pressure for the core to contract, heat up and ignite Carbon burning.

This is true for stars up to about 8 solar masses.

Stars with masses greater than 8 solar masses go through successive phases of nuclear burning following the sequence:

1) exhaustion of element X in the core

2) contraction and heating of core

3) ignition of shell source burning X

4) ignition of new element Y in core

5) repeat steps (1)-(4) for Y

Each nuclear burning phase is shorter than the last:

Final structure of a massive star looks like an onion…...

This sequence continues until iron (Fe) is reached.

“exothermic” “endothermic”

9.2 Supernovae and Nucleosynthesis

Once the core is entirely Fe the star cannot produce more energy by nuclear burning so there is nothing to halt the collapse of the core.

As the core contracts and heats up electron degeneracy pressure becomes important but is only sufficient to halt the collapse if the core mass is less than 1.4 Solar Masses (called the Chandraesekhar Mass).

As the temperature sky-rockets many of the heavy elements produced earlier are destroyed by photodisintigration. E.g.

npHenHeFe 22413 42

42

5626 +→++→+ +γγ

At these extreme densities and temperatures the free electrons are captured by free protons:

enep ν+→+ −+

As the electrons are destroyed the electron degeneracy pressure support is lost and a catastrophic collapse begins.

The collapse is halted only when densities are high enough for neutron degeneracy pressure to become important.

This occurs so suddenly that the core rebounds, sending a massive shock wave that explodes through the envelope.

In the explosion elements heavier than Fe are created as the massive release of energy creates conditions where this is feasible.

In stars less massive than 25 solar masses the core remains behind as a neutron star.

In stars more massive than 25 solar masses even neutron degeneracy pressure isn’t enough to stop the collapse of the core and it implodes to form a black hole.

These explosions are known as Type II Supernovae.

During the explosion the star can outshine a galaxy!

(See 9.3 for supernovae Type Ia.)

Light curves for the central object in SN Type II.

The gas can be seen for thousands of years afterwards, glowing in synchrotron radiation

Evolutionary tracks of high mass stars

9.3 Supernovae Type Ia

Consider a close binary pair consisting of two stars of relatively low mass (e.g. 1 and 2 solar masses)

• After a little more than a billion year the 2 Msun star will become a red giant.

• If the pair is very close it can expand sufficiently to transfer mass to the 1 Msun star

• Eventually it will lose its envelope entirely and leave behind a white dwarf star

•After another 10 billion years the 1 Msun star becomes a red giant

• If the binary is close enough it can start transferring matter back to the white dwarf

During the transfer stage the white dwarf can be observed as

• a cataclysmic variable - most of luminosity due to accretion of material onto surface of dwarf

• a nova - outburst due to the ignition of thermonuclear reactions of material on the surface of the star

• If enough matter transfers to the white dwarf its mass will go over the Chandrasekhar limit, EDP will no longer be enough to sustain the structure

• The white dwarf rapidly collapses, starts fusion processes and explodes in a Type Ia supernova

• There is no remnant - the star blows itself completely apart.