Chapter 13 Post Main Sequence Stellar Evolution

The Sun

The Sun (continued)

Internal Evolution of the Sun

The Sun 5 Billion years from Now

Evolution in the H-R Diagram

Main Sequence Turn-Off indicates the age of a cluster

Many Mature Stars Pulsate

Which causes periodic changes in brightness

Which in turn is correlated with the stars luminosity – extremely useful as a distance indicator

The Death of Stars Depends on Mass. For a 1 Mo star…

Planetary Nebulae (with a White Dwarf at the center)

White Dwarfs W.D progenitors are stars with main sequence masses < 8 M . W.D’s are Carbon-Oxygen cores, exposed during the planetary nebula stage. W.D cores are supported by degenerate electron pressure, according to the Pauli exclusion principle, not by thermal pressure. So the pressure is independent of the internal temperature. As a result, W-D’s have approximately uniform density, so, in this case we can reliably use the equation of hydrostatic equilibrium to estimate the the central pressure of a W.D. Additionally, the mass-radius relation for W.D. is M R 3 = constant, which means that as the mass increases, the size decreases!

The Chandrasekhar Limit However, there is a maximum mass for W.D’s – the maximum that can be supported by electron degeneracy pressure - and that mass corresponds to 1.4 M  Any more mass than 1.4 M  will cause further collapse until Neutron degeneracy is reached, leading to a neutron star. A Neutron star is essentially a giant iron nucleus comprised of protons and degenerate neutrons and electrons with enough free electrons to produce zero net charge. Neutron stars obey the same mass-radius relation as W.D.’s, so they too shrink as more mass is added! When the mass of a Neutron star exceeds 3 M  it will collapse into a singularity, a point of infinite density, where the known laws of physics break down.

White Dwarfs cool and fade into obscurity

High Mass Stars (> 5 Mo) process H into Fe becoming Red Supergiant stars

Endothermic vs. Exothermic Reactions All thermonuclear reactions occurring in the cores of stars are exothermic, that is, they release energy, but only up until the Fusion of Iron (Fe). Iron takes more energy to fuse than can be obtained from it, and is an example of an endothermic process, which does not occur in stars. As stars produce nuclei with masses progressively nearer the iron peak of the binding energy curve, less and less energy is produced per kg of fuel, until none is produced at all, marking the onset of a supernova explosion. As the star collapses, the core grows until it reaches the Chandrasekhar limit, and then collapses into a rapidly rotating neutron star.

The Supernova Explosion The supernova explosion is caused when the overlying layers of the stellar atmosphere free-fall onto the core and literally bounce – off, creating a shock wave that blows off all the other overlying layers, in a spectacular explosion, that we know as a Supernova. The supernova explosion produces so much light that it can temporarily outshine an entire galaxy. The photons destroy the heavy nuclei through a process of photodisintegration to produce a flood of neutrinos. For all supernovae, the source of light during the decline in brightness after the explosion is the decay of radioactive isotopes created in the supernova explosion.

SN 1987A - Before and After Interestingly, no compact remnant has been found, only a flood of neutrinos following the explosion has been detected.

Supernova Remnants

Pulsars are magnetized Rotating Neutron Stars

The Crab Nebula

Pulsar light curves

Pulsar periods increase as the neutron star looses rotational energy

White Dwarfs, Neutron Stars, and Black Holes

Summary Maximum mass of a White Dwarf is 1.4 Mo, above that it collapses into a Neutron star. If the mass of a Neutron star exceeds 3 Mo, it will likely collapse into a Black Hole.