Stellar Life - Nebulae to Death

Overview

Once a star has used the core's hydrogen and converted it to helium, it can no create energy that way, and must use other resources. This point is the trigger for the advanced evolution of the star, and over the next few millennia, it will evolve and then eventually turn into a neutron star or black hole.

Late-Stage Nuclear Processes

In a star whose mass is greater than 8 M_Sun, the Carbon/Oxygen core can undergo further nuclear processes, starting with carbon burning:

This creates an "ash pile" of other elements. As it contracts, its temperature and density rises, causing oxygen burning:

Thus, the star forms an onion-like structure, with iron at the core, a layer of silicon fusion, a silicon-rich layer, oxygen fusion, oxygen-rich layer, carbon fusion, carbon-rich layer, helium fusion, helium-rich layer, hydrogen fusion, and then an outer hydrogen/helium envelope.

Each process produces less energy than the less. The table at the right shows the effective time scales for the different processes for a 15 M_Sun star.

Silicon fusion results in an end product of iron. Iron is the most stable nucleus, and cannot be fused to produce energy. The star must now turn to something completely different.

Supernova

With nothing less to fuse, gravity wins out against a now non-existent outward pressure, and compresses the core. As it is squeezed, it becomes degenerate, and is held up by degeneracy pressure. However, now photons have enough energy to destroy nuclei:

This is known as photodisintegration, but it uses energy rather than creates it. In the extreme conditions at the core of such a star, free protons capture the free electrons, producing a neutron and an electron neutrino. However, remember that the electrons were what was holding up the core against gravity.

With the sudden release of the degeneracy pressure, the core again starts to collapse, but this time there is nothing to stop it. The effective free-fall depends on density, and since the density increases the closer to the center, the inner portions of the star react first. With collapse speeds of about 70,000 km/s, the inner core collapses from about 1 R_Earth to 50 km in one second.

As the core collapses, the outer layers have nothing to support them, and they collapse inwards, too. Once the density reaches about 10¹⁵ gm/cm³, the collapse is again halted. This is about the density of an atomic nucleus. The stop of collapse is caused by neutron degeneracy.

The core rebounds a little, sending out a shock wave. As this shockwave travels out, it eventually meets the shockwave of material falling in. When this occurs, there is a tremendous explosion called a Type II supernova.

The outer layers are blast off in a violent explosion. When they become optically thin, the energy escapes and we can see it. The peak optical luminosity for a typical Type II supernova is approximately 10⁹ L_Sun. Approximately 100 times more energy is released in the form of neutrinos. The net effect is a brightness that outshines an entire galaxy.

The most famous supernova occurred on A.D. July 4, 1054, was so bright that it could be seen in broad daylight for 23 days. The nebula it created is called the Crab Nebula, and is the picture to the left.

End Products

The outer layers that were expelled will become visible from the radiation emitted from the newly exposed core. This results in a type of nebula called a supernova remnant.

As to what happens to the core, the path is divided. If the star is less than about nine (but more than 1.4) solar masses, the core will collapse into a neutron star - a star made entirely of neutrons. If the star possesses more mass, it will continue to collapse into a black hole - a point of theoretically infinite density that possesses such a strong gravitational pull that not even light can escape its pull.