Star Death

One of the most interesting studies in astronomy is star death. After the main sequence phase when stars convert hydrogen to helium, stars enter a sequence of processes that ultimately end with the death of the star. Large stars end as super red giants and then supernovae. Stars the size of the sun become red giants before ending as white dwarfs.

During the sun’s 9-billion-year main sequence phase, hydrogen is converted to helium. After approximately nine billion years as a main sequence star, the sun will run out of hydrogen in the core and will leave the main sequence phase. Several processes over a span of 200 million years will cause the sun to balloon into a red giant, shed its outer layers as a planetary nebula and become a white dwarf.

The death of stars is sequential because of the different required fusion temperatures for different elements. Helium will not fuse at the temperature that hydrogen fuses at because it has two charges rather than one, and it is more difficult to push the nuclei together. This is called the Coulomb barrier. Helium is also heavier than hydrogen, which means that it has the potential to become denser; thus, the core begins to contract again. As it contracts, gravitational energy is converted to thermal energy. When the temperature in the contracting helium ash core rises, this increases the temperature in a shell around the core, and it ignites hydrogen fusion in this shell. The rate of hydrogen fusion in the shell continues to increase, which in turn drives a greater radiation pressure outward. This radiation pushes on the outer layers of the star, literally pushing them outward by 10 to 100 times, and the star begins to balloon out as a red giant. The evolutionary track of the sun in Figure 3‑25 shows that during the red branch phase the luminosity initially increases 100-fold (larger radius) but the surface temperature dips making the star red.

During the horizontal branch phase (Figure 3‑25), the core contracts and the temperature reaches 10⁸ K (100 million K), which is hot enough to cause helium to fuse into carbon. Normally, the core would expand due to the increased heat, but electrons control the size of the core (degenerate electron pressure) based on quantum mechanics, so the core does not respond to this increased heat. This causes a helium flash that releases more power than an entire galaxy. During the asymptotic branch phase, the star’s radius increases so much that the star can barely exert a gravitational pull on itself from one side to the other. Eventually the outer layers of the star are ejected, and the star becomes a planetary nebula such as the Cat’s Eye Nebula (Figure 3‑26). The name ‘planetary’ nebula is a misnomer. Planetary nebulae have nothing to do with planets. When they were first detected, astronomers thought the white dwarves at the centers of these glowing clouds of gas were planets. The white hot, electron degenerate carbon core will become a white dwarf. If they are by themselves, white dwarfs drift harmlessly in space, slowly cooling over the rest of the life of the universe. However, if the white dwarf has a large nearby companion star, it causes one of the most energetic and spectacular events in astronomy - a Type Ia supernova.

Stars greater than 8 solar masses end their lives as supernovae. High mass stars rapidly consume the hydrogen in their core, and when they exhaust that, they transition to helium fusion much like the sun. Once a massive star has fused its helium ash core into carbon, it keeps going on forming heavier and heavier elements. It also fuses lighter elements in concentric shells around the core. The process keeps going until the core is filled with iron, which has endoergic fusion.

A reaction that results in a net energy gain is called exoergic (“exo-” out, “ergic” energy). Hydrogen and helium fusion are the most exoergic fusion processes. Carbon fusion releases less energy than helium, and nitrogen fusion still less than that and so on until finally the star attempts to fuse an iron ‘ash’ core. The big problem with this is that iron fusion requires input energy, which means it is endoergic (“endo-” absorbing). Iron fusion in the core drains the star of radiative energy. The iron ash core contracts, just as the helium ash core did, but this core heats to around 100 billion Kelvin and forms a neutron core. Meanwhile the outer layers of the star come crashing down, hit the now degenerate neutron core, and then bounce out in a Type II supernova explosion.

The banner shows the Crab nebula, which is the result of a supernova explosion observed in the year 1054 AD. This explosion was so bright it could be seen during the day. A closer look reveals a rapidly spinning neutron star in the middle of the structure. Supernovae form all of the heaviest elements. For example, gold forms in supernovae explosions.

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A supernova explosion. The Crab Nebula. Credit: J Hester, A. Loll, NASA, ESA.