The death of high mass stars

In Chapter 10 we saw how stars evolve to the red giant or red supergiant stages, and how low mass stars (less than 5 M0) lose enough mass to leave behind a white dwarf as the final stellar remnant. We also saw that electron degeneracy pressure can only support a 1.44 M0 remnant. In this chapter we will see what happens to higher mass stars.

It is important to remember that stars lose mass as they evolve. This mass loss can be through winds, or the ejection of planetary nebulae. (In the next chapter, we will see that stars in close binary systems can transfer mass to a companion.) Though we only have estimates for the total amount of mass loss, it seems likely that massive stars can lose more than half of their mass by the time they pass through the red supergiant phase. A star's evolution will depend on how much mass it starts with, and how much mass it loses along the way.

11.11 Supernovae

11.1.1 Core evolution of high mass stars

In the core of a high mass star the buildup of heavier elements continues. If we look at nuclear binding energies (Fig. 9.3) we see that the isotope of iron 56Fe has the highest binding energy per nucleon. This makes it the most stable nucleus. This means that any reaction involving 56Fe, be it fission or fusion, requires an input of energy. When all of the mass of the core of the star is converted to 56Fe (and other stable elements, such as nickel), nuclear reactions in the core will stop.

At this stage, the core will start to cool and the thermal pressure will not be sufficient to support the core. As long as the mass of iron in the core is less than the Chandrasekhar limit, the core can be supported by electron degeneracy pressure. However, once the core goes beyond that limit, there is nothing to support it, and it collapses. In the collapse, some energy, previously in the form of gravitational potential energy, is liberated. Since that energy is available, the 56Fe can react by using up the energy. This means that the core does not get any hotter. It continues to collapse. A runaway situation develops in which the iron and nickel consume liberated energy. As the iron is destroyed, protons are liberated from nuclei. The electrons in the star can combine with these protons to form neutrons and neutrinos. This reaction can be written e~ + p s n + v (11.1)

The core is driven to a very dense state in a short time, about one second. What happens next is not completely understood, but the collapse results in an explosion in which most of the mass of the star is blown away. The neutrons created in reaction (11.1) probably play a role in this. They also obey the exclusion principle, and exert a degeneracy pressure (the details of which we will discuss in the next section). This pressure can stop the collapse and cause the material to bounce back. In addition, so many neutrinos are created, and the material is so dense, that a sufficient number of neutrinos interact with the matter forcing the material outward.

Such an exploding star is called a supernova. This type of supernova is actually called a type II supernova. Another type of supernova, type I, seems to be associated with older objects in our galaxy. (The mechanism for type I supernovae probably involves white dwarfs in close binary systems,

A supernova in another galaxy can be almost as bright as the whole galaxy.This shows a supernova in the spiral galaxy NGC4603. [STScI/NASA]

and is discussed in Chapter 12.) During the explosion, nuclear reactions take place very rapidly, and elements much heavier than iron are created. This material is then spread out into interstellar space, along with the results of the normal nucleosynthesis during the main sequence life of the star. This enriched material is then incorporated into the next generation of stars.

The light from a supernova explosion can exceed that of an entire galaxy (Fig. 11.1). The energy output in a type II supernova is about 1053 erg. About 1% of this shows up as kinetic energy of the shell, and 0.1% as light. (Most of the energy is in the escaping neutrinos.) After a rapid increase in brightness, the supernova fades gradually, on a time scale of several months (Fig. 11.2).

SNII MV light curves

11.1.2 Supernova remnants

The material thrown out in a supernova explosion is called a supernova remnant. It contains most of the material that was once the star. In young supernova remnants we can actually see the expansion of the ejected material. These remnants are important because they spread the products of nucleosynthesis in stars throughout the interstellar medium. There, this material enriched in "metals" will be incorporated into the next generation of stars. This explains why stars that formed relatively recently in the history of our galaxy have a higher metal abundance than the older stars. In the later stages of a supernova remnant's expansion, we still see a glowing shell, like those in Fig. 11.3. These shells also serve to stir up

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