Rick's Cosmology Tutorial: Chapter 17 Abstract

Stars spend most of their lives burning hydrogen in their centres into helium. This defines the Main Sequence. Previous chapters were able to calculate in some detail the rates of the nuclear reactions during the Main Sequence, albeit with the aid of simplifying assumptions. It was even possible to produce crude models for the hydrostatic and heat transport condition in this phase. When the hydrogen in a star's core is exhausted, the evolution of the star beyond the Main Sequence becomes far more complicated. It is no longer practicable to produce simple analytical models of the stellar structure. Instead it is necessary to deploy computer models. Consequently, in this chapter, we abandon attempts to calculate, and instead resort to describing verbally the evolution of stars beyond the Main Sequence. This will be based on the outcome of decades of experience with such computer models, suitably constrained by observations.

It will be convenient to consider stars in various different ranges of initial mass. In particular, the star's mass determines the end point of its evolution. An isolated star of less than about 8 solar masses ends ceases its nuclear fusions after helium burning. It ends its days as a dead white dwarf, having first sloughed off its envelope as a planetary nebula. Such white dwarf stars in binary systems can produce explosive ejecta by accretion of material from the companion star. For example there might be a nova (which leaves the white dwarf intact) or a Type Ia supernova (which destroys the white dwarf completely).

Stars not exceeding about 8 solar masses can enrich the interstellar medium (ISM) with chemical elements up to nickel. This occurs during the red giant phase of their life, in which stellar winds, stellar oscillations and ultimately the emission of a planetary nebula, provide the means by which the created elements escape from the star. For binary systems, a nova and/or supernova can also provide the mechanism for release of elements into the ISM.

Stars with initial masses above about 12 solar masses will continue their fusion reactions up to the point where iron and nickel are produced by burning silicon. Fusion cannot proceed further. Such a star dies in a supernova which involves the collapse of the core. These stars will also contribute to the enrichment of the ISM with elements up to nickel during their steady evolution. During the supernova, however, heavier elements beyond the iron group are formed, including radioactive isotopes. The supernovae of such massive stars is the only source of elements beyond nickel.

The fact that oxygen and carbon occur early in the chain of fusion reactions beyond helium is consistent with these two elements being respectively the third and fourth most common elements in the universe (after hydrogen and helium). The relative contributions of stars of different mass to the abundance of the lighter elements is not completely settled. However, the broad consensus is that low and intermediate mass stars produce most of the carbon and iron, and probably nitrogen, whereas massive stars produce most of the oxygen. However, all mass ranges contribute to elements up to the iron group.

The Moon with a stary background. There's a dark side all right.