Introduction.

  When stars are plotted in the H-R diagram, the number of stars in and out of the main sequence, together with models of stellar evolution provides a description of the possible ways in which stars are born, evolve and, eventually, die. During this process the star ``move about'' in the HR diagram (see Fig. 9.1). Since most stars are in the main sequence it is reasonable to suppose that during their life most stars stay in the main sequence, evolving into it when they are born and out of it when they are about to die. Models of stellar evolution confirm this.

 

Figure 9.1: Diagram illustrating the evolution of a sun-like star. Born from a gas cloud it moves towards the main sequence (1) where it spends most of its life. After all Hydrogen is consumed in its core, the star burns Helium and becomes a red giant (2). Finally, when the Helium is consumed nuclear reactions subside and the star becomes a white dwarf (3) where it will spend its remaining (billions of) years.  

For large objects (such as stars, galaxies, etc) the one ever-present force is gravity. This is always an attractive force which tends to condense stars and such into smaller and smaller objects. There are (fortunately) other effects which, at least temporarily, can balance gravity and stop this contraction. These effects are generated by the material which makes up the star and are always associated with various kinds of pressure (which tends to enlarge objects); a familiar example is the usual gas pressure

A less known type of pressure is produced by electrons when they are brought in very close contact. Under these circumstances there is a very strong repulsion between the electrons, not only because they have equal charges (and hence repel each other), but because electrons, by their very nature, detest being close to each other: they require a relatively large breathing space. This repulsion between electrons is called degenerate electron pressure . This effect has a quantum origin and has many interesting consequences, to mention two, thanks to this strong dislike of electrons for occupying near-by locations, the floor supports your weight, and atoms have different chemical properties.

Electrons are not the only kids of particles that dislike being in close contact with one another. For example, the nucleus of a Hydrogen atom, called a proton also exhibits this property. Finally, and this is important for stellar evolution, other particles called neutrons also dislike being close to each others. Neutrons have no electric charge and are slightly heavier than protons; they are also found in atomic nuclei and are, in fact, a common sight in nature. All the atomic nuclei (except for Hydrogen) are made of protons and neutrons, with the neutrons serving as buffers, for otherwise the electric repulsion of the protons would split the nuclei instantly. When close to each other neutrons produce a degenerate neutron pressure and protons a degenerate proton pressure (see Fig 9.2).

 

Figure 9.2: List of the most important particles which generate a degenerate pressure when in close contact. Also in the picture, the places where these particles are most commonly found.  

The various stages of stellar evolution are classified according to the origin of the pressure which counterbalances gravity's pull. For most stars a balance is reached in the final stages of the star's life; there are some objects, however, for which gravity's pull overwhelms all repulsion in the stellar material, such objects are called black holes.

The mass of the star largely determines its history, light stars (such as our Sun) will end in a rather benign configuration called a White Dwarf; heavier stars (with masses below 3-4 solar masses but larger than one solar mass) end as neutron stars after some spectacular pyrotechnics. Very massive stars end their lifes as black holes. This will detailed below, but before we need to understand what makes stars tick.