Basic properties of stars

From our personal viewpoint a crucial property of stars is their long-term stability, which astrophysicists recognize as being due to a number of lucky 'accidents'. For example, if the di-proton (two-proton nucleus) were stable, stars would race through their life cycle in millions rather than billions of years. The reactions that take place are actually rather improbable, and this is why a star like the Sun can spin out the

fuel supply for ten billion years. If the reactions had a higher probability, stellar evolution would proceed much faster. One of the major findings of stratigraphic geology has been the discovery of the huge time span, measured in billions of years, that preceded the emergence of hominids on this planet. Our presence in the universe is made possible only by the long time scale associated with stellar evolution.

The rate at which a star evolves depends strongly on its mass. Stars a few times more massive than the Sun have significantly higher central temperatures. Consequently they use their fuel faster and reach the later stages of stellar evolution sooner than solar-mass stars.

When the hydrogen fuel is depleted, the end point of a star's life is determined mainly by its mass. Low-mass stars start to shrink as the temperature falls. This can ignite hydrogen-burning reactions in the unprocessed outer layers. The contracting core of the star becomes hot enough to sustain further reactions: it burns helium to form carbon and oxygen, carbon to form neon and magnesium, oxygen to form - silicon, and silicon is burnt to make iron. Meanwhile the burning in the outer layers can lift these layers away to form a planetary nebula (so called because its appearance in an astronomical telescope resembles the disc presented by a planet, in contrast with the point image of a star: there is no suggestion that this gas subsequently forms planets). The remnant core finally cools down and collapses until

forces between electrons are high enough to stave off further gravitational contraction. At this stage it has become a white dwarf star, with a density some million times greater than ordinary matter at 10⁹ kg/m³. Ultimately it cools to an inert mass that plays no further part in the life of the Galaxy.

Stars that are somewhat more massive than the Sun probably finish their evolution with a stupendous explosion. The dense reactor core runs through a series of reactions that form elements as far as the iron group in the periodic table. Eventually the contraction causes the internal pressure of this core to reach a stage in which its electrons combine with its protons to form neutrons.

This change removes the electron pressure support at the centre and instantaneously triggers a catastrophic implosion. The outer layers of the star suddenly find that the core has collapsed beneath them: they, in turn, rain down towards the centre. Temperatures now rise, out of control, and in less than a second runaway nuclear reactions are detonated in the unspent fuel of the outer layers. A gigantic explosion throws most of the stellar material back into interstellar space. Super-nova explosions arc thought to be the major site of element synthesis in the universe, the means by which the elements beyond hydrogen and helium have been created since the primeval big bang.

The ejecta from a supernova outburst form a supernova remnant (Figure 2.9). Initially the remnant may expand at a good fraction of the velocity of light, but ultimately this slows to the ambient velocity

as the matter ploughs into the interstellar medium. At the site of the explosion is the collapsed core of the original star.

2.9: Vela supernova remnant is a blast wave of debris cast into space when a star exploded

1 00000 years ago. Eventually these glowing ashes will merge with the interstellar medium and become the matter that will form a future star

For a stellar relic with a mass between 0, 1 and 1 .4 solar masses the final stable configuration is a neutron star, when the object has the density associated with nuclear matter, some 10¹⁸ kg/m³. It is then supported by neutron pressure against any further collapse beyond its radius of 10 km or so.

When they are made in supernova explosions, these neutron stars rotate rapidly, preserving the angular momentum of the pre-supernova star, which had a far larger radius. They are observed by radio astronomers as pulsars. If the central relic exceeds 1.4 solar masses it collapses indefinitely and forms a black hole, a region of space from which nothing, not even light, can escape.