The lives of Stars

This ‘burning’ of hydrogen to form helium continues for a long time around 10¹⁰ years for a star like the sun, which is currently about halfway through this burning process. Most observable stars are in this state and are referred to as main sequence stars. However, after some 10% or so of the star’s mass has been converted into helium, the high temperature core begins to contract and the outer layers of the star undergo massive expansion and cool down. In this state, the star is very large and, correspondingly, it is referred to as a red giant. The contraction of the core means that more gravitational energy is released, leading to a further increase in its temperature to around 10⁸K. This enables ‘helium burning’ to take place, which leads to the formation of very stable carbon () nuclei these are, as it were, a combination of three helium nuclei. Here it should be noted that the combination of two helium nuclei (beryllium ) is unstable and is only formed transitorily in the build-up of the carbon nuclei. It is also remarkable that the formation of the carbon nuclei only happens at a significant rate because there is an energy level in the carbon nucleus at just the right energy to enable a transitory beryllium nucleus to capture a helium nucleus by a strong resonant process. This fortuitous energy level is a key factor in the existence of life and will be referred. Many other nuclear reactions in stars can take place, leading. To the synthesis of even heavier nuclei such as silicon and iron as further collapse of the core takes place and higher temperatures are reached, but the nature of this collapse and the further evolution of a star depends on its mass and there is a variety of possible scenarios.

Brown Dwarfs. First it should be mentioned that if the mass of the matter gathered together by gravitational attraction is less than about 1/15 of the mass of the sun then the gravitational energy released in the collapse and the resultant core temperature is too low to sustain a fusion reaction. Such stars are known as brown dwarfs. Because of their low temperatures they hardly radiate and so are difficult to detect; only a few have been identified.

White Dwarfs. If a stars mass is greater than 1/15 but less than about 3/2 times the mass of the sun (known as the Chandresekhar (Nobel Laureate, 1983) limit) there comes a point at which the gravitational force causing the collapse of the core is balanced by the effective repulsion between electrons in the core due to the exclusion principle no two electrons can be in exactly the same state. Thus the core stops collapsing and settles down into a final state with a size similar to that of the earth but with a density some 10⁶ or l0⁷ times greater. Such a star is known as a white dwarf. The external envelopes of these stars are ejected and become what are known as planetary nebulae.

Neutron Stars and Pulsars. If the star’s mass is between the Chandresekhar limit and about two solar masses then another end state is possible. Because of the larger mass and the resultant increased effect of gravity, the exclusion principle repulsion between the electrons in the core is overcome and they are captured by protons. This process is due to the weak interaction and leads to the formation of neutrons and neutrinos. The latter escape and the core is then simply like a very large nucleus consisting basically of neutrons. The exclusion principle repulsion between the neutrons now stops complete collapse. Even so, the final neutron star only has a radius of a few kilometres and a density some l0¹⁵ or more times that of the earth hundreds of millions of tons per cubic inch. Many neutron stars are rotating rapidly and, as a result of this and their magnetic fields, emit regular pulses of radio waves like a lighthouse. They are referred to as pulsars. The first to be discovered, in 1967, rotated once a second but many more have now been identified rotating with frequencies up to 1000 times a second.

Black Holes. For even larger masses the repulsion between the neutrons due to the exclusion principle loses out to the gravitational force and further collapse of the core takes place. The gravitational field at the surface is now so strong that light signals cannot escape. The light can only go so far to the event horizon and is then pulled back like a ball being pulled back to the earth. Anything trespassing within the event horizon can never escape. Since no light emerges from it, the core is referred to as a black hole and, in general relativity language, space time in its vicinity is curved back on itself and emitted light follows this curve. The gravitating matter in the black hole is concentrated into a point of infinite density a singularity and the only information we can have about it is its mass, angular momentum and electric charge. Since black holes cannot be observed directly their presence in the universe has been deduced from the effects of their huge gravitational fields which extend beyond the event horizon. For example, x-rays are produced as matter is dragged into a black hole in binary star systems (pairs of stars held together by their gravitational attraction) in which a black hole is one (invisible) component. Supermassive black holes having masses of up to a billion solar masses are almost certainly the explanation of quasars (quasi-stellar objects). These are extremely bright and distant objects at the centre of some galaxies, including, possibly, our own. They are sometimes 1000 times the brightness of a whole galaxy, and this brightness can be attributed to the radiation emitted as galactic material is continually sucked into such a black hole at the galactic centre. The whole of the central region of the galaxy is presumed to be in a continual state of gravitational collapse into the black hole. It should also be mentioned that primordial black holes of mass less than that of the sun and which would require gigantic compression of matter may have been created during the initial stages of the big bang but there is, as yet, no concrete evidence for them. Finally, although black holes suck everything into them there is a mechanism, suggested by Hawking in 1973, whereby they can lose energy. Was pointed out that particle-antiparticle pairs are continually appearing and disappearing in space. When this happens near a black hole the particle could, for example, be sucked into the hole by its gravitational field whilst the antiparticle escapes, taking energy from the black hole with it, thus reducing its mass. So, gradually, all black holes evaporate!

Supernovae. Although the shedding of the outer regions of a star as it becomes a white dwarf is relatively peaceful this is not necessarily so with stars whose mass is greater than the Chandresekhar limit. When the core of such a star collapses into a neutron star or a black hole the fusion process for creation of energy has come to an end and the outward pressure on the external regions of the star decreases so that these regions collapse in towards the core. Their temperature then increases massively and, since they still contain combustible material, nuclear fusion again sets in. The resultant release of energy up to l0⁹ times the previous output leads to a catastrophic explosion in which the outer layers are blown into space carrying with them many of the elements synthesized during the fusion processes that have taken place and in subsequent neutron induced reactions. Some of this debris, consisting of both light and heavy elements, will be absorbed as second  and third generation stars are formed from the interstellar gas and dust. The sun and its planetary system were formed in this way and, indeed, but for this debris the earth (and we!) would not be here. There is also a huge release of electromagnetic radiation and a supernova can be brighter than the whole of its galaxy for a few weeks. Many radio sources have been identified as supernovae but only a few optical sources. Here the most famous is the supernova observed in 1034 whose remnants are what we now know as the Crab nebula.