As a star evolves and burns its ever-diminishing supply of nuclear fuel it becomes less and less able to support itself against the inexorable force of gravitation. The material in the centre of our Sun is only about 100 times as dense as water, but the centres of some collapsed stars in which the supplies of nuclear fuel have been exhausted contain matter so dense that a pinhead made of it would weigh over a million tonnes. Some of the most energetic phenomena in the Universe involve such collapsed stars. In this chapter we consider the properties of matter at high densities and of the objects in the Universe that are made of it.

The volume of an atom comprises mainly empty space. The bulk of the atomic mass resides in the central nucleus, which is composed of proton* and neutrons, each of which is about 1840 times more massive than one of the surrounding electrons. An atom is about 10~10m in diameter, but the atomic nucleus, which contains most of the mass of the atom, measures a mere 10~15m across. This is the distance at which the nuclear forces, as opposed to the electromagnetic force, have dominant influence. The size of the atom is determined by the electron cloud – with comparatively negligible mass – which is about 10-10m in diameter. It is the electrons that decide the physical and chemical properties of ordinary matter as we experience it in our everyday lives; the nuclear force has no effect for most practical purposes on the behaviour of ordinary matter. For everyday purposes we may consider the protons in the nucleus to be present merely to keep the matter electrically neutral arid the neutrons to act as stabilizers for the nucleus.

The factor of 105 in linear size between the atom and the nucleus means that there is approximately a factor of 1015 in density between the two. Since the density of ordinary solid matter, which consists of closely-packed atoms, is a few tonnes per cubic metre, we can conclude that the density of nuclear matter is of the order of It)15 tonnes m 3 (or 1018kgmr3). To emphasize this point, it may be noted that 99.95 per cent of the mass of the Earth resides in nuclei which, if they were packed tightly together, would fill a sphere with a diameter of about 200 metres ! It is difficult to visualize that this is so, because the obvious rigidity and opacity of the Earth give the impression that ordinary matter is completely solid. How¬ever, as we explain below, all of the obvious properties of ordinary matter are due to the electrons, not the very dense nucleus.

What would happen to the structure of matter on Earth if the Earth’s mass increased? Imagine a situation in which rocks are piled on the Earth from outside. Initially hills and valleys arise, but as they become too extreme these structures would crumble under their own weight and the material supporting them would flow like a liquid because of the enormous pressure of over-lying rocks. Mountains formed out of -ordinary rock cannot be much higher than about 20 kilometres on Earth. This is why the Earth has no ‘corners’. In fact because of its own gravity a lump of rock m space cannot appreciably deform from a sphere unless its diameter is less than about 400 km .

If the combined mass of the Earth and the added material were to become much greater than that of the giant planet Jupiter, the atomic electrons would no longer be able to maintain any pretence of structure. Immense internal pressures would crush the normal atomic structure, and the electrons would become a free electron gas! The radius of the Earth would now decrease if more material were to be added. Eventually, with the addition of more matter, it would have a radius similar to that of the original Earth again but would have some of the properties associated with the white –dwarf stars Application of extreme pressure might cause it to become a neutron star, by which time it would be only about l5km in radius.The density would then be similar to that of nuclear matter- and 1 he object would resemble one enormous nucleus. Finally as more matter is added, exotic effects associated with gravitation would build up: it would disappear from view and become a black hole.!

We have, of course, passed over many important points in this imaginary experiment, no! least of which is the release of gravitational energy. If you drop a stone on the ground it releases energy, mostly in the form of heat. The addition of all the hypothetical material to the Earth would release an enormous amount of energy that might well prevent the experiment from continuing at certain stages. Nevertheless, it should be stressed that many of the superficial appearances of matter at the densities to which we are normally accustomed are determined by the atomic electrons. If enough cold matter is contained in a sphere held together by its own gravity, the effect of the electrons may be completely washed out and nuclear densities prevail throughout. Some fraction of a dying star will eventually pass into at least one of the three terminal states of evolution (white dwarf, neutron star or black hole) unless it is completely dispersed. Many observed objects are identified as white dwarfs and neutron stars, and there are even some good candidates for black holes. Studies of these extreme objects, reveal properties of matter unattainable on Earth. Many of the violent and highly active phenomena in space involve compact objects. As we have mentioned, a small stone dropped onto the surface of the Earth makes a noise. Drop a small stone onto a neutron star and it will release as much energy as a hydrogen bomb! Discoveries made by astronomers in the late-1960s and continuing in the 1970s inspired a great upsurge in the study of very dense matter, for which the relevant experimental results come mainly from astronomical observation

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