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The Universe contains even more extreme forms of matter. Degenerate matter is, in a simplified way, extremely dense matter (Figure 3.21). Degenerate matter was first described for a mixture of ions and electrons in 1926 by Ralph Fowler (1889–1944). In electron degenerate matter, the material is so compressed that electrons are forced to occupy the lowest energy levels, and they become delocalized from their nuclei. However, as we discovered earlier, the Pauli exclusion principle prevents them from occupying identical quantum states, and this generates a pressure, the electron degeneracy pressure, which inhibits the material from being compressed further. The resulting material has a very high density (∼1000 kg cm⁻³).

Figure 3.21 Electron and neutron degenerate matter. In electron degenerate matter, the electrons become delocalized from the nuclei of the atoms. In neutron degenerate matter, the electrons are forced to combine with protons to form neutrons.

To understand this type of matter, it is instructive to move from the atomic and molecular scale, which has generally attracted our attention so far, to the astronomical scale. It is within astrophysical objects that we find this material. This also gives us an opportunity to explore some astronomy.

Electron degenerate matter can be found in white dwarf stars. White dwarfs are the final state of low mass stars such as our Sun. Inside a white dwarf, indeed any star, there is a tug of war. On the one hand, gravitational forces have the effect of collapsing the star, but on the other hand, the pressure of the matter tends to prevent this gravitational collapse from occurring. In a white dwarf, this balance is such that the pressures inside the star are sufficient to form electron degenerate matter. The electron degeneracy pressure prevents further collapse. However, there is an upper limit to the mass of an electron degenerate object, the Chandrasekhar limit, beyond which electron degeneracy pressure cannot support the object against collapse under its own gravity. The limit is approximately 1.44 times the mass of the Sun (solar masses) for objects with compositions like the Sun. If we have a higher mass than this, then the star will collapse further.

If we continue compressing matter, the pressure increases to the point where it is energetically favorable for electrons to combine with protons to produce neutrons (Figure 3.21), and neutron degenerate matter is formed. The density of this material is even greater than electron degenerate matter (>10⁵ kg cm⁻³). This is the material from which neutron stars are constructed. A neutron star has a diameter in the order of one-thousandth of a white dwarf. The interior structure of these objects is uncertain, but one model is shown in Figure 3.22.

Figure 3.22 A hypothetical internal structure of a neutron star.

Neutron stars spin very rapidly with their enormous magnetic fields generating beams of radio or light energy that, if pointing in the direction of Earth, can be detected as pulsars and have a frequency between about 5 and 650 seconds.

Amazingly, even neutron stars have not escaped the attentions of astronomers and planetary scientists as abodes for life. The popular article by Frank Drake, “Life on a neutron star,” published in Astronomy in December 1973 has become something of a classic. This was followed by science fiction stories, for example Robert Forward's books Starquake and Dragon's Egg. These are depictions of the “cheela,” a civilization of tiny beings that live on the surface of a neutron star under its intense gravity. They intervene to help some hapless humans in orbit around their star who are suffering a malfunction on their spaceship. These ideas are fascinating and thought-provoking. However, neutron stars are unlikely places for life. As this textbook progresses, you can consider some of the factors that might cause you to agree or disagree with this statement. You might also like to consider the Discussion Point.