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.
Discussion Point: Can Life be Made from Different States of Matter to Terrestrial Life?
The neutron star dwelling cheela raise an important point about whether life can be made of different states of matter, or even exclusively made from one state of matter. We are made up of solids and liquids, and we exchange gases with the environment. Consider a liquid-only life form. How would information be encoded to allow such a life form to reproduce or repair itself? What about a gas cloud intelligence, such as that found in astronomer Fred Hoyle's science fiction novel The Black Cloud? Would gas molecules be too disordered to allow for information storage, movement, and processing, and how would such an entity evolve in the first place? Can clouds of gas ever reproduce and make similar copies of themselves while adapting to their environment, or are gas molecules generally either too energetic or too dispersed to make systems of reproducing, evolving matter? One could continue with a list of such ideas, and you might want to list every conceivable different type of matter life form you can think of and list what the limitations of such life forms might be. An alternative, and fun, exercise is to consider our own liquid–solid make-up from the point of view of an alien made of a different state of matter and think about some of the things that might make our physics seem unlikely to them. Maybe the unpredictable interactions at solid–fluid interfaces make our own construction unlikely? These sorts of deliberations bring us back to the basic question: Is life on Earth a reflection of universal principles governing the evolution of living things or are we simply narrow-minded in our outlook?
Neutron degenerate matter is associated with huge gravitational forces. When a neutron star has a close companion, it pulls material to it. This material flies down to the surface of the star and collides with the surface, releasing energy. This energy is emitted mostly as X-rays and is modulated with the neutron star spin.
The typical characteristics of a neutron star are that they have a mass greater than 1.4 solar masses, a radius of 10–80 km, and a density of ∼1011 kg cm−3.
There is an upper limit to the mass of a neutron degenerate object, the Tolman–Oppenheimer–Volkoff limit, which is analogous to the Chandrasekhar limit for white dwarfs. The precise limit is unknown. Above this limit, a neutron star may collapse into a black hole (Figure 3.23).
Figure 3.23 A photograph of a black hole. The supermassive black hole is at the center of the galaxy M87. The orange halo is superheated material being swept up into black hole.
Source: Reproduced with permission of Event Horizon Telescope collaboration.
The composition of the interior of a black hole is unknown. Does the interior of black holes contain quark degenerate matter, where the neutrons have themselves disintegrated into their constituent subatomic quarks? Quark degenerate matter has also been suggested to occur in hypothetical “quark stars.”
At the center of a black hole is thought to be a singularity or singularity ring where density is infinite. This can be regarded as some of the most extreme matter in the Universe.
These extreme types of matter are not very relevant for life, although as we have seen, that has not stopped speculation about possible life forms inhabiting objects made of these types of matter. These ideas aside, one reason for investigating these unusual types of matter is that they remind us that the ordinary matter from which known life is made is a tiny subset of the matter in the known Universe. If ordinary matter constitutes say 5% of the matter in the Universe and 99% of that is plasma, then the gases, liquids, and solids that make up biological systems are made from about 0.05% of the types of matter in the Universe.
3.14
