Читать книгу Astrobiology - Charles S. Cockell - Страница 35

The matter from which stars, planets, and life are made is probably less than 5% of all the matter in the Universe. That may come as a surprise. Most of the rest of the matter of the Universe is thought to be in more elusive forms such as Dark Matter. The properties of so-called Dark Matter are not known, but this matter was hypothesized to exist to explain certain properties of galaxies, such as their rotation, that cannot be explained without assuming there is a lot more matter in the Universe than the 5% we are familiar with. It is called Dark Matter because of its apparent lack of interaction with electromagnetic radiation (which is one factor that makes it difficult to detect).

In addition, Dark Energy is an elusive form of energy thought to permeate the Universe and has been hypothesized to exist to complete theories about the expansion rate of the Universe. These other types of matter and energy that are thought to exist in the Universe are cosmological areas of science that are hugely interesting. However, as far as we know, they have no direct relevance to life, although of course they are thought to influence the characteristics of the Universe in which life exists.

We will not consider these other forms of matter further in this textbook. Instead, this whole book focuses on the part of the Universe that we call ordinary matter or baryonic matter. It is usually just called “matter.”

The basic building blocks of matter are atoms, themselves constructed from subatomic particles. An atom has a nucleus, which is surrounded by electrons. Apart from hydrogen, which has a single proton, the nucleus of atoms is made of two subatomic particles: protons, which are positively charged, and neutrons, which have no charge. They have very small masses: the proton is 1.672 × 10⁻²⁷ kg; the neutron is 1.675 × 10⁻²⁷ kg. Protons and neutrons are themselves made of elementary particles, called quarks. An elementary particle is a particle that cannot be broken down any further, so they represent the basic building blocks, if you like, of matter. Quarks fall into the domain of the scientific field of particle physics. Although, like the cosmological questions raised earlier, this is an enormously interesting area of science, we will not spend any more time on it here. The electrons that surround the nucleus are negatively charged and they are elementary particles (Figure 3.1).

Figure 3.1 A very simplified depiction of the structure of a typical atom showing the nucleus and electrons. Note that this is not to scale. The electrons are in orbitals about 10 000 times further out than the diameter of the nucleus. Electrons occupy energy levels, depicted here as circles.