NASA's Cosmicopia -- Basics -- Composition

The particles studied in cosmic and heliospheric science are atoms or pieces of atoms. Atoms are composed of three major building blocks: protons (with a positive charge), neutrons (with no charge), and electrons (with a negative charge). The nucleus of an atom contains the protons and neutrons, while the electrons orbit the nucleus. The number of protons determines to which element the atom belongs: hydrogen has one proton, carbon has six, etc.

The Particle Adventure (by the Particle Data Group) introduces the theory of fundamental particles and forces, called the Standard Model. It explores the experimental evidence and the reasons physicists want to go beyond this theory. In addition, it provides information on particle decay and a brief history section.

The Science of Matter, Space, and Time (by the Fermilab) contains another good particle physics "tour". Hydrogen, deuterium, and tritium models

New elements are created in stars by a process called nucleosynthesis. (More about nucleosynthesis...)

The number of neutrons tells us which isotope of the element is present. The isotope number is the total of the number of protons plus the neutrons. Take carbon, for example, which is a very common element necessary for life, and is found in nature as diamonds and graphite. We find several different isotopes of carbon in nature. Carbon-12 has an equal number of protons and neutrons, six of each. Carbon-14 contains two more neutrons (eight) than carbon-12, but still has only six protons. This makes carbon-14 an isotope of carbon, but it is different from carbon-12. While carbon-12 is a stable isotope, carbon-14 is unstable, or radioactive. It is much less common in nature but is found where carbon-12 is found. Because of its radioactive nature, carbon-14 is used to date archaeological artifacts. Shown in this figure, deuterium and tritium are isotopes of hydrogen. This image is from the Princeton Plasma Physics Laboratory,.

The periodic table of the elements provides some information about each of the different elements. Each element has distinctive properties, and those with similar properties are grouped into families, shown by the different colors. Follow the link on the periodic table here to learn more about it and the different elements.

In an atom, the number of electrons that orbit the nucleus equals the number of protons in the nucleus, thus making an atom electrically neutral. When bombarded by ultraviolet (UV) radiation or after being struck by energetic particles, atoms can lose one or more of their electrons. The positively charged remains of these atoms are called ions. The number of electrons lost determines the charge state of the particle. For example, alpha particles are helium nuclei with a double positive charge; they have two protons and two neutrons, but no electrons.

Together with their freed electrons, the ions form a plasma. Plasma is a fourth state of matter, not a liquid, solid, or gas. Matter in the Sun is in a plasma state. Plasma is the most common state of matter in the Universe. More than 99% of all matter is plasma, so what we see on Earth is the exception. Since plasmas consist of electrically-charged particles, electric and magnetic forces affect a plasma.

When we measure the elemental, isotopic, and charge composition of ions, it helps us to understand how nature selected the particles and accelerated them to the energies at which we find them. The "composition" of electrons is not interesting, since all electrons look the same. But their number, energy, and direction of travel are important.

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