Carbon	A chemical element with six protons that is the basis for all known life on Earth.
Chemical Element	A substance whose atoms are all the same (that is, each atom contains the same number of protons as each of the other atoms in the substance). Sometimes, the word “element” is used to refer to the atoms or atomic nuclei themselves, as in the statement “Many elements are formed as products of dying stars.”
Chemistry	The scientific study of the composition, structure, properties, and reactions of different forms of matter.
Cluster	A group of galaxies held together by their mutual gravitational pulls.
Cosmic Horizon	The distance in our Universe beyond which we cannot see (46-billion to 47-billion light- years from Earth). Light from beyond the cosmic horizon has not yet had enough time (in the history of the Universe) to reach us.
Density	The mass per unit of volume of a substance.
Fusion	The combining of lighter atomic nuclei into heavier atomic nuclei. This process can release a great deal of energy, and is what powers most stars.
Galaxy	A huge system of stars, interstellar dust, and dark matter, held together by mutual gravitational pull.
Ion	An atom that has a different number of protons than electrons, giving it an overall positive or negative charge.
Iron	A chemical element with 26 protons. The most common chemical element in the planet Earth, iron forms the majority of Earth’s inner and outer core. The process of creating new elements through nuclear ​fusion in stars ends with iron, since fusing atomic nuclei together to produce elements heavier than iron does not produce energy.
Milky Way Galaxy	The spiral-shaped galaxy that contains our Solar System.
Neutron Star	One possible end product of supernovae. When a star much more massive than our Sun runs out of fuel, its core may collapse to produce a ball of neutrons more dense than virtually anything else in the Universe.
Periodicity	Regular, recurring trends. For example, a Cepheid variable star exhibits periodicity because its brightness changes in a regular, predictable way that repeats over time.
Periodic Table of Elements	The generally accepted system for organizing the known chemical elements. Russian chemist Dmitri Mendeleev first used this method of arrangement in 1869. As new elements are discovered, they are added to the table.
Periodization	An approach to history in which time is divided into distinct and identifiable periods (for example, the Jurassic or Paleolithic.).
Plasma	A state of matter in which protons and electrons are not bound together. This was the state of the entire Universe roughly before 380,000 years after the Big Bang, and is the normal state inside stars.
Radioactivity	The breakdown of an unstable atomic nucleus, such as uranium, through the spontaneous emission of subatomic particles.
Star	A huge, glowing ball of plasma held together by its own gravity. Stars, the first complex entities in the Universe, have structure, stability, and a sustained flow of energy due to nuclear fusion at their centers.
Supercluster	A large group of galaxy clusters that together form some of the largest known structures in the Universe.
Supernova	The explosion of a large star at the end of its life; most chemical elements are created by supernova explosions.