Glossary — Space physics

Artificial satellite — an object built by people and put into orbit around a planet or another body.

Big Bang — the model that the universe began from a very small region that was extremely hot and dense, and has been expanding ever since.

Cosmic microwave background radiation (CMBR) — a faint microwave signal that comes from every direction in space, left over from the hot, dense early universe.

Dark energy — the unknown cause of the speeding up of the expansion of the universe.

Dark matter — matter that does not emit electromagnetic radiation but pulls on galaxies. Its nature is not known.

Dwarf planet — a small, roughly spherical body that orbits the Sun but is not one of the eight planets.

Galaxy — a huge group of stars. The Sun is one star in the Milky Way galaxy.

Main sequence — the long stage in the life of a star when it fuses hydrogen into helium, such as the Sun.

Milky Way — the galaxy that contains the solar system.

Moon — a natural satellite that orbits a planet.

Nebula — a cloud of dust and gas. Stars form from a nebula.

Neutron star — the small, very dense core left behind after a massive star explodes as a supernova.

Orbit — the path of an object around a larger body, kept in place by gravity.

Planet — a large, roughly spherical body that orbits a star.

Red giant — the stage in the life of a Sun-sized star when it expands after the main sequence.

Red-shift — the increase in the wavelength of light from a source that is moving away from the observer.

Solar system — the Sun and everything that orbits it: the planets, the dwarf planets, the moons, the asteroids and the comets.

Spectrum — the range of wavelengths in light, crossed by dark lines at fixed wavelengths.

Supernova — the explosion of a massive star at the end of its life. Elements heavier than iron are made in a supernova.

White dwarf — the small, dense, cooling core left behind after a Sun-sized star becomes a red giant.