Definitions (Universe)

UNIVERSE

Universe: The vast region of space containing all celestial bodies — galaxies, stars, planets, satellites, comets, meteors, and meteorites.

Celestial body: Any natural object found in space, such as a star, planet, satellite, comet, meteor, or meteorite.

Galaxy: A gigantic system of billions of stars, plus clouds of gas and dust, all bound to one another by gravity.

Milky Way: Our home galaxy, a spiral of stars roughly 100,000 light years across, with the Sun about 30,000 light years from its centre; it holds an estimated 100 to 400 billion stars (some textbooks give 1.5 billion, which is far too low).

Light year: A unit of length used in astronomy: how far light goes in a year, about 9.46 trillion kilometres.

Astrophysics: The part of astronomy that uses physics to explain how stars, galaxies and the universe as a whole began, how they are built and how they will change.

Big Bang theory: The most widely accepted theory of the universe's origin, stating that the universe began about 13.8 billion years ago from the explosion of a single, extremely dense and energetic point.

Fundamental forces: The four basic forces of nature: gravity, the electromagnetic force, the strong nuclear force and the weak nuclear force. Scientists think all four acted as one single force at the very start of the Big Bang.

Hubble's Law: The law stating that the velocity at which a galaxy moves away from Earth is directly proportional to its distance from Earth, expressed as v = Hd.

Hubble's constant (H): The constant of proportionality in Hubble's Law, with an approximate value of 73 km/s per megaparsec.

Critical density: The exact average density at which gravity would be just strong enough to bring the universe's expansion to a halt.

Open universe: A universe whose average density is below the critical density; gravity is too weak to stop it, so it goes on expanding forever.

Flat universe: A universe whose average density is exactly the critical density; its expansion keeps slowing down and comes to a stop only after an endless time.

Closed universe: A universe whose average density is above the critical density; gravity eventually stops the expansion and pulls everything back inward, ending in a Big Crunch.

Big Crunch: The hypothetical collapse of a closed universe back into a single point, essentially the Big Bang happening in reverse.