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The Big Bang Theory -

The Big Bang Theory describes the origin of the universe. It explains how the universe expanded from a high-density, high-temperature state into the cosmos we see today. 

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Asteroid -

Asteroids are chunks of rock or metal that used to be parts of other small planets that have broken off due to a collision in space and now orbit the sun. 

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Galaxy -

We all know that we live in the Milky Way galaxy, but what exactly is a galaxy? Well, a galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity.

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Red giant -

A red giant is a star that's reaching its final days and approaching death. Our own sun will eventually expand and turn into a red giant, but not for a few billion years. 

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Celestial sphere -

A celestial sphere is a tool used in spherical astronomy. It's a sphere with a large radius that's concentric with Earth. The sky surrounding the Earth is projected on the sphere, which helps astronomers understand positions in situations where distances aren't important.

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Escape velocity -

Escape velocity is the minimum speed needed for an object to escape from a planet or moon's gravitational pull. For example, the escape velocity for a spaceship to leave the surface of Earth is about 25,000 miles per hour. 

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Black hole -

Made famous by Albert Einstein's theory of general relativity, a black hole is the small, dense core remnant of a dead star.

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Light-year -

A light-year is an astronomical distance that measures the distance that light travels in a year. One light-year roughly equates to six trillion miles.

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Solar system -

A solar system is a gravitationally bound system of a sun and the objects that orbit it.

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Dark matter -

Dark matter is a component of the universe whose presence is noticed from its gravitational attraction rather than its luminosity. Although it covers a large chunk of the universe, it still remains quite mysterious to scientists.

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Quasar -

Quasars are large distant objects in space, which are powered by large, powerful black holes. They tend to shine so brightly that their light eclipses the old galaxies that they exist within. 

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Milky Way -

The Earth's galactic home, the Milky Way is a spiral galaxy that's constantly rotating. Our solar system lies about 30,000 light-years from the galaxy's center.

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Comet -

Comets are icy, frozen balls of gas, rock, and dust, which get their bright glow by emitting gas as their orbit passes close to the sun.

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Andromeda Galaxy -

Also known as M31, this galaxy is the closest to the Milky Way, where the Earth exists. It has a similar structure to the Milky Way, and can be seen from Earth with the unaided eye, particularly on fall and winter nights. 

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Gravity -

Gravity is a natural phenomenon by which a planet or other body draws objects toward its center. The force of gravity is what keeps us planted on Earth's surface and also keeps the Earth orbiting the sun. 

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Eclipse -

An eclipse refers to one body in space moving into the shadow of another body. Eclipses can exist in two forms: solar and lunar.

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Hypergalaxy -

A hypergalaxy consists of one large dominant galaxy surrounded by various smaller tertiary galaxies. The Milky Way and the neighboring Andromeda galaxies are both hypergalaxies.

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Equinox -

During an equinox, Earth experiences an even 12 hours of daytime and 12 hours of nighttime. This occurs twice a year: The March equinox marks spring's arrival in the Northern Hemisphere and fall in the Southern Hemisphere. The September equinox marks the fall's arrival in the north and spring in the south.

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Supernova -

A supernova is the largest explosion that can be seen with the human eye. A supernova occurs as the last step of a dying massive star, which emits an extremely bright light.

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Meteor -

Known as shooting stars, meteors are rocky objects or asteroids that vaporizes when entering the Earth's atmosphere. 

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Nebula -

A nebula is a large cloud of dust and gas in space that is produced when a star dies. The gas and dust eventually clump together into a mass large enough to form its own gravity.

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Orbit -

An orbit is the circular motion in which objects in space travel due to gravity's pull. The moon orbits around the Earth, and the Earth orbits around the sun in a circular motion. An object that orbits another is called its satellite.

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Alpha Centauri -

Alpha Centauri is the name of the closest star system to Earth. It comprises two main stars, Alpha Centauri A and Alpha Centauri B, as well as the closest of the three stars, Proxima Centauri. 

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Star cluster -

Star clusters are large groups of stars held together by self-gravitation. Two types of star clusters can be determined: globular clusters and open clusters. 

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Gamma-ray burst -

Gamma-ray bursts are the most powerful and violent explosions in the known universe. When it happens, it's the brightest source of light in the observable universe. Satellite evidence suggests gamma-ray bursts are the result of matter collapsing into black holes.

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Constellation -

Constellations map out a group of stars that form shapes in the sky. Explorers first used constellations to map directions, as well as a tool for storytelling.

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Magellanic clouds -

The Magellanic clouds consist of two irregular galaxies that orbit the Milky Way. Orbiting about 200,000 light-years away, the Magellanic clouds are formed by the Large Magellanic Cloud and the Small Magellanic Cloud.

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Neutron star -

Neutron stars are small, but dense objects that are birthed by the explosive death of a giant star. There are two types of neutron stars. The first is a pulsar, which emits a narrow radiation beam. The second is a magnetar, which has a powerful magnetic field that has the force to distort atoms.

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Exoplanet -

When planets exist beyond our solar system and orbit a star other than the sun, they're called an exoplanet. Extremely hard to see, the first exoplanet wasn't recognized until 1992.

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Aberration of light -

An aberration of light is the phenomenon where stars or other celestial bodies appear from Earth to be slightly off from their true position. This happens because the interaction of the Earth's movement with the speed of light.
Sources: (Stacker) (Space) (NASA) (Wired) 
See also: A history of famous firsts in space

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