The term "asteroid" either refers to a small Solar System body, excluding comets, or to a small body of rock inside the Jupiter’s orbit.

An astronomical object is any object that exists in the Universe. Examples of astronomical objects include the Moon, the Sun, stars, and galaxies.

The Big Bang is the very first moment when the Universe began to expand. 

A black hole is a place in space where so much mass is concentrated into a tiny point that even light cannot escape from it. The only black holes ever discovered are between a few to a billion times as massive as our Sun.

A comet is a small body that ejects gas and dust (small particles) in the Solar System.

Cosmic microwave background radiation is the light (electromagnetic waves) left over from the earliest moments of the Universe when it was in a hot thermal state. It permeates the entire Universe.

Although being known to have mass and thus exert gravitational effects, dark matter is still an unknown collection of particles, which generally do not react with normal matter. It is believed that it outweighs normal matter by several times in the entire Universe.

A dwarf planet is a planet that is smaller than planets yet large enough to have rounded its shape substantially , such as the asteroid Ceres and Pluto.

An exoplanet is a planet that orbits a star outside the Solar System. 

The Universe has been expanding since it began. The expansion of the Universe does not just mean the distance between astronomical objects is getting larger, but all of space itself, which encompasses those objects, is expanding.

A galaxy is a gravitationally bound system of numerous numbers of stars and gas.

A galaxy cluster is a group of more than a hundred bright galaxies.

A galaxy group is a set of galaxies comprising several to dozens of galaxies. The Milky Way Galaxy is part of a galaxy group that contains dozens of galaxies.

Gravity is a force that pulls an object in proportion to its weight (mass). All objects attract any other object with a force of gravity.

Infrared (infrared light) is electromagnetic waves with wavelengths longer than those of visible light yet shorter than those of radio waves. Because Earth's atmosphere prevents most infrared radiation from reaching the ground, high-altitude mountains provide suitable locations for infrared observations. 

The Universe has large-scale structure with a myriad of galaxies residing in clusters. A supercluster of galaxies forms a part of the structure.

A light-year is the distance that light travels in one year. It is equivalent to about 9.5 trillion kilometers.

The Milky Way is the galaxy that contains our Solar System. It is also called as the “Galaxy” with a capital G. From Earth, the disk-shaped part of the structure appears as the Milky Way band all across our sky. The constellation of Sagittarius in the summer Milky Way marks the direction toward the galactic center.

See Satellite (natural satellite)

A nebula is a region of higher density of gas and dust than surrounding areas, and appears like a cloud. Some nebulae shine with their own light or reflect light from nearby stars, whereas some others, called dark nebulae, absorb light from behind because of dust inside the nebula.

A planet is an astronomical object that orbits a star, without generating energy. The Solar System has 8 planets, such as Earth and Jupiter. Many planets orbiting stars other than the Sun have been found in recent years.

A planetary nebula is a glowing nebula that is formed by outer layers expelled from a red giant. It eventually disappears, at which point, the core remains as a new white dwarf.  The term originates from the planet-like shape of nebulae observed through early telescopes, but it is unrelated to planets.

A quasar is the most luminous object  that emits strong light (electromagnetic waves) from the central region of a galaxy. It is believed that a supermassive black hole exists at the center of a quasar.

A red giant is a type of star that has expanded from its original size, decreasing in surface temperature.  Roughly 5 billion years from now, the Sun will turn into a red giant.

A satellite is a small astronomical body that orbits a planet or dwarf planet, such as the Moon, which is Earth’s satellite. Neither small rock nor ice, as found in Saturn’s rings, is called a satellite.

See Small Solar System body.

A small Solar System body is an astronomical object in the Solar System that is neither the Sun, a planet, a satellite, nor a dwarf planet. It includes asteroids of rock in asteroid belts, comets of ice, astronomical objects of ice that orbit the Sun at greater distances than Neptune.

A star is an astronomical object that shines by generating energy. Most objects visible in the night sky are stars.

A supercluster of galaxy is a large structure of galaxy clusters and groups.

A supernova is a large explosion of a star that is far more massive than our Sun, or a white dwarf that accumulates too much mass. The term supernova was given because a star appears extremely bright all of a sudden when the supernova takes place, but it does not mean new star formation. 

A trans-Neptunian object is a small object in the Solar System that orbits the Sun at a greater distance than Neptune.

Visible light is a form of electromagnetic waves that the human eye can view. It covers the range of wavelengths from 0.4 (violet) to 0.8 (red) micrometers. Stars like the Sun, emit the most energy in the visible portion of the spectrum. 

A white dwarf is an astronomical object that solely consists of a core remnant after a red giant ejects its outer layers. It is hot when it forms but it gradually cools down.