Terminologies of Astronomy

We have looked into our past and saw how humanity understood the heavens in a much detailed way and true nature of the cosmos. We will be diving in more detailed stuffs later on, but before that we need to understand the basic terminologies used in Astronomy.

Astronomical terms can be a little technical and difficult to understand. Not to worry anymore! Below are few Astronomical terms which will help you understand the future blogs in this series.

Absolute magnitude

It is defined as the apparent magnitude of a celestial body would show if it were located at a distance of 10 parsecs, or 32.6 light-years.

Apparent Magnitude

It is defined as the measure of the brightness of a celestial body observed from earth

Astronomical Unit

The average distance between the centers of the Earth and the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometer or about 150 million kms (93 million miles)

Accretion Disk

A circular mass of diffused material in orbit around a heavy object usually a star or a black hole


A common center of mass about which any two or more celestial bodies orbit. Its location is dependent on mass of the bodies in the system and their relative distance.

Escape Velocity

The minimum speed that must be achieved by an object to escape from the gravitational influence of a massive body.

Galactic Year

 The time taken by an astronomical body within a galaxy to complete one orbit around the galactic center. Galactic year of our solar system is around 225 to 250 million years.


The point at which a body is orbiting the earth (moon or artificial satellite) is farthest from the earth.


The point at which a body is orbiting the earth (moon or artificial satellite) is closest to the earth.


The point at which a body orbiting our Sun is farthest from the sun.


The point at which a body orbiting our Sun is closest from the sun.

Light Year

Distance travelled by a beam of light in a year. It is approximately 9.46 trillion kilometers


Unit to measure large astronomical distances outside solar system. It is 3.26 light year or 30.83 trillion kilometers

Circumstellar disc

A ring shape where there is accumulation of matter composed of gas, dust, asteroids etc around a star.


The scientific study and origin of celestial objects and phenomenon involved in their evolution. It deals with studies such as formation of solar system, black holes, neutron star etc.


The scientific study of the origin, evolution and fate of universe. It deals with theories such as Big Bang, Big Freeze, Big Rip etc.


The scientific study of early origin, evolution and distribution of life in the universe.


The path of an astronomical body revolving around another astronomical body.

Extrasolar Object

Any astronomical object that exists outside the solar system. This term is generally not used for stars or any other astronomical body larger than a star or solar system. For eg: Oumuamua can be considered as extrasolar object but proxima centauri can’t be considered as a extrasolar object as it’s a star.


Any planet which is outside the solar system.

Galactic Nucleus

The region at the center of a galaxy, usually home to very dense concentration of stars and dust. Every galactic nucleus contains a supermassive black hole around which the dense concentration of stars and dust move along with all the stars in the galaxy

Gamma-Ray Burst (GRB)

A catastrophic event that generates intense outburst of gamma ray radiation whose source is theorized to be supernova or Hypernova explosion of massive stars. Short GRBs may also be a result of collision of neutron stars.

Gas Giant

A giant planet composed mainly of hydrogen and helium gases rather than heavier elements. Saturn and Jupiter as two gas giants in our solar system

Interstellar Medium

The matter and radiation that exists between the stars in a galaxy. This medium mainly consists of hydrogen and helium.


The region around a star or planet which is formed when plasma such as solar wind interacts with the magnetic field of that body.


A piece of meteor which manage to successfully survive its passage through planet’s or Moon’s atmosphere and touch down its surface. All the rocks that are falling down to earth from space are meteorite and not meteors


A small rock or boulder that has entered a planet’s or moon’s atmosphere. If it reached to the surface, then it is termed as Meteorite.


A relatively small, icy body that displays extended features (tail) when it approaches the sun. As the comet approaches the sun, solar radiation vaporizes the volatile material of its surface creating a tail. The tail will always point away from the sun due to solar radiation pressure.


Asteroids are minor planets, especially inside our solar system. Larger asteroids have also been termed as Planetoids. Asteroids are generally in different size and not usually round like a normal planet.


The total amount of energy emitted by a star, galaxy or any other astronomical object. It is measures in Joules per second or Watts per second. Our sun emits 3.83 x 10^26 watts per second. In comparison, total energy consumption of the world in a year is approximately 4.75 x 10^17 watts

Near-Earth Object

Any small solar system body such as asteroid or comet whose proximity with earth is less than 1.3 AU from sun at its closest approach

Orbital Plane

The imaginary plane defined by the orbit of an astronomical body around another body.


An increase in the wavelength and decrease in the frequency and energy of photon. This happens when an object is moving away from the observer.


A decrease in the wavelength and increase in the frequency and energy of photon. This happens when an object is moving towards the observer.

Retrograde Motion

Orbital or rotational motion of an astronomical body is opposite to the rotation of primary’s object.

Roche Limit

The distance from an astronomical body at which the tidal force of the primary object will cause the orbiting body to disintegrate and usually form a ring.

Surface Gravity

Gravitational acceleration experienced at the equatorial surface of an astronomical body. The surface gravity of earth is 9.806 m/s^2

Rogue Planet

A planetary mass object which revolves directly the galactic center instead of revolving around a star or substellar object.