Astronomy Glossary


Absolute Magnitude: It is a measurement of absolute brightness of star. It can be said as the apparent magnitude of star.

Accretion Disk: It is a circular structure of mass or diffuse material orbiting around a giant object like a star or black hole. The material has been acquired from a external source to the central object.

Active Galactic Nucleus (AGN): It is a central region of a galaxy containg higher mass and displaying a higher luminosity than normal. AGN being strong emitters of Ultraviolet rays, X-rays and radio waves.

Albedo: It is a ratio of incident light with reflected light by the surface typically that of a planet or moon.

Apparent Magnitude: It is a measurement of brightness of a celestial body like star, planet, comet as seen from Earth. The brighter the object appears, the lower(negative) the value of its magnitude.

Appulse: It is an closest apparent separation between two celestial bodies as viewed from a third body.

Asterism: It is a pattern of stars recognized from night sky. It can be a part of an constellation or be composed of stars from more than one.

Astronomical Unit: It is the distance between Earth and the Sun which is known also as 1 AU.

Autumnal Equinox: It is the point when Sun crosses the plane of celestial equator and making equal length of day and night.

Azimuth: It is an angular measurement of an object relative to the north direction.


Black hole: It is concentration of the mass so dense that it creates a region from which not even light can escape.

Break-up Velocity: It is the surface velocity of rapidly spinning star at which the centrifugal force matches with the force of Newtonian gravity. It is also known as critical velocity. Star would begin to eject matter after this point from surface.

Brown Dwarf: It is a substellar object which generates energy from gravitational contraction and by the fusion of deuterium. Such object have low mass and can not sustain the nuclear fusion of hydrogen-1.


Celestial Sphere: It is an appearent sphere which covers the entire sky and is stationary with respect to the background stars.

Chromospheric Activity Index: It is a parameter used in indicating the magnetic activity of chromosphere of Star.

Color Index: It is a value which is used to compare the brightness of a star measured with different frequency bands of the multiwavelength (electromagnetic spectrum), as the energy output of a star varies by frequency.

Comets: These are small and icy celestial objects, shows extended features when they approach the Sun. The energy from the Sun produces a visible coma around the comet by vaporizing volatiles on the surface of comet. Comets revolves around Sun in highly eccentric elliptical orbits.

Commensurability: It is a rational proportion of period between two celestial bodies orbiting to the same object. For example, the orbital periods of Neptune and Pluto around the Sun is very nearly 2:3.

Constellation: It is a set of stars in celestial sphere. The constellation names are assigned by tradition and mythologies.


Declination: It is the angular measurement of the celestial object from Equator of Earth in the equatorial coordinate system. Coordinates north of the celestial equator are measured in positive degrees from 0° to 90°, while negative to the south.

Decretion Disk: It is a circumstellar disk formed from gas ejected from the central star. This type of disk can be found around many Be stars.

Double Star: It is a group of two or more stars appearently close to each other.

Dwarf Star: A star relatively small in size and from main sequence in HR diagram. A star like the Sun in contrast to evolved giant stars like Betelgeuse and Antares.


Early-type Star: Such type of Star is a hotter and more massive. These stars began their early life at a high temperature then cooled off gradually as they aged.

Eccentricity: It is a parameter which used to determine how much an orbit deviates from a perfect circle. For an elliptical orbit, the eccentricity ranges from zero to less one.

Ecliptic Plane: It is the imaginary plane containing the Earth’s orbit around the sun. Path of Sun appearing from earth lies in this plane. Other planetary bodies also follows this path.

Effective Temperature: It is the temperature of a star or planet calculated from the radiation it emits. It is the total amount of electromagnetic radiation.

Evolutionary Track: It is the expected movement of a star in HR (Hertzsprung–Russell) diagram. This Evolutionary track predicts the variations in luminosity and temperature in a lifecycle of that star.

Extinction: It is the most indicative phenomenon revealing the presence of diffuse dark matter. Atmospheric extinction varies by the wavelength of the radiation.


A Field Galaxy: It is a galaxy which lies gravitationally alone and that does not belong to a cluster of galaxies.

A Field Star: An individual star which does not belong any star cluster.


Galactic Tide: This is a process in which an object of small mass is distorted gravitationally by object with larger mass.

Geometric Albedo: It is the ratio of brightness of an astronomical body at zero phase angle to the brightness of a perfectly diffusing disk. It is a measure of how much of the incoming illumination is being scattered back toward an observer.


H II Region: It is a region from space which consist of ionized hydrogen. It is hot and massive region with high energy.


Inferior Planet: Inferior planet is that planet whose orbit lies inside the other planet’s orbit around the Sun. For Earth, Mercury and venus are two inferior planets.

Interstellar Medium: The matter like gas in diffused medium and dust lies between stars in galaxy is known as Interstellar Medium. This may contain Neutral Hydogen, Molecular Hydrogen, ionized Hydrogen, Helium and Dust.

Interstellar Reddening: It is the phenomenon of absorption and scattering of electromagnetic spectrum between source of emission and observer.

Isochrones: It is the expected movement of a star in HR (Hertzsprung–Russell) diagram which represents evolutionary positions of stars having the same age but different masses.


Jeans Instability: It is a physical state of interstellar cloud consisting of gas. When it starts collapsing, it can be convert into star.

Jovian Planets: These planets are Giant gas planets like Jupiter, Saturn, Uranus and Neptune.


Magnetosphere: It is a region aroud planet and forms when plasma like solar wind interacts with magnetic field of that planet.

Main Sequence: It is a category of stars in HR (Hertzsprung–Russell) diagram which fuse hydrogen atoms to helium atoms in their core region. The Sun is an example of main sequence star.

Meteor: When a small body of matter from outer space enters into earth’s atmosphere, it appears as a flare of light as a result of friction.

Meteoroid: It is a small rock or boulder travels through space.

Meteorite: If meteoroid survives to reach the ground while entering earth’s atmosphere, it is then termed a meteorite.

Meteor Shower: When multiple number of meteors from same radiant point observed, its a Meteor shower. It happens when Earth passes through the path of a comet.


Nebula: It is an interstellar cloud of hydrogen, helium, dust and other ionized gases. This is a region where new stars are being formed or reminant of dead star. Historically it was referred as source of luminosity.

Neutron Star: It is a supernova explosion remnant of a massive star and composed of neutrons. Neutron Star can have a mass between 1.35 to 2.0 times the mass of the Sun and with a very short radius of around 12 km.


Open Cluster: It is a set of stars that formed in the same molecular cloud and loosely gravitationally bound to each other.

Opposition: When two celestial objects are on opposite sides in the sky, that moment is known as opposition.

Orbital Elements: These are parameters that uniquely define an orbit.


Parsec: It is the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth’s orbit. It
is combination of two words, parallax and second. It is equal to 3.2616 light years.

Periapsis: It is the closest approach point between two orbiting objects.

Phase Angle: It is the angle between an celestial body and the Sun as viewed Earth or from a particular point from solar system. It determines the amount of visible surface that lies in shadow for a specific planet or moon.

Precession Motion: The rate of change in the orientation of an object’s axis of rotation is known as Precession Motion. Precision cycle of Earth is 25,765 years.

Proper Motion: It is the rate of angular motion of an celestial object over an interval of time.

Protostar: When interstellar cloud starts contracting, the central regions is known as protostar. Once central region collects sufficient mass, it becomes ready for main sequence star.


Quasar: A quasar is an active galactic nucleus which consists of a super-massive black hole surrounded by an accretion disk of gas. These are most distant objects and strong radio sources in the known universe. Some times it also referred as quasi-stellar object.


Radial Velocity: The velocity of an celestial object along the line of sight to the observer.

Right Ascension (RA): It is the celestial equivalent of terrestrial longitude in the equatorial coordinate system. It divides the celestial equator into 24 hours, each of 60 minutes.

Roche Limit: The distance where tidal force matches with a gravitational attraction. Inside the limit, body starts to deform by
the tidal forces.