Here is the astronomy glossary i.e., some key terms that being used frequently in the field of astronomy:
Astronomy glossary is a collection of terms and definitions related to the study of astronomy. These terms are used to describe astronomical phenomena, objects and concepts in the field of astronomy.
We need an astronomy glossary because it helps to standardize the language used in the field of astronomy and allows for clear communication between astronomers. It also helps non-experts to understand astronomical concepts and terms that they may come across in popular science articles or books. Additionally, an astronomy glossary can serve as a useful reference tool for students or anyone interested in learning more about astronomy.
A
Absolute Magnitude: It is a measurement of the absolute brightness of the star. It can be said as the apparent magnitude of a 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 an external source to the central object.
Active Galactic Nucleus (AGN): It is a central region of a galaxy containing higher mass and displaying a higher luminosity than normal. AGN is a strong emitter 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.
Angular diameter: The angle subtended by an object as seen by an observer.
Aphelion(Apogee): The point in an object’s orbit at which it is farthest from the Sun.
Apparent Magnitude: It is a measurement of the brightness of a celestial body like a star, planet, or comet as seen from Earth. The brighter the object appears, the lower(negative) the value of its magnitude.
Appulse: It is the closest apparent separation between two celestial bodies as viewed from a third body.
Arcminute: A unit of angular measurement equal to 1/60 of a degree.
Arcsecond: A unit of angular measurement equal to 1/3600 of a degree.
Asteroid: A small, rocky object that orbits the Sun.
Astrometry: The branch of astronomy that deals with the measurement of the positions and motions of celestial bodies.
Atmosphere: The layer of gases surrounding a planet or other celestial body.
Atom: The basic building block of matter, consisting of a nucleus of protons and neutrons surrounded by electrons.
Asterism: It is a pattern of stars recognized from the night sky. It can be a part of a 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 the Sun crosses the plane of the celestial equator and makes equal lengths of day and night.
Azimuth: It is an angular measurement of an object relative to the north direction.
B
Big Bang: The theory that the universe began as an extremely hot and dense state and has been expanding ever since.
Black hole: It is the 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 a 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 the surface.
Brown Dwarf: It is a substellar object which generates energy from gravitational contraction and by the fusion of deuterium. Such objects have low mass and can not sustain the nuclear fusion of hydrogen-1.
C
Celestial Sphere: It is an apparent sphere that 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 the chromosphere of a Star.
Color Index: It is a value that 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, show 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 the comet. Comets revolve around Sun in highly eccentric elliptical orbits.
Commensurability: It is a rational proportion of the period between two celestial bodies orbiting to the same object. For example, Neptune and Pluto’s orbital periods around the Sun are nearly 2:3.
Constellation: It is a set of stars in a celestial sphere. The constellation names are assigned by tradition and mythologies.
Cosmic rays: High-energy particles that travel through space.
Cosmology: The study of the origin, structure, and evolution of the universe.
D
Dark matter: Matter that does not emit, absorb, or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from its gravitational effects.
Declination: It is the angular measurement of the celestial object from the 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.
Doppler shift: A change in the wavelength of light or other electromagnetic radiation caused by the motion of the source or the observer.
Double Star: It is a group of two or more stars appear close to each other.
Dwarf Star: A star relatively small in size and from the main sequence in the HR diagram. A star like the Sun in contrast to evolved giant stars like Betelgeuse and Antares.
E
Early-type Star: Such type of Star is hotter and more massive. These stars began their early life at a high temperature and 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 than one.
Eclipse: An event that occurs when one celestial body moves into the shadow of another.
Ecliptic Plane: It is the imaginary plane containing the Earth’s orbit around the sun. The path of the Sun appearing from earth lies in this plane. Other planetary bodies also follow 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.
Electromagnetic radiation: Energy that travels through space in the form of waves.
Evolutionary Track: It is the expected movement of a star in the HR (Hertzsprung–Russell) diagram. This Evolutionary track predicts the variations in luminosity and temperature in the lifecycle of that star.
Exoplanet: A planet that orbits a star other than the Sun.
Extinction: It is the most indicative phenomenon revealing the presence of diffuse dark matter. Atmospheric extinction varies by the wavelength of the radiation.
F
A Field Galaxy: It is a galaxy that lies gravitationally alone and that does not belong to a cluster of galaxies.
A Field Star: An individual star that does not belong to any star cluster.
G
Galactic Tide: This is a process in which an object of small mass is distorted gravitationally by an object with a larger mass.
Galaxy: A large collection of stars, gas, and dust held together by gravity.
General Relativity: A theory of gravity proposed by Albert Einstein in which gravity is described as the curvature of spacetime caused by mass and energy.
Geocentric: A model of the solar system in which the Earth is at the center.
Geometric Albedo: It is the ratio of the 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
Heliocentric: A model of the solar system in which the Sun is at the center.
H-R diagram(Hertzsprung–Russell): A graph introduced by Ejnar Hertzsprung and by Henry Norris Russell which plots the luminosity of stars against their surface temperature.
H II Region: It is a region from space that consists of ionized hydrogen. It is a hot and massive region with high energy.
I
Inertia: The property of an object that tends to resist a change in motion.
Inferior Planet: Inferior planet is a 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 a galaxy is known as Interstellar Medium. This may contain Neutral hydrogen, Molecular Hydrogen, ionized Hydrogen, Helium, and Dust.
Interstellar Reddening: It is the phenomenon of absorption and scattering of the electromagnetic spectrum between the source of emission and the observer.
Isochrones: It is the expected movement of a star in the HR (Hertzsprung–Russell) diagram which represents the evolutionary positions of stars having the same age but different masses.
J
Jeans Instability: It is the physical state of an interstellar cloud consisting of gas. When it starts collapsing, it can be converted into a star.
Jovian Planets: These planets are Giant gas planets like Jupiter, Saturn, Uranus, and Neptune.
K
Kuiper Belt: A region of the solar system beyond Neptune that is populated by small, icy objects.
L
Light year: It is the distance light travels in one year. It is around 5.88 trillion miles (9.46 trillion kilometers).
Luminosity: The amount of energy emitted by a celestial object per unit time.
M
Magnetosphere: It is a region around the planet and forms when the plasma-like solar wind interacts with the magnetic field of that planet.
Main Sequence: It is a category of stars in HR (Hertzsprung–Russell) diagram which fuses hydrogen atoms to helium atoms in their core region. The Sun is an example of a main-sequence star.
Magellanic Clouds: Two small galaxies that orbit the Milky Way.
Magnitude: A measurement unit of the brightness of a celestial object.
Main sequence star: The region on the H-R diagram where most stars fall.
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 that travels through space.
Meteorite: If a meteoroid survives to reach the ground while entering the earth’s atmosphere, it is then termed a meteorite.
Meteor Shower: When multiple meteors from the same radiant point are observed, it’s a Meteor shower. It happens when Earth passes through the path of a comet.
Milky Way: The galaxy in which the Earth and the solar system are located.
N
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 remnants of a dead star. Historically it was referred as a 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.
Nova: A star that suddenly increases in brightness.
Nuclear fusion: A process in which atomic nuclei come together to form heavier nuclei, releasing energy in the process.
O
Oort Cloud: A hypothetical cloud of comets that surrounds the solar system.
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.
P
Parallax: The apparent shift in the position of an object when viewed from different locations.
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 a combination of two words, parallax and second. It is equal to 3.2616 lightyears.
Penumbra: The outer part of a shadow where only part of the light source is partially blocked.
Periapsis: It is the closest approach point between two orbiting objects.
Phase Angle: It is the angle between a celestial body and the Sun as viewed from Earth or from a particular point in the solar system. It determines the amount of visible surface that lies in shadow for a specific planet or moon.
Photon: A elementary particle of light.
Precession Motion: The rate of change in the orientation of an object’s axis of rotation is known as Precession Motion. The precision cycle of Earth is 25,765 years.
Proper Motion: It is the rate of angular motion of a celestial object over an interval of time.
Protostar: When the interstellar cloud starts contracting, the central region is known as a protostar. Once the central region collects sufficient mass, it becomes ready for the main sequence star.
Pulsar: A rapidly spinning neutron star that emits regular pulses of radio waves.
Q
Quasar: A quasar is an active galactic nucleus that consists of a super-massive black hole surrounded by an accretion disk of gas. These are the most distant objects and strong radio sources in the known universe. Sometimes it is also referred as a quasi-stellar object.
R
Radial Velocity: The velocity of an celestial object along the line of sight to the observer.
Red giant star: A large, cool, luminous star that is approaching the end of its life.
Redshift: A shift in the wavelength of light to longer wavelengths caused by the expansion of the universe or the motion of the source.
Retrograde motion: The apparent motion of a planet in the opposite direction to the motion of the other planets.
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.
S
Satellite: It is a natural or artificial object that orbits around a planet.
Sidereal Month: A sidereal month is a time it takes for the Moon to complete one orbit around the Earth with respect to the fixed stars. It is approximately 27.32 days long.
Solar Cycle: The solar cycle, also known as the sunspot cycle, is the periodic variation in the number of sunspots that occur on the surface of the Sun. The solar cycle typically lasts around 11 years, during which time the number of sunspots increases and decreases in a regular pattern.
Solar Eclipse: A solar eclipse is an event that occurs when the Moon passes between the Sun and the Earth, casting a shadow on the Earth’s surface. This causes the Sun to appear partially or completely blocked from view for a short period of time.
Solar Nebula: The solar nebula is a hypothetical cloud of gas and dust from which the solar system is thought to have formed.
Solar Wind: Solar wind is a stream of charged particles that are continuously emitted from the Sun’s upper atmosphere, called the corona. These particles, mostly protons and electrons, are continuously being expelled from the Sun at high speed.
Solstice: Solstice refers to the moment when the tilt of the Earth’s axis is most inclined towards or away from the sun, causing the longest or shortest day of the year.
Spectroscopy: The study of the spectrum of celestial objects like stars, planets, comets, etc.
Spiral Galaxy: A spiral galaxy is a type of galaxy that has a flat disk-like structure and spiral arms that contain stars, gas, and dust. These arms are thought to be formed by the density waves that rotate around the galaxy’s center.
Star Cluster: A star cluster is a group of stars that are located in close proximity to each other and are gravitationally bound.
Supergiant star: A star that is very large, and luminous that is approaching the end of its life.
Supermoon: A full moon that occurs when the Moon is at its closest point to the Earth in its elliptical orbit, also known as the perigee.
Supernova: The explosion of a star at the end of its life.
T
Telescope: An instrument that is used to observe distant celestial bodies.
Terrestrial Planet: A terrestrial planet is a type of planet that is primarily composed of rock or metal, as opposed to gas or ice. Examples of terrestrial planets in our solar system include Earth, Mars, Venus, and Mercury.
Terminal Velocity: Terminal velocity is the highest velocity that an object can reach as it falls.
Tesla: The Tesla unit (abbreviated as T) is the SI unit of magnetic flux density, also known as magnetic induction. It is named after the inventor and electrical engineer Nikola Tesla.
Tidal Force: Tidal force is the gravitational force exerted by one celestial body, such as a planet or moon, on another celestial body, such as an ocean or another moon.
Trans-Neptunian Object (TNO): Objects that orbit the Sun at a distance greater than that of Neptune, the outermost planet in our solar system. TNOs include the dwarf planets Pluto and Eris, as well as many other smaller objects.
Transit: It is a phenomenon when an observer sees a celestial body across the face of another(bigger) celestial body.
Triple Star: A triple star system is a system of three stars that are gravitationally bound to each other and orbit a common center of mass. Some famous examples include Alpha Centauri and Mizar.
Trojans: Trojans are a group of small celestial bodies that share an orbit with a larger celestial body, such as a planet or a moon.
Twilight: Twilight is the period of time just before and after sunrise and sunset, when the sky is partially illuminated by the sun, but the sun is below the horizon. Twilight is caused by the scattering of sunlight by the Earth’s atmosphere.
U
Ultraviolet: Ultraviolet (UV) refers to a type of electromagnetic radiation with a wavelength shorter than that of visible light but longer than X-rays. It is invisible to the human eye.
Umbra: An Umbra shadow is the dark central region of a shadow cast by an object, where the object completely blocks the light source.
Universe: All of the matter and energy that exists, including all galaxies, stars, and planets.
Universal Time (UT): Universal Time (UT) is a time scale that is based on the rotation of the Earth. It is also known as Greenwich Mean Time (GMT), as it is based on the time at the Prime Meridian (0 degrees longitude) which passes through the Royal Observatory in Greenwich, London, England.
V
Vacuum: A vacuum is a region of space that is empty of matter.
Vernal Equinox: The Vernal equinox, also known as the spring equinox, is the moment when the sun is directly above the Earth’s equator, resulting in equal amounts of daylight and darkness for all locations on Earth.
Variable star: A star that changes in brightness over time.
Virgo Supercluster: The Virgo Supercluster is a massive group of galaxy clusters that includes the Virgo Cluster, which is the closest cluster of galaxies to the Milky Way, and is located about 60 million light-years away from Earth.
W
Waning Phase: The Waning Phase is a phase of the lunar cycle, where the illuminated portion of the Moon as seen from Earth is decreasing, after the full Moon and before the new Moon.
Waxing Phase: The Waxing Phase is a phase of the lunar cycle, where the illuminated portion of the Moon as seen from Earth is increasing, after the new Moon and before the full Moon.
Wavelength: In astronomy, wavelength refers to the distance between consecutive peaks or troughs of a wave of electromagnetic radiation.
White dwarf: A small, dense, hot star that is the remnant of a low-mass star that has exhausted its nuclear fuel.
X
X-ray: Electromagnetic radiation with a wavelength shorter than ultraviolet but longer than gamma rays.
X-ray Star: An X-ray star is a type of astronomical object that emits X-rays, which are a form of high-energy electromagnetic radiation.
Y
Yellow Dwarf: A yellow dwarf is a type of star that is slightly cooler and less massive than the Sun. These stars are called “dwarfs” because they are smaller in size.
Z
Zenith: The point on the celestial sphere that is directly overhead.
Zodiac: The band of the sky that contains the twelve constellations through which the Sun, Moon, and planets appear to move.