Astronomy Dictionary
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Absolute Magnitude: A measure of a celestial object's intrinsic brightness, regardless of its distance. It calculates how bright an object would appear if it were exactly 10 parsecs (approx. 33 light-years) away from Earth. On this scale, the Sun's absolute magnitude is +4.8, while its Apparent Magnitude is -26.7 because it is so close.Absolute Zero: The temperature at which all atomic and molecular motion stops and no heat is emitted. This is 0 Kelvin or -273.16° Celsius.Accretion: The process by which particles stick together to form larger objects; for example, solar system dust gathered to form Chondrules, and Planetesimals merged to form planets.Achondrite: A stony meteorite that represents differentiated planetary material.Albedo Feature: Dark or light markings on an object's surface that may or may not be geological or topographical. Albedo is a measure of a planet's reflectivity, measured on a scale of 0 to 1. An albedo of 0 means the planet absorbs all light, while 1 means it reflects all light.Alpha Centauri: The closest bright star system to our solar system.Altitude: The angle measured in degrees above the horizon.Angstrom (Å): A unit of length equal to $10^{-8}$ cm (one hundred-millionth of a centimeter). An Angstrom is roughly the size of an atom.Antimatter: Matter in which the particles have the opposite electrical charge of normal matter; protons have a negative charge and electrons have a positive charge.Antipodal Point: A point on the exact opposite side of a planet; for example, Earth's North Pole is the antipodal point of the South Pole.Apastron: The point of maximum separation between two stars in a binary star system.Aperture: The size of the opening through which light passes in optical instruments like cameras or telescopes. A larger number indicates a smaller aperture, and a smaller number indicates a larger aperture.Aphelion: The point in a planet's orbit that is farthest from the Sun.Apoapsis: The point in an orbit farthest from the host planet.Apogee: The point in an orbit farthest from the Earth.Apparent Magnitude: How bright an object appears in the sky to an observer on Earth. Brighter objects have lower (or negative) numbers, and dimmer objects have higher numbers.Arc Second: A unit of angular measurement; there are 60 arc seconds in 1 arc minute and 3,600 arc seconds in 1 arc degree. On the Sun, one arc second is approximately 725 km.Arc Degree: A unit of angular measurement, where a full circle contains 360 arc degrees.Arc Minute: 1/60th of a degree.Asteroid: A small object orbiting the Sun, larger than a meteoroid but smaller than a planet. Most are found in the belt between Mars and Jupiter.Astrochemistry: The branch of science that studies chemical reactions between dust and gases in interstellar space.Astronomical Unit (AU): The average distance from the Earth to the Sun; 1 AU is 149,597,870 km (92,960,116 miles).Atmosphere: One atmosphere is 14.7 pounds per square inch, which is the average pressure at sea level on Earth. An atmosphere is also the layer of gases surrounding a planet, moon, or star.Aurora: Light produced in the ionosphere by the interaction between a planet's magnetic field and charged particles from the Sun.Aurora Borealis: Light visible in the Northern Hemisphere caused by solar winds and Earth's magnetic field. In the Southern Hemisphere, this is called Aurora Australis.Autumnal Equinox: The point on the celestial sphere where the Sun crosses the celestial equator from north to south. This occurs around September 22 each year, marking the beginning of autumn.Average Orbital Speed: A measure of how fast a planet moves (in km/h) as it orbits the Sun in space.Axial Inclination: The angle at which a planet's axis is tilted relative to its orbit; this tilt is responsible for seasons on Earth.Axis: An imaginary line passing through the center of rotation of an object.Azimuth: The angular distance of an object measured along the horizon from a specific zero point.B
Bar: A unit of atmospheric pressure. One bar equals 0.987 atmospheres or 14.7 pounds/square inch.Barycenter: The center of mass of a system of objects.Basalt: Dark-colored igneous rocks rich in iron and magnesium.Big Bang: The theory of the universe's origin, which suggests the universe began with a massive explosion approximately 18 billion years ago.Binary Stars: A system of two stars rotating around a common center of gravity.Black Hole: The collapsed core of a massive star. Its gravity is so powerful that even light cannot escape, and all matter is compressed into a Singularity.Blueshift: When an object moves toward the observer, the lines in its spectrum shift toward the blue end.Bolide: An exploding meteor.C
Caldera: A large circular crater formed by a volcanic explosion or the collapse of its surface.Carbonaceous Chondrite: A type of primitive meteorite that contains evidence of nebular processes.Celestial Equator: The projection of Earth's equator onto the celestial sphere.Celestial Pole: Points on the celestial sphere where Earth's axis meets the sphere.Celestial Sphere: An imaginary sphere with Earth at its center, upon which all stars appear to be located.Cepheid: A pulsating variable star. Its brightness changes at regular intervals, making it a powerful tool for measuring distances in astronomy.CGS: Centimeter-Gram-Second; a system of measurement for distance, mass, and time.Chondrite: A meteorite containing Chondrules and other components of the solar nebula.Chondrule: Small glass-like spheres found in meteorites.Chromatic Aberration: A lens defect that causes colored edges to appear because the lens focuses different colors of light at different points.Chromosphere: The layer of the Sun's atmosphere located above the photosphere and below the corona.Circumpolar Star: A star that never sets and always stays above the horizon; such as Polaris (the North Star) in the Northern Hemisphere.Coma: The cloud of gas surrounding the nucleus of a comet, which can extend for millions of miles.Comet: A body of frozen gases, ice, and rock that orbits the Sun. As it nears the Sun, a long tail becomes visible behind it.Conjunction: When two or more objects appear very close to each other in the sky.Constellation: A group of stars in the sky that forms an imaginary pattern; there are 88 recognized constellations (Note: The Gujarati text mentioned 28, likely referring to Nakshatras).Convection Zone: The layer of a star where energy moves outward via convection currents. In the Sun, this begins below the photosphere.Corona: The outermost layer of the Sun's atmosphere, made of extremely hot gases, visible during a solar eclipse.Coronal Mass Ejections (CMEs): Massive bubbles of gas ejected from the Sun, connected to magnetic field lines.Cosmic Ray: Ultra-high energy atomic nuclei (mainly protons) that collide with Earth's atmosphere.Cosmic String: A tube-like structure of energy theorized to have existed in the early universe.Cosmology: The science studying the origin, structure, and nature of the universe.Crater: A bowl-shaped depression formed by the impact of an asteroid or meteor; or a volcanic vent.Crystalline: A rock made of mineral crystals rather than glass.D
Dark Matter: Matter that cannot be seen but can be detected through its gravitational effects.Declination: The angular distance of an object from the celestial equator.Density: The amount of matter in a given volume (measured in g/cc). The density of water is 1.0.Differentiated: When a body melts and separates into layers based on composition; such as Earth's iron-nickel core and silicate surface.Disk: The surface of the Sun or other objects as seen in the sky.Doppler Effect: The apparent change in wavelength due to an object's motion, used to determine an object's speed and direction.Double Star: A pair of stars that appear close together, whether they are physically orbiting each other or just aligned by chance.Double Asteroid: Two asteroids orbiting each other due to gravity.Dust Grains: Extremely small pieces of carbon or silicate in interstellar space.E
Eclipse: An event where one celestial body partially or fully blocks the path of another.Ejecta: Material thrown out from below the surface due to a meteorite impact.Electromagnetic Spectrum: The entire range of wavelengths from gamma rays to radio waves.Electron: A negatively charged particle located outside the nucleus of an atom.Electron Volt (eV): A unit of energy.Ellipse: An oval shape; Kepler discovered that planetary orbits are elliptical.Elliptical Galaxy: A galaxy with a simple structure and an elliptical shape.Elongation: The angular distance between the Sun and a planet as seen from Earth.Emission Nebula: Gas clouds that emit their own light (usually red).Enstatite: A type of primitive meteorite composed of silicate minerals.Ephemeris: A table showing the positions of the Sun, Moon, and planets by date.Equinox: The two days of the year (March 21 and September 22) when the Sun is on the celestial equator and day and night are of equal length.Escape Velocity: The minimum speed required to break free from a planet's gravity.Event Horizon: The invisible boundary around a black hole beyond which even light cannot escape.Evolved Star: A star that has reached the final stages of its life.F
Faculae: Bright patches visible on the Sun's surface.Filament: Strands of cool gas floating on the Sun's surface that appear dark.Finder: A small telescope attached to a main telescope that helps in locating objects.Fireball: An extremely bright meteor, often brighter than the Moon.Solar Flare: A sudden and intense explosion of energy from a specific region of the Sun.Flare Star: A star whose brightness increases suddenly.Free Electron: An electron that is free from the bond of an atom.Frequency: The number of repetitions of a wave; measured in Hertz (Hz).Fusion: The process where atomic nuclei join together and release massive energy; this is how stars shine.G
Galactic Nucleus: The dense cluster of stars and gas at the innermost part of a galaxy.Galactic Halo: The spherical region surrounding the nucleus or center of a galaxy.Galaxy: A massive system of about 100 billion stars. Our Sun is a member of the Milky Way galaxy.Galilean Moons: Jupiter's four largest moons—Io, Europa, Callisto, and Ganymede.Gamma Ray: Photons with the highest energy (shortest wavelength) in the electromagnetic spectrum.Geomagnetic Storm: A worldwide disturbance in Earth's magnetic field associated with solar activity.Geosynchronous Orbit: An orbit of a satellite moving west to east above Earth's equator at the same speed as Earth's rotation, making it appear stationary. Its altitude is approx. 35,900 km.Giant Molecular Cloud (GMC): Massive clouds in interstellar space composed primarily of hydrogen molecules.Globular Cluster: A dense, spherical group of hundreds of thousands of stars, usually old and found in the central parts of a galaxy.Granulation: Small cell-like structures seen on the Sun's surface caused by the convection of hot gases.Gravitational Lens: A concentration of matter (like a galaxy) that bends light from a background object, creating duplicate images.Gravity: The mutual physical force of nature by which two objects attract each other.Greenhouse Effect: The increase in temperature caused by the trapping of outgoing thermal radiation by the atmosphere.H
Heliocentric: Calculations or ideologies centered around the Sun.Heliopause: The point where the solar wind meets the interstellar medium or solar winds from other stars.Heliosphere: The region within the boundary of the heliopause containing the Sun and solar system.Helium: The second lightest and most abundant element in the universe.I - K
Ice: In astronomy, a term used for frozen substances like water, methane, and ammonia.Igneous Rocks: Rocks formed from the cooling of molten material (magma or lava).Infrared: Electromagnetic radiation with wavelengths between 0.75 and 1000 microns.Interstellar Medium: The gas and dust located in the space between stars.Ionosphere: Part of a planet's atmosphere that is ionized by solar radiation.Kelvin: A unit of temperature measurement. 0 Kelvin is absolute zero.Kuiper Belt: An area beyond Neptune's orbit inhabited by comets and small icy bodies.L - O
Lagrange Points: Five points in space where the gravitational forces of two large bodies keep a third smaller body stable relative to them.Light Year: The distance light travels in a vacuum in one year (approx. 9.46 trillion k m).Limb: The outer edge or rim of a celestial body like the Sun or Moon.Luminosity: The total energy emitted by an object per second.Magnetic Field: The area around a magnet or electric current where magnetic force can be felt.Magnetosphere: The region around a planet where its magnetic field dominates the solar wind particles.Main Sequence: The stage in a star's life where it converts hydrogen into helium.Meteor: A body that enters Earth's atmosphere and burns up.Meteorite: The portion of a meteor that reaches the Earth's surface.Nebula: Clouds of gas and dust in space.Neutron Star: An extremely dense object formed after the death of a massive star.Orbit: The path of one object's motion around another.P - Z
Parsec: A unit of distance; 1 Parsec = 3.26 light-years.Photosphere: The visible surface of the Sun.Planetesimal: Early rocky fragments of planet formation.Quasar: The extremely bright center of a galaxy that emits radio waves.Red Giant: A massive star in the final stage of its life.Redshift: The increase in light's wavelength when an object moves away.Singularity: The point of infinite density at the center of a black hole.Solar Wind: A constant stream of charged particles coming from the Sun.Sunspot: Dark spots on the Sun's surface that are cooler than their surroundings.Supernova: The massive explosion of a large star.Terraforming: The process of making another planet habitable like Earth.Transit: When a smaller celestial object passes in front of a larger one.Variable Star: A star whose brightness changes over time.White Dwarf: The dense, hot core left after the death of a small star.Zenith: The point exactly above the observer's head.Zodiac: An imaginary belt in the sky where the Sun, Moon, and planets are found.Zodiacal Light: Faint light seen on the horizon after sunset or before sunrise, caused by sunlight reflecting off dust particles.
