General Astronomy Terminology
Universe & Cosmos
- Astronomy: The scientific study of celestial objects, space, and the universe as a whole
- Astrophysics: Branch of astronomy concerned with the physical nature of celestial bodies and phenomena
- Cosmology: Study of the origin, evolution, and structure of the universe
- Universe: All of space-time and its contents, including planets, stars, galaxies, and all forms of matter and energy
- Observable Universe: The portion of the universe that can be observed from Earth
- Light-Year: Distance light travels in one year (approximately 9.46 trillion kilometers or 5.88 trillion miles)
- Parsec: Unit of astronomical distance equal to about 3.26 light-years
Celestial Coordinates & Navigation
- Celestial Sphere: Imaginary sphere surrounding Earth on which celestial objects appear to be located
- Celestial Equator: Projection of Earth’s equator onto the celestial sphere
- Ecliptic: Apparent path of the Sun across the celestial sphere over the course of a year
- Celestial Poles: Points where Earth’s axis of rotation intersects the celestial sphere
- Right Ascension: Celestial equivalent of longitude, measured in hours (0-24)
- Declination: Celestial equivalent of latitude, measured in degrees north (+) or south (-) of celestial equator
- Azimuth: Horizontal angular distance from north, measured clockwise (0-360°)
- Altitude: Angular distance above the horizon, measured in degrees (0-90°)
- Zenith: Point directly overhead (altitude = 90°)
- Meridian: Great circle passing through the zenith and celestial poles
Astronomical Measurements
- Magnitude: Logarithmic scale used to specify the brightness of celestial objects
- Apparent Magnitude: Brightness as seen from Earth
- Absolute Magnitude: Intrinsic brightness (as seen from a distance of 10 parsecs)
- Angular Size: Apparent size of an object in the sky, measured in degrees, arcminutes (1/60°), or arcseconds (1/3600°)
- Astronomical Unit (AU): Average distance from Earth to the Sun (about 150 million kilometers or 93 million miles)
- Redshift: Increase in wavelength of light from distant galaxies, indicating they are moving away
- Albedo: Measure of reflectivity of a celestial object (0 = completely dark, 1 = perfectly reflective)
Solar System
Solar System Components
- Solar System: The Sun and all objects orbiting it, including planets, dwarf planets, moons, asteroids, comets, and other bodies
- Sun: The star at the center of our solar system, a G-type main-sequence star (G2V)
- Planet: A celestial body that orbits the Sun, has sufficient mass for self-gravity to maintain a nearly round shape, and has cleared its orbit of other objects
- Dwarf Planet: Similar to a planet but has not cleared its orbital path of other objects
- Moon: A natural satellite that orbits a planet or other celestial body
- Asteroid: Rocky remnant from the early formation of the solar system, primarily found in the asteroid belt between Mars and Jupiter
- Comet: Icy body that releases gas and dust when heated by the Sun, forming a coma and tail
- Meteoroid: Small rocky or metallic body traveling through space
- Meteor: The visible streak of light when a meteoroid enters Earth’s atmosphere (also called a “shooting star”)
- Meteorite: A meteoroid that survives passage through the atmosphere and reaches Earth’s surface
Orbital Mechanics
- Orbit: Path of an object around another due to gravity
- Perihelion: Point in an orbit closest to the Sun
- Aphelion: Point in an orbit farthest from the Sun
- Perigee: Point in an orbit closest to Earth
- Apogee: Point in an orbit farthest from Earth
- Eccentricity: Measure of how elliptical an orbit is (0 = perfect circle, 1 = parabola)
- Inclination: Angle between the orbital plane and a reference plane
- Conjunction: Alignment of two celestial bodies as viewed from Earth
- Opposition: Alignment of a planet with Earth on opposite sides of the Sun
- Transit: Passage of one celestial body in front of another, as viewed from Earth
- Occultation: When one celestial body passes in front of another, completely hiding it from view
Stars & Stellar Evolution
Star Properties
- Star: Luminous sphere of plasma held together by gravity, generating energy through nuclear fusion
- Main Sequence: The phase in a star’s life when it fuses hydrogen into helium in its core
- Stellar Classification: Classification of stars based on spectral characteristics (O, B, A, F, G, K, M)
- Luminosity: Total amount of energy emitted by a star per unit time
- Spectral Type: Classification of stars based on their spectra (absorption lines)
- Binary Star: System of two stars orbiting around their common center of mass
- Variable Star: Star whose brightness changes over time
- Parallax: Apparent shift in position of a star due to Earth’s orbit, used to determine stellar distances
Stellar Evolution
- Nebula: Cloud of gas and dust in space, often a region of star formation
- Protostar: Early stage in the formation of a star, before nuclear fusion begins
- Red Giant: Late stage of stellar evolution when a star expands and cools
- Red Dwarf: Small, cool, low-mass main-sequence star
- White Dwarf: Remnant of a star after it has exhausted its nuclear fuel
- Neutron Star: Extremely dense stellar remnant composed mostly of neutrons
- Black Hole: Region of spacetime with gravitational effects so strong that nothing can escape, not even light
- Supernova: Powerful explosion that occurs at the end of a star’s life
- Nova: Sudden brightness increase in a binary star system
- Pulsar: Rotating neutron star that emits beams of electromagnetic radiation
Galaxies & Deep Space Objects
Galaxy Types & Structure
- Galaxy: Vast system of stars, gas, dust, and dark matter bound together by gravity
- Spiral Galaxy: Galaxy with a flat, rotating disk of stars and gas, a central bulge, and spiral arms
- Elliptical Galaxy: Galaxy with an ellipsoidal shape and little gas or dust
- Irregular Galaxy: Galaxy without a regular shape
- Lenticular Galaxy: Galaxy with characteristics of both spiral and elliptical galaxies
- Active Galaxy: Galaxy with an unusually high energy output from its nucleus
- Quasar: Extremely luminous active galactic nucleus powered by a supermassive black hole
Deep Space Objects
- Star Cluster: Group of stars bound together by gravity
- Open Cluster: Loosely bound group of relatively young stars
- Globular Cluster: Tightly bound, spherical collection of old stars
- Nebula: Cloud of gas and dust in interstellar space
- Emission Nebula: Glowing gas excited by nearby hot stars
- Reflection Nebula: Dust cloud reflecting light from nearby stars
- Dark Nebula: Dense cloud of dust blocking light from behind it
- Planetary Nebula: Shell of gas ejected from a dying star
- Supernova Remnant: Structure resulting from the explosion of a star in a supernova
- Galaxy Cluster: Group of galaxies bound together by gravity
- Supercluster: Group of galaxy clusters
- Void: Vast empty regions between filaments of galaxies
Telescopes & Observation
Telescope Types
- Refractor: Telescope that uses lenses to focus light
- Reflector: Telescope that uses mirrors to focus light
- Catadioptric: Telescope that uses both lenses and mirrors
- Radio Telescope: Instrument designed to receive radio waves from space
- Space Telescope: Telescope located in outer space
Telescope Components & Terms
- Aperture: Diameter of the telescope’s primary light-gathering element (lens or mirror)
- Focal Length: Distance from a lens or mirror to the point where light rays converge
- Focal Ratio (f/number): Ratio of focal length to aperture
- Eyepiece: Lens assembly used to view the image formed by the telescope
- Magnification: Degree to which a telescope enlarges an object, determined by dividing the focal length of the telescope by the focal length of the eyepiece
- Resolution: Ability of a telescope to separate closely spaced objects
- Field of View: Angular size of the sky visible through a telescope
Observational Terms
- Light Pollution: Excessive artificial light that reduces visibility of the night sky
- Seeing: Measure of atmospheric turbulence affecting image quality
- Transparency: Clarity of the atmosphere
- Averted Vision: Technique of looking slightly to the side of an object to better see faint details
- Star Hopping: Technique of finding celestial objects by starting from known stars
- Limiting Magnitude: Faintest stars visible under given conditions
Celestial Events & Phenomena
Planetary Phenomena
- Retrograde Motion: Apparent backward motion of planets as seen from Earth
- Planetary Conjunction: Close visual alignment of two or more planets
- Great Red Spot: Persistent high-pressure storm on Jupiter
Lunar Phenomena
- Lunar Phase: Changing appearance of the Moon as it orbits Earth
- New Moon: Phase when the Moon is not visible from Earth
- Full Moon: Phase when the entire face of the Moon is illuminated
- First Quarter: Phase when the right half of the Moon appears illuminated
- Last Quarter: Phase when the left half of the Moon appears illuminated
- Waxing: Moon phases between New and Full, when visible portion is increasing
- Waning: Moon phases between Full and New, when visible portion is decreasing
- Libration: Apparent wobbling of the Moon that allows viewing of slightly more than 50% of its surface over time
Eclipses & Occultations
- Solar Eclipse: When the Moon passes between Earth and the Sun, blocking sunlight
- Total Solar Eclipse: When the Moon completely blocks the Sun’s disk
- Partial Solar Eclipse: When the Moon blocks only part of the Sun’s disk
- Annular Solar Eclipse: When the Moon appears smaller than the Sun, creating a ring of fire
- Lunar Eclipse: When Earth passes between the Sun and the Moon, casting a shadow on the Moon
- Total Lunar Eclipse: When the Moon passes completely into Earth’s umbra
- Partial Lunar Eclipse: When only part of the Moon passes into Earth’s umbra
- Occultation: When one celestial body passes in front of another, blocking it from view
Meteor Phenomena
- Meteor Shower: Increased frequency of meteors appearing to originate from one point in the sky
- Radiant: Point in the sky from which meteors in a shower appear to originate
- Bolide: Extremely bright meteor, often exploding in the atmosphere
- Fireball: Very bright meteor, brighter than any planet or star
Modern Astrophysics Concepts
Cosmological Concepts
- Big Bang: Prevailing cosmological model for the universe’s origin about 13.8 billion years ago
- Cosmic Microwave Background (CMB): Electromagnetic radiation remnant from the early universe
- Dark Matter: Hypothetical form of matter that doesn’t interact with light but affects gravity
- Dark Energy: Hypothetical form of energy causing the accelerated expansion of the universe
- Inflation: Theory that the universe expanded exponentially in the first fraction of a second after the Big Bang
- Hubble’s Law: Observation that galaxies are moving away from us at speeds proportional to their distance
- Cosmic Web: Large-scale structure of the universe, consisting of filaments of galaxy clusters and voids
Relativity & Quantum Concepts
- General Relativity: Einstein’s theory describing gravity as the curvature of spacetime
- Special Relativity: Theory relating space and time for objects moving at constant velocities
- Spacetime: Unified model of space and time as a four-dimensional continuum
- Gravitational Waves: Ripples in spacetime caused by accelerating massive objects
- Event Horizon: Boundary around a black hole beyond which no light or information can escape
- Singularity: Point of infinite density believed to exist at the center of black holes
- Hawking Radiation: Theoretical radiation emitted by black holes due to quantum effects
Exoplanets & Astrobiology
- Exoplanet: Planet orbiting a star other than the Sun
- Transit Method: Technique for detecting exoplanets by measuring the dimming of a star as a planet passes in front
- Radial Velocity Method: Technique for detecting exoplanets by measuring a star’s wobble
- Habitable Zone: Region around a star where conditions might allow liquid water on a planet’s surface
- Biosignature: Feature that suggests the presence of past or present life
- Technosignature: Evidence for the presence of technology created by extraterrestrial intelligence
- SETI: Search for Extraterrestrial Intelligence, efforts to detect signs of technological civilizations