Astronomy Glossary

Astronomy Concepts

Absolute magnitude measures intrinsic luminosity—how bright an object would appear if placed at standard distance of 10 parsecs (32.6 light-years).

Astronomy Concepts

Absorption lines are dark lines in a spectrum where atoms in cooler foreground gas absorbed specific wavelengths from background continuous source.

Galaxies

An Active Galactic Nucleus is a compact, extraordinarily luminous central region of a galaxy powered by accretion onto a supermassive black hole.

Astronomy Concepts

Albedo is the fraction of incoming light reflected by a surface, ranging from 0 (perfect absorber) to 1 (perfect reflector).

Telescopes Equipment

An altazimuth mount (alt-az) moves in altitude (up-down) and azimuth (side-to-side), like a camera tripod.

Units Distance

Angular size is the apparent angle an object spans in the sky as viewed from Earth, regardless of its physical size or distance.

Telescopes Equipment

Aperture is the diameter of a telescope's primary light-collecting element (lens or mirror), the single most important specification determining teles...

Solar System

Aphelion is the point in an object's orbit farthest from the Sun, where orbital velocity is slowest.

Solar System

Apogee is the point in an object's orbit around Earth that is farthest from Earth (analogous to aphelion for Sun orbits).

Astronomy Concepts

Apparent magnitude measures how bright an object appears from Earth, with lower/negative values being brighter.

Units Distance

An arcminute (symbol: ' or arcmin) is 1/60th of a degree, used for medium-scale angular measurements.

Units Distance

An arcsecond (symbol: " or arcsec) is 1/3600th of a degree, the standard small-angle measurement in astronomy.

Solar System

Asteroids are rocky remnants from the Solar System's formation, too small to be rounded by gravity, predominantly found in the asteroid belt between M...

Solar System

The asteroid belt (main belt) is a torus-shaped region between Mars and Jupiter containing hundreds of thousands of rocky bodies, remnants from Solar ...

Units Distance

The average distance from Earth to the Sun.

Units Distance

An Astronomical Unit is the average distance between Earth and the Sun, serving as a fundamental measuring stick for the Solar System.

Telescopes Equipment

A Barlow lens is a negative (diverging) lens placed between telescope and eyepiece to multiply magnification, typically by 2× or 3×.

Galaxies

Barred spiral galaxies have a prominent linear bar structure of stars extending through the nucleus, with spiral arms originating from bar ends.

Cosmology Universe

The Big Bang theory describes the Universe's evolution from an extremely hot, dense initial state ~13.8 billion years ago, expanding and cooling to th...

Stellar Objects

Binary star systems consist of two stars orbiting their common center of mass, gravitationally bound to each other.

Telescopes Equipment

Binoculars offer wide-field, two-eyed viewing ideal for casual astronomy, comet hunting, and surveying large sky regions.

Stellar Objects

A region of spacetime with extreme gravity.

Galaxies

Blazars are AGN with relativistic jets pointed directly at Earth, producing extreme luminosity, rapid variability, and strong polarization.

Stellar Objects

Blue giants are massive, hot, luminous stars on or near the main sequence with surface temperatures 10,000-50,000+ K.

Telescopes Equipment

Cassegrain telescopes use a primary mirror that reflects light to a convex secondary mirror, which reflects it back through a hole in the primary to a...

Stellar Objects

Cepheid variables are luminous pulsating stars with periods of 1-100 days, exhibiting a precise relationship between pulsation period and intrinsic lu...

Telescopes Equipment

Coma is an optical aberration in parabolic mirrors (like Newtonians) that makes off-axis stars appear as tiny comets pointing toward the field edge.

Solar System

Comets are icy bodies from the outer Solar System that develop spectacular tails when approaching the Sun as their ices vaporize.

Solar System

A Coronal Mass Ejection is an enormous burst of plasma and magnetic field ejected from the Sun's corona into space, carrying billions of tons of mater...

Cosmology Universe

Cosmic horizons define boundaries beyond which information cannot reach us due to finite light speed and Universe's expansion history.

Cosmology Universe

Cosmic inflation is a period of exponential expansion in the first 10^-36 to 10^-32 seconds after the Big Bang, proposed to solve several cosmological...

Cosmology Universe

The Cosmic Microwave Background is the thermal radiation from 380,000 years after the Big Bang, when Universe cooled enough for neutral atoms to form ...

Galaxies

The cosmic web is the large-scale structure of the Universe, consisting of filaments of galaxies surrounding vast empty voids, resembling a sponge-lik...

Cosmology Universe

Dark energy is a mysterious component causing the Universe's expansion to accelerate, comprising ~68% of the Universe's energy density.

Cosmology Universe

Dark matter is non-luminous matter detectable only through gravitational effects, comprising ~85% of matter in the Universe.

Astronomy Concepts

Declination (Dec, δ) is celestial latitude—angular distance north (+) or south (-) of the celestial equator, analogous to latitude on Earth.

Telescopes Equipment

A Dobsonian mount is a simple altitude-azimuth mount using Teflon bearings on a rotating base, revolutionizing amateur astronomy by making large apert...

Galaxies

Dwarf galaxies are small galaxies with 10^7 to few billion stars, the most common galaxy type in the Universe.

Solar System

Dwarf planets meet criteria 1 and 2 of planethood but have NOT cleared their orbital neighborhoods—they share orbital space with similar-sized objects...

Solar System

The ecliptic is the apparent path of the Sun across the celestial sphere over a year, representing Earth's orbital plane projected onto the sky.

Galaxies

Elliptical galaxies are smooth, featureless systems ranging from nearly spherical to highly elongated, containing predominantly old stars with little ...

Astronomy Concepts

Emission lines are bright lines at specific wavelengths in a spectrum, produced when excited atoms/ions emit photons.

Telescopes Equipment

Equatorial mounts align one axis parallel to Earth's rotation axis, allowing simple tracking of celestial objects with single-axis motion.

Astronomy Concepts

An equinox occurs when the Sun crosses the celestial equator, making day and night approximately equal length everywhere on Earth.

Cosmology Universe

The expansion of the Universe is the increase in distance between galaxies over time, a fundamental prediction of General Relativity and observational...

Telescopes Equipment

An eyepiece is the small optical system at the telescope's focus that magnifies the image for viewing, essentially a sophisticated magnifying glass.

Telescopes Equipment

A field flattener is a corrective lens that eliminates the curved focal plane inherent in many telescopes, producing sharp stars across the entire ima...

Telescopes Equipment

Specialized filters block light pollution and enhance contrast for emission nebulae by isolating specific wavelengths.

Telescopes Equipment

A finder scope is a small, low-power telescope or sighting device mounted on the main telescope to help locate objects in the sky.

Telescopes Equipment

Focal length is the distance from a telescope's primary lens or mirror to the point where light converges to form an image.

Telescopes Equipment

Focal ratio is the telescope's focal length divided by its aperture (f-ratio = focal length / aperture diameter).

Galaxies

A galaxy is a gravitationally bound system of stars, gas, dust, and dark matter, ranging from dwarfs with millions of stars to giants with trillions.

Galaxies

Galaxy clusters are the largest gravitationally bound structures in the Universe, containing hundreds to thousands of galaxies plus hot intracluster g...

Stellar Objects

Globular clusters are ancient, spherical concentrations of 100,000-1 million stars, tightly gravitationally bound and orbiting in galactic halos.

Telescopes Equipment

Go-To mounts use computerized motors and databases to automatically locate and track celestial objects, dramatically simplifying observing sessions.

Telescopes Equipment

A guide scope is a small telescope piggybacked on the main imaging telescope, paired with a camera to provide real-time tracking corrections during lo...

Astronomy Concepts

The Hertzsprung-Russell diagram plots stars' luminosity (or absolute magnitude) vs temperature (or spectral type/color), revealing stellar populations...

Solar System

The heliosphere is the vast bubble of solar wind and magnetic field surrounding the Solar System, separating it from interstellar space.

Telescopes Equipment

A space telescope launched in 1990.

Cosmology Universe

Hubble's Law states that galaxies' recession velocities are proportional to their distances: v = H₀ × d.

Stellar Objects

A hypernova is an exceptionally energetic supernova (10-100× normal), producing relativistic jets and associated with long-duration gamma-ray bursts.

Galaxies

Irregular galaxies lack organized structure—no spiral arms or elliptical symmetry—appearing chaotic and asymmetric.

Units Distance

A kiloparsec equals 1,000 parsecs (3,260 light-years), used primarily for measuring structures within galaxies.

Solar System

The Kuiper Belt is a disk-shaped region beyond Neptune (30-50 AU) containing thousands of icy bodies, short-period comets, and dwarf planets.

Cosmology Universe

The ΛCDM model (Lambda Cold Dark Matter) is the standard cosmological model describing Universe's large-scale structure and evolution.

Astronomy Concepts

A light curve is a graph of brightness vs time, revealing periodic variations, eclipses, transits, and explosive events.

Telescopes Equipment

Light pollution filters selectively block wavelengths from artificial lighting (sodium, mercury vapor) while passing starlight, improving contrast in ...

Units Distance

The distance light travels in one year.

Astronomy Concepts

Luminosity is the total energy radiated by an object per unit time (intrinsic brightness), measured in watts or solar luminosities (L☉ = 3.828 × 10²⁶ ...

Stellar Objects

A magnetar is a neutron star with an extraordinarily powerful magnetic field (10^14-10^15 Gauss), the strongest magnetic fields known in the Universe.

Solar System

A magnetosphere is the region around a planet dominated by its magnetic field, where the field deflects the solar wind and traps charged particles.

Stellar Objects

Main sequence stars are in the stable, longest phase of stellar life, fusing hydrogen into helium in their cores.

Telescopes Equipment

Maksutov-Cassegrain telescopes use a thick meniscus corrector lens instead of Schmidt's aspheric plate, creating compact, sealed instruments.

Units Distance

A megaparsec equals one million parsecs (3.26 million light-years), the standard unit for cosmological distances.

Solar System

A meteor is the visible light phenomenon ("shooting star") when a meteoroid enters Earth's atmosphere at high speed and burns due to friction, typical...

Solar System

A meteorite is a meteoroid fragment that survives atmospheric entry and lands on Earth's surface, providing direct samples of Solar System materials.

Solar System

A meteoroid is a small rocky or metallic object in space, typically debris from asteroids or comets, ranging from dust grains to boulder-sized chunks.

Astronomy Concepts

The nadir is the point directly below the observer (altitude = -90°), opposite the zenith.

Solar System

Near-Earth Objects are asteroids or comets with orbits bringing them within 1.3 AU of the Sun, potentially crossing Earth's orbit and posing impact ha...

Stellar Objects

A neutron star is the ultra-dense remnant of a massive star's supernova explosion, composed almost entirely of neutrons and supported by neutron degen...

Telescopes Equipment

The Newtonian design uses a parabolic primary mirror at the tube's base and a flat secondary mirror near the top to reflect light to a side-mounted ey...

Cosmology Universe

The observable Universe is the spherical region containing everything we can see or detect, limited by the finite speed of light and Universe's age.

Astronomy Concepts

An occultation occurs when one object passes in front of another from an observer's perspective, temporarily blocking it.

Solar System

The Oort Cloud is a hypothetical spherical shell of icy planetesimals surrounding the Solar System at extreme distances (2,000-100,000 AU), serving as...

Stellar Objects

Open clusters are loose groups of 100-1,000 young stars born together from the same molecular cloud, weakly gravitationally bound.

Solar System

Eccentricity (e) quantifies how elongated an orbit is, ranging from 0 (perfect circle) to 1 (parabola—unbound).

Solar System

Inclination is the angle between an object's orbital plane and a reference plane (usually the ecliptic for Solar System objects or galactic plane for ...

Astronomy Concepts

Parallax is the apparent shift in a star's position when viewed from opposite sides of Earth's orbit, the primary direct distance measurement method.

Units Distance

A unit of astronomical distance equal to about 3.26 light-years.

Solar System

Perigee is the point in an object's orbit around Earth that is nearest to Earth.

Solar System

Perihelion is the point in an object's orbit nearest to the Sun, where orbital velocity is fastest.

Solar System

A planet is a celestial body orbiting a star, massive enough to be rounded by its own gravity, and has cleared its orbital neighborhood of other debri...

Stellar Objects

A planetary nebula is a glowing shell of gas ejected by a dying low/medium-mass star, ionized by ultraviolet radiation from the exposed hot core (futu...

Astronomy Concepts

Polarization describes the orientation of electromagnetic wave oscillations, providing unique astrophysical diagnostics unavailable from intensity alo...

Stellar Objects

A protostar is the earliest stage of star formation, a dense core within a collapsing molecular cloud that has not yet initiated sustained nuclear fus...

Stellar Objects

A pulsar is a highly magnetized, rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles.

Galaxies

Quasars (quasi-stellar radio sources, though most aren't radio-loud) are the most luminous persistent objects in the Universe, powered by supermassive...

Stellar Objects

RR Lyrae stars are older, less luminous pulsating variables with periods 0.2-1 day, found in globular clusters and galactic halos.

Astronomy Concepts

Radiation pressure is the force exerted by electromagnetic radiation on matter, important for understanding stellar structure and astrophysical dynami...

Stellar Objects

A red giant is an evolved star that has exhausted core hydrogen and expanded dramatically while cooling, fusing hydrogen in a shell around an inert he...

Stellar Objects

Red supergiants are the largest stars by volume, evolved massive stars (>8 solar masses) with radii up to 1,000-1,500 times the Sun.

Units Distance

Redshift quantifies how much light has been stretched to longer (redder) wavelengths, serving as both a velocity and distance indicator.

Telescopes Equipment

Reflector telescopes use curved mirrors to collect and focus light, offering the most aperture per dollar.

Telescopes Equipment

Refractor telescopes use lenses to bend (refract) light to a focus, the classic "spyglass" design.

Astronomy Concepts

Right Ascension (RA, α) is celestial longitude—angular distance eastward along celestial equator from the vernal equinox, analogous to longitude on Ea...

Telescopes Equipment

Schmidt-Cassegrain Telescopes (SCTs) combine a Schmidt corrector plate at the front with Cassegrain optics, creating a versatile, compact system.

Galaxies

Seyfert galaxies are spiral galaxies with bright, compact nuclei showing strong emission lines, a nearby moderate-luminosity class of AGN.

Cosmology Universe

A singularity in General Relativity is a point where spacetime curvature becomes infinite and physical laws break down.

Telescopes Equipment

Solar filters provide the ONLY safe way to observe the Sun through telescopes or binoculars, protecting eyes from permanent retinal damage.

Solar System

A solar flare is a sudden, intense burst of electromagnetic radiation from the Sun's surface, caused by magnetic energy release in the corona.

Solar System

The solar wind is a continuous stream of charged particles (mostly electrons and protons) flowing outward from the Sun's corona at high speeds through...

Astronomy Concepts

A solstice occurs when the Sun reaches its northernmost or southernmost point in the sky, marking the longest and shortest days of the year.

Astronomy Concepts

Spectral classification categorizes stars by surface temperature via absorption line patterns, using OBAFGKM sequence.

Astronomy Concepts

A spectrum is light dispersed by wavelength, revealing composition, temperature, velocity, and physical conditions of astronomical objects.

Galaxies

Spiral galaxies are disk-shaped galaxies with prominent spiral arm patterns, active star formation, and significant gas content.

Stellar Objects

A star is a self-luminous sphere of plasma held together by gravity, generating energy through nuclear fusion in its core.

Telescopes Equipment

A star diagonal is a mirror or prism at 90° that redirects the light path, making viewing more comfortable by positioning the eyepiece at right angles...

Galaxies

Starburst galaxies undergo exceptionally intense star formation at rates 100-1,000× higher than normal galaxies, consuming gas rapidly.

Galaxies

Superclusters are massive structures containing multiple galaxy clusters and groups, spanning hundreds of millions of light-years, though not gravitat...

Stellar Objects

A supernova is a catastrophic stellar explosion, briefly outshining entire galaxies and producing most elements heavier than iron.

Stellar Objects

Supernova remnants (SNRs) are expanding shells of gas and magnetic fields produced by supernova explosions, observable for tens of thousands of years.

Telescopes Equipment

A T-ring is a camera-specific adapter that attaches a camera body to telescopes via the standard T-thread (M42×0.75mm) mounting system.

Solar System

Trans-Neptunian Objects are bodies orbiting beyond Neptune (>30 AU), including Kuiper Belt objects, scattered disk objects, and detached objects.

Astronomy Concepts

A transit is when a smaller object passes in front of a larger one (opposite perspective from occultation, but often used interchangeably).

Solar System

Trojan asteroids are objects trapped at a planet's L4 and L5 Lagrange points—stable gravitational equilibrium positions 60° ahead and behind the plane...

Stellar Objects

Variable stars are stars whose brightness changes over time, either intrinsically (physical changes) or extrinsically (eclipses, rotation).

Stellar Objects

A white dwarf is the dense, hot remnant core of a low/medium-mass star (0.5-8 solar masses) that has shed its outer layers as a planetary nebula.

Astronomy Concepts

The zenith is the point directly overhead (altitude = 90°), the highest point in an observer's sky.