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.

Astronomy Concepts

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

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 ...

Galaxies

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

Stellar Objects

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

Stellar Objects

A region of spacetime with extreme gravity.

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

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...

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...

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.

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

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

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

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.

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.

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 ...

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 ...

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.

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

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...

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...

Astronomy Concepts

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

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.

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...

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

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

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.

Telescopes Equipment

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

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

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.

Stellar Objects

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

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.