Space & Astronomy Glossary

A rotating disk of gas, dust, and other matter that forms around a massive body such as a black hole, neutron star, or young star. The material spirals inward due to gravitational forces and friction, often emitting intense radiation. Accretion disks are among the most luminous objects in the universe.

The point in the orbit of a planet, asteroid, or comet at which it is farthest from the Sun. Earth reaches its aphelion around July 4th each year, when it is approximately 152.1 million kilometers from the Sun. The opposite of aphelion is perihelion.

A small rocky body orbiting the Sun, primarily found in the asteroid belt between Mars and Jupiter. Asteroids range from a few meters to hundreds of kilometers in diameter. They are remnants from the early solar system that never coalesced into a planet.

A region of the solar system between the orbits of Mars and Jupiter where most asteroids are found. Despite popular depictions, the belt is mostly empty space, with asteroids separated by large distances. Its total mass is less than 4% of the Moon.

A unit of measurement equal to the average distance between Earth and the Sun, approximately 149.6 million kilometers (93 million miles). It is commonly used to express distances within our solar system. Jupiter orbits at about 5.2 AU from the Sun.

A natural light display in Earth's sky, predominantly seen in high-latitude regions. Auroras are caused by charged particles from the solar wind interacting with atmospheric gases, guided by Earth's magnetic field. The aurora borealis and aurora australis are the two types.

The prevailing cosmological model explaining the origin of the universe from an initial state of extremely high density and temperature approximately 13.8 billion years ago. Evidence includes cosmic microwave background radiation, light element abundance, and the observed expansion of the universe.

A star system consisting of two stars orbiting around their common center of mass. Binary stars are very common, with more than half of all star systems being binaries or multiples. They are important for determining stellar masses.

A region of spacetime where gravity is so strong that nothing, not even light, can escape once past the event horizon. Black holes form when massive stars collapse at the end of their life cycle. Supermassive black holes exist at the centers of most galaxies.

A decrease in wavelength and increase in frequency of electromagnetic radiation from an object moving toward the observer. This is the opposite of redshift. When a star or galaxy moves toward Earth, its light shifts toward the blue end of the spectrum.

An imaginary sphere of arbitrarily large radius centered on Earth, upon which all celestial objects are projected. Useful for locating objects using coordinates like right ascension and declination, though objects are at vastly different actual distances.

The largest object in the asteroid belt and the only dwarf planet in the inner solar system. Ceres has a diameter of about 940 km and contains about a third of the asteroid belt's total mass. NASA's Dawn spacecraft orbited Ceres from 2015 to 2018.

An icy small body that, when passing close to the Sun, releases gases in outgassing, producing a visible coma and sometimes a tail. Comets originate from the Kuiper Belt and the Oort Cloud at the edges of the solar system.

A group of stars forming a recognizable pattern traditionally named after its apparent form or a mythological figure. The IAU recognizes 88 official constellations covering the entire sky. Stars in a constellation are usually not physically related.

Electromagnetic radiation left over from the early universe, about 380,000 years after the Big Bang. It is the oldest light in the universe, now cooled to about 2.725 Kelvin. The CMB is crucial evidence for the Big Bang theory.

A hypothetical form of energy that permeates all of space and drives the accelerating expansion of the universe. It constitutes approximately 68% of the total energy content of the universe. Its nature remains one of the biggest mysteries in modern physics.

A hypothetical form of matter that does not emit, absorb, or reflect light. Its existence is inferred from gravitational effects on visible matter, such as galaxy rotation curves. Dark matter makes up about 27% of the universe's total mass-energy content.

The change in frequency or wavelength of a wave relative to an observer moving relative to the source. In astronomy, it determines whether objects are moving toward or away from Earth by measuring spectral line shifts.

A celestial body orbiting the Sun with enough mass for a nearly round shape but that has not cleared its orbital neighborhood. The five recognized dwarf planets are Pluto, Eris, Haumea, Makemake, and Ceres. Established by the IAU in 2006.

An astronomical event when one celestial body moves into the shadow of another. Solar eclipses occur when the Moon passes between Sun and Earth; lunar eclipses when Earth passes between Sun and Moon. Total solar eclipses briefly turn day into night.

The boundary around a black hole beyond which no light or radiation can escape. Once matter crosses it, it is inevitably pulled toward the singularity. The size depends on the black hole's mass and is described by the Schwarzschild radius.

A planet orbiting a star outside our solar system. Thousands have been found since the first confirmed discovery in 1992, primarily by NASA's Kepler and TESS missions. Exoplanets show a wide variety of sizes, compositions, and orbital configurations.

A gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range from dwarfs with billions of stars to giants with trillions. The Milky Way contains an estimated 100-400 billion stars.

The most energetic electromagnetic events in the universe, lasting milliseconds to minutes. They release more energy in seconds than the Sun will in its entire lifetime. They originate from massive star collapse or neutron star mergers.

Einstein's 1915 theory describing gravity as curvature of spacetime caused by mass and energy. Confirmed by light bending, gravitational time dilation, and gravitational wave detection. It remains the foundation of modern gravitational physics.

The bending of light from a distant source by the gravitational field of a massive intervening object. Predicted by general relativity, it can magnify, distort, or multiply images of distant galaxies. It is also used to detect dark matter.

Ripples in spacetime caused by accelerating massive objects, such as merging black holes. Predicted by Einstein in 1916 and first detected by LIGO in 2015. They opened a new window for observing the universe.

The region around a star where liquid water could exist on a planet's surface, often called the Goldilocks zone. It varies with the star's size and temperature. Finding exoplanets here is key to the search for extraterrestrial life.

A value describing the rate of the universe's expansion, named after Edwin Hubble. Its precise value remains debated, with the discrepancy between measurement methods known as the Hubble tension being a major unsolved problem in cosmology.

The matter and radiation between star systems, consisting primarily of hydrogen and helium gas with dust particles. It plays a crucial role in star formation, as new stars form from the collapse of dense regions within it.

A circumstellar disc in the outer solar system from Neptune's orbit (30 AU) to about 50 AU. Home to Pluto and many icy bodies, it is similar to the asteroid belt but far larger, composed mainly of frozen volatiles like methane, ammonia, and water.

The distance light travels in one year in a vacuum: approximately 9.461 trillion km. Used to express astronomical distances outside the solar system. Alpha Centauri, the nearest star system, is about 4.37 light-years away.

The region of space surrounding a planet dominated by its magnetic field. Earth's magnetosphere extends tens of thousands of kilometers, shielding the planet from harmful solar wind particles. Without it, solar radiation would strip the atmosphere.

A band on the Hertzsprung-Russell diagram representing stars fusing hydrogen into helium. Stars spend the majority of their lifetime on the main sequence. The Sun is a main-sequence star and will remain so for about 5 billion more years.

The streak of light when space debris burns up in Earth's atmosphere. Called a shooting star, it becomes visible at heights of 80-120 km. If the object survives and reaches the ground, it is called a meteorite.

A solid piece of debris from a meteoroid or asteroid that survives passage through Earth's atmosphere. Classified as stony, iron, or stony-iron, meteorites provide valuable information about the solar system's composition and age.

A small rocky or metallic body in space, from grain-of-sand to one-meter size. When entering Earth's atmosphere, it creates a meteor. Meteoroids originate from comets, asteroids, and collisions between solar system bodies.

The barred spiral galaxy containing our solar system, with an estimated 100-400 billion stars spanning about 100,000 light-years in diameter. Its name derives from its appearance as a milky band of light across the night sky from billions of faint distant stars.

The different appearances of the Moon over a synodic month (about 29.5 days). The eight phases are new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent, caused by changing Sun-Moon-Earth angles.

Any small solar system body whose orbit brings it close to Earth, with a perihelion distance less than 1.3 AU. Includes near-Earth asteroids and comets. NASA's Planetary Defense Coordination Office tracks potentially hazardous NEOs larger than 140 meters.

A giant cloud of dust and gas in space, often spanning light-years. Nebulae are star birthplaces, forming when regions collapse under gravity. They also form from dying star remnants, such as planetary nebulae and supernova remnants.

The collapsed core of a massive star after a supernova, with 1.4-2 solar masses in a sphere about 20 km across. Incredibly dense; a teaspoon would weigh about a billion tons. Some emit radiation beams and are called pulsars.

A theoretical spherical cloud of icy objects surrounding the Sun at 2,000-100,000 AU. Thought to be the source of most long-period comets. Never directly observed, but its existence is inferred from comet orbits.

The curved path of a celestial object around a star, planet, or moon under gravity's influence. Orbits can be circular, elliptical, parabolic, or hyperbolic. Johannes Kepler first described planetary orbit laws in the early 1600s.

The apparent displacement of a nearby object against a distant background viewed from two positions. Stellar parallax measures distances to nearby stars by observing their shift as Earth orbits the Sun. It is the most direct method of measuring astronomical distances.

A unit of distance equal to about 3.26 light-years (30.9 trillion km). Derived from parallax of one arcsecond. Professional astronomers often prefer parsecs because the measurement relates directly to the parallax technique.

The point in an orbit at which the object is closest to the Sun. Earth reaches perihelion around January 3rd at approximately 147.1 million km. The opposite is aphelion.

A dwarf planet in the Kuiper Belt, reclassified from planet status by the IAU in 2006. It has five moons, the largest being Charon. NASA's New Horizons flew by in 2015, revealing mountains of water ice and plains of nitrogen ice.

A highly magnetized rotating neutron star emitting beams of radiation from its magnetic poles. As it rotates, the beams sweep like a lighthouse, creating a pulsing signal. Pulsars can rotate up to hundreds of times per second.

An extremely luminous active galactic nucleus powered by a supermassive black hole. Quasars can outshine entire galaxies and are among the most distant observable objects. Short for quasi-stellar radio source.

A luminous giant star that has exhausted its core hydrogen. The outer layers expand and cool, giving a reddish appearance. The Sun will become a red giant in about 5 billion years, expanding to potentially engulf the inner planets.

An increase in wavelength of radiation from an object moving away from the observer. The redshift of distant galaxies provides evidence for cosmic expansion. Greater redshift indicates faster recession and greater distance.

A point where matter density and spacetime curvature become infinite, as predicted by general relativity. Singularities exist at black hole centers and at the Big Bang's initial point. Their physics is one of the hardest problems in theoretical physics.

The outermost layer of the Sun's atmosphere, extending millions of kilometers. Much hotter than the surface at 1-3 million degrees Celsius (the coronal heating problem). Visible during total solar eclipses as a pearly white halo.

A sudden, intense burst of electromagnetic radiation from the Sun near sunspots. Releases energy equivalent to millions of nuclear bombs, can disrupt communications, damage satellites, and cause power failures. Classified as A, B, C, M, or X class.

The gravitationally bound system of the Sun and everything orbiting it: eight planets, dwarf planets, moons, asteroids, comets, and small bodies. Formed 4.6 billion years ago from a molecular cloud collapse. Extends to the Oort Cloud at roughly 100,000 AU.

A continuous stream of charged particles from the Sun's corona at 300-800 km/s. It shapes planetary magnetospheres, drives auroral displays, and creates the heliosphere enclosing our solar system.

The study of the interaction between matter and electromagnetic radiation. In astronomy, it reveals composition, temperature, density, mass, distance, luminosity, and motion by analyzing emitted or absorbed light spectra.

The speed light travels in vacuum: approximately 299,792 km/s. According to special relativity, this is the universal speed limit for anything with mass. Light from the Sun takes about 8 minutes 20 seconds to reach Earth.

A massive, luminous sphere of hot gas held together by gravity, generating energy through nuclear fusion. Stars range from red dwarfs to supergiants over 100 solar masses. A star's life cycle depends primarily on its initial mass.

A powerful explosion at the end of a massive star's life, briefly outshining an entire galaxy. Supernovae scatter heavy elements throughout space, seeding future stellar and planetary formation. Two main types: core-collapse and Type Ia.

Gravitational forces arising because gravitational pull varies across a body's extent. On Earth, the Moon and Sun cause ocean tides. Near black holes, extreme tidal forces stretch objects in spaghettification.

A technique for detecting exoplanets by measuring a star's dimming as a planet passes in front. The most successful detection method, with Kepler alone discovering thousands. It reveals planet size, orbital period, and sometimes atmospheric composition.

Two doughnut-shaped zones of energetic charged particles trapped by Earth's magnetic field. The inner belt is at 1,000-5,000 km altitude, the outer at 15,000-25,000 km. Discovered by James Van Allen in 1958 using Explorer 1 data.

A stellar core remnant of electron-degenerate matter, Earth-sized but with a mass comparable to the Sun. The final state of stars not massive enough for neutron stars or black holes. They slowly cool and fade over billions of years.

A belt-shaped sky region extending about 8 degrees north and south of the ecliptic. It includes the constellations through which the Sun, Moon, and planets appear to move over the year. Used historically for both astronomical observation and astrological practice.