Cosmic Glossary
Master the language of the universe. From Absolute Magnitude to Zenith, explore our comprehensive A-Z dictionary of astronomical terms.
A
Absolute Magnitude
A measure of the intrinsic brightness of a celestial object. It is the apparent magnitude an object would have if it were located at a standard distance of 10 parsecs (32.6 light-years) from Earth.
Accretion Disk
A rotating disk of structured gas and dust spiraling towards a central massive body, most commonly a black hole or young star.
Apparent Magnitude
How bright a star or celestial object appears from Earth. The lower or more negative the number, the brighter the object. Sirius has an apparent magnitude of -1.46.
Asteroid
A small, rocky object that orbits the Sun. Most asteriods in our solar system are found in the asteroid belt between Mars and Jupiter.
Astronomical Unit (AU)
A unit of length representing the average distance between the Earth and the Sun, approximately 150 million kilometers (93 million miles).
B
Big Bang
The prevailing cosmological model for the observable universe, describing how the universe expanded from an initial state of high density and temperature about 13.8 billion years ago.
Binary Star
A star system consisting of two stars orbiting around their common center of mass.
Black Hole
A region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it.
Brown Dwarf
A substellar object that is larger than a gas giant planet but not massive enough to trigger thermonuclear fusion in its core. Often called 'failed stars'.
C
Comet
An icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes a tail.
Constellation
A recognizable pattern of stars in the night sky. The International Astronomical Union officially recognizes 88 constellations.
Cosmic Microwave Background (CMB)
The thermal radiation left over from the time of recombination in Big Bang cosmology. It is faint cosmic background radiation filling all space.
D
Dark Matter
A hypothetical form of matter thought to account for approximately 85% of the matter in the universe. It does not interact with the electromagnetic field, meaning it does not absorb, reflect, or emit light.
Dwarf Planet
A celestial body resembling a small planet but lacking certain technical criteria that are required for it to be classed as such. Pluto is a prominent example.
E
Eclipse
An astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer.
Ecliptic
The apparent path of the Sun on the celestial sphere, and the basis for the ecliptic coordinate system.
Event Horizon
A boundary beyond which events cannot affect an observer. Most commonly associated with black holes, it is the point of no return.
Exoplanet
A planet outside the Solar System. The first confirmed detection occurred in 1992.
G
Galaxy
A gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The Milky Way is the galaxy that contains our Solar System.
Gas Giant
A large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. Jupiter and Saturn are the gas giants of our Solar System.
Globular Cluster
A spherical collection of extremely old stars that orbit a galactic core as a satellite.
Goldilocks Zone (Habitable Zone)
The range of planetary orbits around a star where conditions are 'just right' to allow for liquid water on a planetary surface.
I
Interstellar Medium
The matter and radiation that exist in the space between the star systems in a galaxy.
K
Kelvin (K)
The base unit of temperature in the International System of Units (SI). Used in astronomy to measure the color temperature of stars. 0 K is absolute zero.
Kuiper Belt
A circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 AU to approximately 50 AU from the Sun.
L
Light-Year (LY)
A unit of length used to express astronomical distances. It is the amount of distance that light travels in a vacuum in one Julian year (about 9.46 trillion kilometers or 5.88 trillion miles).
Luminosity
The total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object.
M
Main Sequence
A continuous and distinctive band of stars that appears on plots of stellar color versus brightness. Our Sun is currently a main sequence star.
Meteorite
A solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon.
N
Nebula
A distinct luminescent part of the interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. They are often star-forming regions.
Neutron Star
The collapsed core of a massive supergiant star. They are the smallest and densest currently known class of stellar objects.
O
Oort Cloud
A theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU.
Orbital Period
The time a given astronomical object takes to complete one orbit around another object. For Earth revolving around the Sun, this is one year.
P
Parsec
A unit of length used to measure the large distances to astronomical objects outside the Solar System, equal to about 3.26 light-years.
Planetary Nebula
A type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
Pulsar
A highly magnetized rotating compact star (usually neutron stars but also white dwarfs) that emits beams of electromagnetic radiation out of its magnetic poles.
Q
Quasar
An extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk.
R
Red Dwarf
The smallest and coolest kind of star on the main sequence. They are by far the most common type of star in the Milky Way.
Red Giant
A luminous giant star of low or intermediate mass in a late phase of stellar evolution.
S
Singularity
A location in spacetime where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system.
Solar System
The gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly.
Supernova
A powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star.
T
Terrestrial Planet
A planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun: Mercury, Venus, Earth, and Mars.
W
White Dwarf
A stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to Earth's.
Z
Zenith
An imaginary point directly 'above' a particular location, on the imaginary celestial sphere.
Zodiac
An area of the sky that extends approximately 8 degrees north or south of the ecliptic. It encompasses the apparent paths of the Sun, the Moon, and the principal planets.
