Accretion Disk
A rotating disk of gas and dust falling toward a massive object such as a young star, white dwarf, neutron star, or black hole.
Space Encyclopedia
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Asteroid
A rocky or metallic small body orbiting the Sun, especially common between Mars and Jupiter.
Astronomical Unit
The average Earth-Sun distance, about 149.6 million kilometers.
Atmosphere
A layer of gases held around a planet, moon, or star by gravity.
Aurora
Colored light in a planet's sky caused when charged particles interact with atmospheric gases near magnetic poles.
Big Bang
The hot, dense early state from which the universe expanded and cooled.
Black Hole
A region where gravity is so intense that nothing crossing the event horizon can escape.
Comet
An icy small body that can grow a glowing coma and tail when warmed by the Sun.
Constellation
A traditional pattern of stars as seen from Earth, used for mapping the sky.
Dark Energy
The name for the unknown cause of the universe's accelerating expansion.
Dark Matter
Matter detected through gravity but not through emitted or reflected light.
Dwarf Planet
A round body orbiting the Sun that has not cleared its orbital neighborhood.
Event Horizon
The boundary around a black hole beyond which return is impossible.
Exoplanet
A planet orbiting a star outside our Solar System.
Galaxy
A large system of stars, gas, dust, dark matter, and stellar remnants bound by gravity.
Gas Giant
A large planet made mostly of hydrogen and helium, such as Jupiter or Saturn.
Gravity
The attractive force between masses that shapes orbits, tides, stars, and galaxies.
Habitable Zone
The orbital region where temperatures could allow liquid water on a planet's surface, if conditions are right.
Kuiper Belt
A distant icy region beyond Neptune that contains Pluto and many small bodies.
Light-Year
The distance light travels in one year, about 9.46 trillion kilometers.
Magnetosphere
The region around a world controlled by its magnetic field.
Nebula
A cloud of gas and dust in space, often connected to star birth or star death.
Neutron Star
An ultra-dense stellar remnant formed when the core of a massive star collapses.
Oort Cloud
A distant, theoretical shell of icy bodies surrounding the Solar System.
Orbit
The curved path one object follows around another because of gravity and motion.
Pulsar
A rotating neutron star whose beams of radiation sweep past Earth like clock ticks.
Quasar
An extremely bright active galactic center powered by a feeding supermassive black hole.
Red Giant
A swollen, cooler stage in the life of many stars after core hydrogen runs low.
Solar System
The Sun and everything gravitationally bound to it, including planets, moons, asteroids, and comets.
Star
A self-gravitating ball of hot plasma that shines mainly by nuclear fusion.
Supernova
A powerful stellar explosion that can create heavy elements and leave behind compact remnants.
Terrestrial Planet
A rocky planet with a solid surface, such as Mercury, Venus, Earth, or Mars.
White Dwarf
The dense exposed core left after a Sun-like star sheds its outer layers.