SPACE TERMS

An astronomical unit, or AU, is the average distance between Earth and the Sun. 1 AU is about 149.6 million kilometers.

An orbit is the curved path an object follows around another object because of gravity.

Gravity is the force that attracts objects with mass toward one another.

An atmosphere is a layer of gases surrounding a planet, moon, or other astronomical object.

A satellite is an object that orbits another object. Natural satellites include moons.

A dwarf planet orbits the Sun and is nearly round, but has not cleared its orbital region.

An asteroid is a rocky or metallic body that orbits the Sun.

A comet is an icy body that can develop a glowing coma and tail when it approaches the Sun.

The Kuiper Belt is a distant region beyond Neptune containing icy bodies and dwarf planets.

The Oort Cloud is a theoretical enormous shell of icy objects surrounding the Solar System.

A light-year is the distance light travels through space in one year: about 9.46 trillion km.

A nebula is a huge cloud of gas and dust in space. Some are regions where stars form.

A galaxy is a massive system of stars, gas, dust, dark matter, and other objects held together by gravity.

A black hole is a region of spacetime where gravity is so strong that nothing, including light, can escape from inside the event horizon.

The event horizon is the boundary around a black hole beyond which escape is impossible.

A singularity is the theoretical central region of a classical black-hole model where spacetime curvature becomes extreme.

An exoplanet is a planet that orbits a star outside our Solar System.

A supernova is an extremely powerful stellar explosion that can briefly outshine an entire galaxy.

A neutron star is the incredibly dense remnant left after certain massive stars explode.

A white dwarf is the hot, dense remnant of a Sun-like star after it has shed its outer layers.

Dark matter is invisible matter inferred from its gravitational effects on visible matter and cosmic structures.

Dark energy is the name given to whatever is driving the accelerated expansion of the Universe.

Solar wind is a continuous flow of charged particles released by the Sun into space.

The heliosphere is the enormous bubble around the Sun created by the solar wind.

The heliopause is the outer boundary of the heliosphere where the solar wind meets interstellar space.

A parsec is an astronomical distance unit equal to about 3.26 light-years.

A red giant is an evolved star that has expanded enormously after exhausting hydrogen in its core.

The main sequence is the stage where a star fuses hydrogen into helium in its core.

The habitable zone is the region around a star where temperatures could allow liquid water on a planet's surface.

The Roche limit is the distance within which tidal forces can break apart a smaller object held together mainly by gravity.

Tidal forces are differences in gravitational pull across an object that can stretch or deform it.