Asteroid
A rocky or metal body orbiting the Sun, smaller than a planet. Most are in the belt between Mars and Jupiter.
Sizes run from metres to hundreds of km.
sourceastronomy dictionary
Space words, explained like a friend would
40 words you hear in space news, in plain English. Every one checked against an official science page (NASA, ESA, the IAU, NOAA or the Nobel committee); the source link is under each.
No word matches that. Try a shorter search.
Astronomical unit (AU)
The average distance from Earth to the Sun, about 150 million km.
Used for distances inside the solar system.
sourceAurora
Lights in the sky near the poles, made when charged particles from the Sun hit gases in Earth's upper atmosphere.
Called the northern or southern lights.
sourceBig Bang
The scientific model in which the universe expanded from an extremely hot, dense state about 13.8 billion years ago.
It describes expansion everywhere, not an explosion at one point.
sourceBlack hole
A region where gravity is so strong that nothing, not even light, can escape once it crosses the edge (the event horizon).
The first image of one (M87*) was released in 2019.
sourceComet
An icy body that grows a glowing head and tails of gas and dust when it comes near the Sun.
Halley's Comet returns about every 76 years.
sourceConstellation
One of 88 areas of the sky defined by the IAU, usually named after the star pattern inside it.
The stars in one pattern are often very far apart from each other.
sourceCosmic microwave background
The faint glow of radiation left from the early universe, seen in every direction.
It dates from about 380,000 years after the Big Bang.
sourceDark energy
The unknown cause of the universe's expansion speeding up.
The discovery that the expansion is speeding up won the 2011 Nobel Prize in Physics.
sourceDark matter
Matter that does not give off or reflect light but shows itself through gravity.
It is thought to make up most of the matter in the universe.
sourceDwarf planet
Orbits the Sun, is round from its own gravity and is not a moon, but has not cleared its orbit of other objects.
Pluto has been one since 2006.
sourceEclipse
When one body blocks the light of another. In a solar eclipse the Moon blocks the Sun; in a lunar eclipse Earth's shadow falls on the Moon.
Never look at a solar eclipse without proper eye protection.
sourceEquinox
The two days a year when the Sun is over the equator, so day and night are about equal everywhere.
Around 20 March and 22-23 September.
sourceEvent horizon
The boundary around a black hole beyond which nothing can return.
It is a boundary, not a solid surface.
sourceExoplanet
A planet that orbits a star other than the Sun.
More than 6,000 have been confirmed.
sourceGalaxy
A huge system of stars, gas, dust and dark matter held together by gravity.
We live in the Milky Way.
sourceGravity
The pull between things with mass. It keeps the Moon around Earth and planets around the Sun.
On the Moon you would weigh about a sixth of your Earth weight.
sourceKuiper Belt
A ring of icy bodies beyond Neptune.
Pluto is one of its largest members.
sourceLight-year
The distance light travels in one year: about 9.46 trillion km. It measures distance, not time.
Light from the Sun takes about 8 minutes to reach us; from the nearest star, about 4.2 years.
sourceMagnitude
A measure of brightness in astronomy. Smaller numbers are brighter.
The Sun is about -27; the faintest stars seen by eye are about +6.
sourceMeteor
The streak of light when a small piece of space rock burns up in Earth's atmosphere. Also called a shooting star.
The rock in space is a meteoroid.
sourceMeteorite
A piece of space rock that survives the fall and lands on the ground.
Most come from asteroids.
sourceMilky Way
Our home galaxy, a barred spiral with hundreds of billions of stars.
On a dark night its disk looks like a faint band across the sky.
sourceMoon (natural satellite)
A natural body that orbits a planet or another body.
Earth has one; Jupiter has over 100 and Saturn nearly 300.
sourceNebula
A cloud of gas and dust in space. Some are where stars are born; some are what dying stars leave behind.
The Orion Nebula can be seen with the naked eye from a dark place.
sourceNeutron star
The collapsed core left after a massive star explodes: more mass than the Sun packed into a ball about 20 km across.
A sugar-cube-sized piece of it would weigh about a billion tonnes on Earth.
sourceOrbit
The curved path of one object around another, held by gravity.
The ISS orbits Earth about every 90 minutes.
sourceParsec
About 3.26 light-years: the distance at which a star's parallax angle (its tiny yearly wobble as Earth circles the Sun) is one arcsecond.
Astronomers use it for distances between stars and galaxies.
sourcePlanet
A body that orbits the Sun, is round from its own gravity, and has cleared its orbit of other objects (IAU 2006 definition).
There are eight in our solar system.
sourcePulsar
A spinning neutron star that sweeps beams of radiation past Earth, so it seems to pulse.
Some spin hundreds of times a second.
sourceRed giant
A late stage of a star like the Sun, when it swells and cools at the surface.
The Sun will become one in about 5 billion years.
sourceRedshift
The stretching of light toward longer, redder wavelengths when its source moves away.
It showed that galaxies are moving apart.
sourceSolar flare
A sudden burst of energy and radiation from the Sun's surface.
Strong ones can cause radio blackouts on Earth.
sourceSolstice
The days when the Sun reaches its highest or lowest point in the sky at noon, giving the longest and shortest days.
Around 21 June and 21 December.
sourceSpectrum
Light spread out by wavelength. It reveals what a star is made of, how hot it is and how it moves.
A rainbow is a spectrum of sunlight.
sourceStar
A huge ball of hot gas, mostly hydrogen and helium, that shines by nuclear fusion in its core.
The Sun is a star.
sourceSupernova
The powerful explosion of a star, which can briefly outshine a whole galaxy.
It spreads heavy elements into space.
sourceTelescope
An instrument that collects light (or other radiation) to see distant objects.
The James Webb Space Telescope observes mostly in infrared.
sourceWhite dwarf
The small, dense, hot core left when a Sun-like star runs out of fuel.
The Sun will become one in billions of years.
sourceZenith
The point in the sky directly above you.
The opposite point, below your feet, is the nadir.
source