Every term explained simply, then deeply
Showing 97 terms
How bright a star truly is, measured as if viewed from a standard distance.
The gradual gathering of matter onto an object by gravity.
A swirling, superheated disk of matter spiraling into a black hole or star.
An extremely bright galactic core powered by a feeding supermassive black hole.
The point in an orbit where an object is farthest from the Sun.
How bright a star looks from Earth, regardless of its true brightness.
A rocky body, smaller than a planet, mostly found in the asteroid belt.
The average distance from Earth to the Sun, about 150 million kilometers.
Ordinary matter made of protons and neutrons — everything visible in the universe.
The rapid expansion from an extremely hot, dense state that began the universe.
A hypothesized reversal of cosmic expansion, collapsing the universe back to a single point.
A hypothesized end where accelerating expansion tears apart all matter, down to atoms.
The compression of light's wavelength toward blue as a source approaches.
An icy body that releases gas and dust into a glowing tail when near the Sun.
The faint afterglow radiation left over from the early hot universe.
The large-scale structure of the universe, with galaxies arranged along filaments around vast voids.
A constant energy density of space, thought to drive the universe's accelerating expansion.
A mysterious force causing the universe's expansion to accelerate.
An invisible substance that exerts gravity but emits no light, making up most of the universe's mass.
The total change in velocity needed to complete a spacecraft maneuver.
A round body orbiting the Sun that hasn't cleared its orbital neighborhood of debris.
The full range of light, from radio waves to gamma rays.
A smooth, rounded galaxy of mostly old stars with little new star formation.
A region outside a spinning black hole where spacetime itself is dragged along.
The minimum speed needed to break free of a body's gravity forever.
The boundary around a black hole beyond which nothing can escape.
A planet that orbits a star other than our Sun.
A long, thread-like structure of galaxies connecting across the cosmic web.
The twisting of spacetime caused by a massive rotating object.
How many wave cycles pass a point each second, measured in hertz.
The merging of atomic nuclei, releasing huge amounts of energy.
A massive, gravitationally bound system of stars, gas, dust, and dark matter.
A massive planet composed mainly of hydrogen and helium, with no solid surface.
An orbit matching Earth's rotation, keeping a satellite over the same longitude.
Using a planet's gravity and motion to boost or redirect a spacecraft's path.
The bending of light around massive objects, distorting what's behind them.
The force by which mass attracts other mass across space.
The orbital range around a star where temperatures allow liquid water to exist.
Faint theoretical radiation that causes black holes to slowly evaporate over time.
A theorized far-future state where the universe reaches maximum entropy and stops changing.
A chart plotting stars by temperature and brightness to reveal their life stage.
The most fuel-efficient elliptical path used to move a spacecraft between two orbits.
The rate at which the universe is currently expanding.
An exceptionally powerful supernova from the collapse of an extremely massive star.
A giant planet rich in water, ammonia, and methane ices, like Uranus and Neptune.
The tilt angle of an orbit relative to a reference plane.
A theorized burst of extremely rapid expansion in the universe's first fraction of a second.
A galaxy with no defined spiral or elliptical shape, often distorted by interactions.
A disk of icy bodies beyond Neptune, including Pluto.
A location where gravity from two bodies balances, letting a spacecraft stay put.
The distance light travels in one year — about 9.5 trillion kilometers.
The cluster of roughly 80 galaxies, including the Milky Way, bound by mutual gravity.
An orbit close to Earth, typically below 2,000 kilometers altitude.
The total amount of energy a star radiates per second.
A neutron star with an extraordinarily powerful magnetic field.
The stable, long phase of a star's life spent fusing hydrogen into helium.
The amount of matter in an object, which determines its gravitational pull.
The streak of light caused by a meteoroid burning up in a planet's atmosphere.
A meteoroid fragment that survives passage through the atmosphere and lands on the ground.
A small rocky or metallic fragment traveling through space.
A natural satellite that orbits a planet or other body.
The hypothesis that our universe is one of many, possibly infinite, universes.
A vast cloud of gas and dust in space, often where stars are born or die.
The ultra-dense crushed core left after a massive star's supernova.
The portion of the universe we can detect, limited by the speed of light and the universe's age.
A theorized vast spherical shell of icy bodies far beyond the Kuiper Belt.
The curved path one object traces around another due to gravity.
The time it takes an object to complete one full orbit.
A distance unit equal to about 3.26 light years, based on stellar parallax.
The point in an orbit where an object is closest to the Sun.
Motion in the same direction as the typical rotation or orbit of a system.
A clump of collapsing gas that hasn't yet ignited fusion to become a true star.
A spinning neutron star that sweeps beams of radiation past Earth like a lighthouse.
An extremely luminous, distant galactic core powered by a supermassive black hole.
The era, around 380,000 years after the Big Bang, when atoms first formed.
A swollen, cooling star late in life after exhausting its core hydrogen.
The stretching of light's wavelength toward red as a source moves away.
Motion in the opposite direction to the typical rotation or orbit of a system.
The radius defining the event horizon for a given mass.
The infinitely dense point at a black hole's center where known physics breaks down.
The stretching of an object into a long thin shape by extreme tidal gravity.
A measure of how efficiently a rocket engine uses its propellant.
A classification of stars by surface temperature and color, labeled O B A F G K M.
A galaxy with a flat, rotating disk and pinwheel-like arms of stars.
The life cycle a star passes through from birth to death.
A dense region within a nebula where new stars are actively forming.
The catastrophic explosion marking the death of a massive star.
A rocky planet with a solid surface, like Earth, Mars, Venus, or Mercury.
The stretching effect caused by gravity's strength differing across an object.
When a body's rotation matches its orbit, keeping the same face toward its partner.
The slowing of time experienced near strong gravity or at high speed.
An asteroid sharing a planet's orbit, held in place at a stable Lagrange point.
Speed in a specific direction — how fast something moves and where.
The vast collection of galaxy groups, including our Local Group, bound loosely together.
A vast region of space containing far fewer galaxies than average.
The distance between successive peaks of a light or sound wave.
The hot, dense leftover core of a low-mass star after it sheds its outer layers.