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Star	A huge ball of hot gas that makes its own light by fusion		space-stars-universe
Nebula	A cloud of gas and dust in space		space-stars-universe
Protostar	A young star still forming inside a nebula		space-stars-universe
Main sequence	The long stage when a star fuses hydrogen into helium		space-stars-universe
Red giant	A swollen, cool star near the end of its life		space-stars-universe
White dwarf	The small hot core left after a medium star ends		space-stars-universe
Planetary nebula	A glowing shell of gas thrown off by a dying star		space-stars-universe
Supernova	The explosion of a massive star at the end of its life		space-stars-universe
Neutron star	The super-dense core left by some supernovae		space-stars-universe
Pulsar	A spinning neutron star that sends out beams of radiation		space-stars-universe
Black hole	A region where gravity is so strong that not even light can escape		space-stars-universe
Event horizon	The boundary around a black hole beyond which nothing returns		space-stars-universe
Red dwarf	A small, cool, long-lived star		space-stars-universe
Supergiant	A very large, luminous star		space-stars-universe
Binary star	Two stars orbiting each other		space-stars-universe
Variable star	A star whose brightness changes		space-stars-universe
Nova	A white dwarf suddenly brightening by pulling in gas		space-stars-universe
Star color and temperature	Blue stars are hottest; red stars are coolest		space-stars-universe
Spectral classes from hot to cool	O, B, A, F, G, K, M		space-stars-universe
Spectral class of the Sun	G		space-stars-universe
Luminosity	The total amount of light a star gives off		space-stars-universe
Apparent magnitude	How bright a star looks from Earth		space-stars-universe
Absolute magnitude	How bright a star would look from a standard distance		space-stars-universe
Hertzsprung-Russell diagram	A graph plotting star brightness against temperature		space-stars-universe
Light-year	The distance light travels in one year, about 9.5 trillion kilometers		space-stars-universe
Parallax	The apparent shift of a nearby star against the background as Earth moves		space-stars-universe
Galaxy	A huge collection of stars, gas and dust held together by gravity		space-stars-universe
Milky Way	Our home galaxy, a spiral with a central bar		space-stars-universe
Spiral galaxy	A galaxy with arms curling out from the center		space-stars-universe
Elliptical galaxy	A smooth oval-shaped galaxy		space-stars-universe
Irregular galaxy	A galaxy with no regular shape		space-stars-universe
Galaxy cluster	A group of many galaxies bound by gravity		space-stars-universe
Local Group	The cluster of galaxies that includes the Milky Way		space-stars-universe
Large Magellanic Cloud	A small galaxy that orbits the Milky Way		space-stars-universe
Quasar	An extremely bright center of a distant galaxy powered by a black hole		space-stars-universe
Dark matter	Unseen matter that adds gravity to galaxies		space-stars-universe
Dark energy	A mysterious force that speeds up the expansion of the universe		space-stars-universe
Big Bang theory	The universe began extremely hot and dense about 13.8 billion years ago		space-stars-universe
Expanding universe	Space is stretching, carrying galaxies apart		space-stars-universe
Redshift	Light from receding objects stretches toward red		space-stars-universe
Who found that galaxies move away from us in 1929?	Edwin Hubble		space-stars-universe
Cosmic microwave background	The faint glow left from the early universe		space-stars-universe
Observable universe	The part of the universe we can see from Earth		space-stars-universe
Exoplanet	A planet orbiting a star other than the Sun		space-stars-universe
Habitable zone	The region around a star where liquid water could exist on a planet		space-stars-universe
Telescope	An instrument that gathers light to see faraway objects		space-stars-universe
Refracting telescope	A telescope that uses lenses		space-stars-universe
Reflecting telescope	A telescope that uses mirrors		space-stars-universe
Radio telescope	A dish that collects radio waves from space		space-stars-universe
Spectrum of starlight	Shows what elements a star contains		space-stars-universe
Astronomy	The study of space and everything in it		space-stars-universe
Astrophysics	The branch of astronomy using physics to explain objects in space		space-stars-universe
Cosmology	The study of the origin and structure of the universe		space-stars-universe
Who proposed the Sun-centered solar system in 1543?	Nicolaus Copernicus		space-stars-universe
Who first used a telescope to study the sky in 1609?	Galileo Galilei		space-stars-universe
Who described the laws of planetary motion?	Johannes Kepler		space-stars-universe
Which star is closest to Earth?	The Sun		space-stars-universe
Which star is the closest after the Sun?	Proxima Centauri		space-stars-universe
Star formation location	Inside dense nebulae		space-stars-universe
Chandrasekhar limit	The maximum mass of a white dwarf, about 1.4 solar masses		space-stars-universe
Solar mass	A unit equal to the mass of the Sun		space-stars-universe
Gravitational waves	Ripples in space-time first detected in 2015		space-stars-universe
First image of a black hole shadow, released	2019, in the galaxy M87		space-stars-universe