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

Free flashcards from Peabody at peabody.kids/flashcards/space-stars-universe