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New to astronomy? We explain every term as you browse, in plain English. Same pages, with the help built in.

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Start simple. Go infinitely deep. A guided path from tonight’s sky to the fate of the universe.

Astronomy 101: the path

Ten steps in order, each one building on the last. Take a week or a year. The universe will wait.

  1. 1
    Earth and the skyWhy the Sun rises, what stars are, and how the sky works
    Start with tonight's real sky →
  2. 2
    The MoonPhases, tides, eclipses, and footprints that last a million years
    Visit Earth's companion →
  3. 3
    The Solar SystemEight planets, ocean moons, and two belts of leftovers
    Take the grand tour →
  4. 4
    StarsHow they ignite, live and die, and where your atoms were made
    The lives of stars →
  5. 5
    GalaxiesCities of stars, and the dark matter that holds them
    Meet the Milky Way →
  6. 6
    GravityThe force that shapes everything, from apples to orbits
    Understand the pull →
  7. 7
    LightWhy telescopes are time machines
    Read the universe's mail →
  8. 8
    Space and timeEinstein's universe: bending space, slowing clocks
    Go relativistic →
  9. 9
    Black holesWhere gravity wins: photographed, weighed, real
    Face the monsters →
  10. 10
    The universeThe Big Bang, dark energy, and how it all may end
    The biggest picture →

The full library, by topic

Cosmology

Cosmic InflationDark EnergyDark MatterHow Galaxies WorkHow the Universe EndsRedshiftThe Big BangThe Cosmic Microwave BackgroundThe Cosmic WebThe Expanding UniverseThe Multiverse

Astrophysics

Falling Into a Black HoleGeneral RelativityGravityHawking RadiationHow Black Holes WorkLight & the Electromagnetic SpectrumNuclear Fusion: Why Stars ShineQuantum Mechanics (for Stargazers)Special RelativitySpectroscopy: Reading StarlightSupernovaeThe Lives of StarsTime DilationWhere the Elements Come From

Exoplanets

Habitable ZonesHow We Find Exoplanets

Space Travel

How Rockets WorkInterstellar Travel: The Honest PhysicsOrbital Mechanics

Observing

How Telescopes Work

Astrobiology

SETI: Listening for NeighborsThe Drake EquationThe Fermi ParadoxThe Science of Life Elsewhere

Speculative Physics

Wormholes

Two other ways in

For kids & familiesYoung Astronomers: big questions answered straight, amazing facts, and a quiz. Nothing dumbed down.
For students & teachersEvery object page layers Basics → Go Deeper, every fact carries its source, and the glossary defines every term, all built to be citable (then verified at the primary source).