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Learning path · 19 parts

Across the Universe

Outward through the cosmos — from the life and death of a single star to the elements it forges, the worlds around others, the extremes of gravity, and the expansion of everything.

About 3 hours of reading · 38 widgets

Start with The Sun: A Nuclear Furnace, Not a Fire
  1. Astronomy & Cosmology10 min

    Part 1: The Sun: A Nuclear Furnace, Not a Fire

    The Sun runs on nuclear fusion, not combustion, converting mass to energy — and that energy takes roughly 100,000 years to escape the core before reaching Earth.

  2. Astronomy & Cosmology10 min

    Part 2: The Life Cycle of Stars

    How a collapsing cloud of gas becomes a star, why its mass decides everything — including the surprising fact that the biggest stars die youngest — and how it ends.

  3. Astronomy & Cosmology10 min

    Part 3: The Hertzsprung–Russell Diagram: Mapping the Stars

    The Hertzsprung–Russell diagram plots stellar luminosity against surface temperature, revealing the main sequence, giants, and white dwarfs — and the tracks stars trace as they evolve.

  4. Astronomy & Cosmology10 min

    Part 4: Where the Elements Come From

    The calcium in your bones and the iron in your blood were built by nuclear fusion in stellar cores — a ladder that climbs to iron and then can climb no further.

  5. Astronomy & Cosmology10 min

    Part 5: How the Solar System Formed

    How a collapsing cloud of gas and dust spun itself into a flat disk 4.6 billion years ago, why the Sun holds almost all the mass, and how the frost line split the planets into small rocky worlds and distant giants.

  6. Astronomy & Cosmology10 min

    Part 6: The Moon: Phases, Faces, and Eclipses

    Why lunar phases come from the geometry of the sunlit half we see, why there is no permanently dark side, and why eclipses are rare shadow events.

  7. Astronomy & Cosmology10 min

    Part 7: Orbital Mechanics: The Art of Falling Sideways

    Why astronauts float despite full-strength gravity, why reaching orbit means going sideways not up, and why a higher orbit is a slower one.

  8. Astronomy & Cosmology10 min

    Part 8: Comets and Asteroids: The Solar System's Leftovers

    Why asteroids are rocky and comets are icy, how a comet grows a coma and tails as its ices sublime, and why a comet's tail always points away from the Sun.

  9. Astronomy & Cosmology10 min

    Part 9: Tidal Forces: Why the Moon Raises Two Bulges

    Why the Moon raises two ocean bulges (not one), why its weaker pull beats the Sun's, and how the same 1/r³ stretch locks moons and shatters them at the Roche limit.

  10. Astronomy & Cosmology9 min

    Part 10: How We Find Other Worlds

    Exoplanets are found not by their light but by the shadow they cast and the tug they exert — a periodic dip in starlight and a Doppler wobble in the star's spectrum.

  11. Astronomy & Cosmology10 min

    Part 11: Black Holes

    How escape velocity reaching the speed of light defines an event horizon, why the Sun would be 3 km across, and what a black hole would — and would not — do to Earth.

  12. Astronomy & Cosmology10 min

    Part 12: Neutron Stars and Pulsars

    How a dying massive star leaves behind a ~20 km ball as dense as an atomic nucleus, why it spins so fast, and how its sweeping radio beam makes a pulsar tick with clock-like regularity.

  13. Astronomy & Cosmology10 min

    Part 13: Gravitational Waves

    How accelerating masses shake spacetime, why the resulting strain is only ~10⁻²¹, and how LIGO's laser interferometers detect a ripple that stretches a 4 km arm by less than a thousandth of a proton's width.

  14. Astronomy & Cosmology10 min

    Part 14: Galaxies and Dark Matter

    Vera Rubin's flat rotation curves, gravitational lensing, and the Bullet Cluster all point to the same conclusion: galaxies are embedded in vast halos of unseen mass that gravitates but does not shine.

  15. Astronomy & Cosmology10 min

    Part 15: Telescopes: Gathering Light, Resolving Detail

    Why a telescope's real job is gathering light and resolving detail — set by aperture area and diffraction — while magnification is cheap, swappable, and often useless.

  16. Astronomy & Cosmology10 min

    Part 16: The Cosmic Distance Ladder

    Distance is not something a telescope sees directly. From nearby stars to the edge of the observable universe, every distance is built on a ladder — geometric parallax, then standard candles, then redshift — where each rung borrows its calibration from the one beneath it.

  17. Astronomy & Cosmology10 min

    Part 17: The Cosmic Microwave Background

    Point a radio antenna at any patch of empty sky and you pick up the afterglow of the hot dense beginning — a near-perfect 2.725 K blackbody, redshifted from a 3000 K glow, whose faint ripples are the seeds of every galaxy.

  18. Astronomy & Cosmology10 min

    Part 18: Dark Energy: Why the Universe Speeds Up

    Dark energy makes up about 68% of the universe and drives its accelerating expansion — a pressure of space itself, opposite in effect to gravitating dark matter.

  19. Astronomy & Cosmology10 min

    Part 19: The Expanding Universe

    Distant galaxies recede faster the farther they are, yet there is no centre and no edge. Run the picture backward and you arrive at a hot dense beginning whose afterglow still fills the sky at 2.725 K.