20 · Galaxies
The island universes
A century ago astronomers argued in public over whether the "spiral nebulae" were inside the Milky Way. They are not: they're other galaxies — hundreds of billions of them, each a hundred billion suns give or take, colliding, merging and evolving on billion-year clocks.
The Milky Way, from inside
A barred spiral roughly 100,000 light-years across, holding on the order of 100–400 billion stars, with Sagittarius A* anchoring the center and our Sun orbiting once every ~230 million years, two-thirds of the way out. We've never seen it from outside — every "photo of the Milky Way" is either the band across our sky or another galaxy standing in.
Andromeda is coming
M31 is approaching at ~110 km/s, and for years the standard line was a merger in about 4.5 billion years. Honest update: a 2025 analysis in Nature Astronomy, folding in the tug of satellite galaxies, put the odds of a merger within 10 billion years at roughly a coin flip. Either way, no star-on-star collisions — galaxies are mostly empty space passing through empty space.
Lensing, in one sentence. Mass bends light, so a foreground cluster acts as a natural telescope — smearing background galaxies into arcs — and the shape of the distortion reveals where the invisible mass sits. It's how the image above maps dark matter without seeing it. You've already met lensing: the arcs in this page's opening Webb deep field are the same physics.
21 · Cosmology
The universe, entire
Cosmology asks the biggest possible questions with the plainest possible data: one baby picture, one expansion rate, one energy budget. Here are all three — including the part where the numbers disagree.
That speckled oval is the entire sky, unrolled. It glows at 2.725 kelvin in every direction — the stretched-out flash of the moment the universe first turned transparent. The speckles are temperature differences of a few parts in 100,000, and every galaxy that exists grew from one of them.
The budget of everything
The gold sliver is everything ever photographed on this page — every star, planet, nebula and person. The other 95% of the universe's energy content is detected only by its gravity (dark matter) or by the acceleration it drives (dark energy). Figures from the Planck mission's fit of the CMB.
The open argument: Hubble tension
Expansion rate predicted from the early universe (Planck CMB fit)
Expansion rate measured nearby, from Cepheids & supernovae (SH0ES)
Why it matters. The two methods should agree, and stubbornly don't — the gap has survived a decade of error-hunting. It may be a subtle systematic. It may be the first crack in the standard model of cosmology. This is what a live scientific controversy looks like, mid-argument. (The Cepheid rung of that ladder was built by Henrietta Leavitt — section 24.)
13 · Worlds in the round
Five worlds in four dimensions
Real spacecraft-derived maps wrapped onto spinning globes — three dimensions of shape plus the fourth, time, as each world turns at its true axial tilt. Saturn wears its rings; Uranus rolls by on its side, exactly as it really does. Drag any globe to explore.