Course overview

Astronomy is the one science where you see the past with your own eyes — and the light-year is the ruler that proves it.

Stars, planets, constellations, the Moon and the Milky Way band — and the trick for telling a steady planet from a twinkling star.

Your real address has eleven lines, and the last one is the observable universe. We zoom out one nested doll at a time.

The analogy ladder — golf ball to galaxy — that turns unimaginable numbers into things you can picture for the rest of the course.

Nearly everything we know arrived here as light — and a single beam secretly carries four separate messages.

Visible light is one octave on a vast cosmic piano — and we had to build a different telescope for every other key.

Every element stamps a unique barcode onto light — which is how helium was found in the Sun before anyone found it on Earth.

The effect that drops a passing siren also reveals the motion of entire galaxies — and plants the seed of the expanding universe.

First milestone review of Modules 1–2, then the night Galileo's telescope quietly demolished the Earth-centered universe.

An orbit is just falling — and missing the ground forever. One law that covers the apple and the Moon alike.

Three laws, discovered with string, tacks and twenty years of stubbornness — and still used to weigh black holes today.

Gravity isn't a pull at all — it's the shape of space and time. Plus why astronauts float, and what escape velocity really means.

A collapsing cloud flattens like spinning pizza dough — and that one image explains why every planet orbits the same way, in the same plane.

It isn't fire, it isn't solid, and a million Earths would fit inside it. Meet the plasma engine that drives everything here.

Four siblings built from the same material that turned out wildly differently — and what that says about the Goldilocks zone.

Storms bigger than Earth, rings of ice, and moons hiding liquid oceans under their crusts — some of the best places to look for life.

The rubble that never became planets — why the asteroid belt is emptier than the movies, and how we practiced deflecting one.

A star is a permanent standoff between gravity squeezing in and fusion pushing out — and it converts four million tons of itself into light every second.

Parallax is just depth perception scaled up to the galaxy — and it's the first rung of every distance we know.

Plot brightness against temperature and the chaos snaps into a pattern. The single most powerful chart in stellar astronomy.

One number set at birth decides everything: how hot, how bright, how long, and how violently it ends.

The most spectacular endings in nature — and the forges that made the calcium in your bones and the iron in your blood.

Second milestone review of Modules 3–5, then the detective work of mapping our own barred spiral while trapped inside it.

Four million Suns crushed into a point at the center of the Milky Way — and the decades of patient orbit-watching that proved it's there.

Galaxies come in families, and they collide — including ours with Andromeda, in a merger that has already begun.

Parallax to standard candles to exploding stars — overlapping rulers, each one calibrated by the last, stretching to the edge of the visible universe.

Galaxies spin far too fast to hold together — unless something invisible, and five times more abundant than everything we can see, is holding them.

In 1998 two rival teams set out to measure how fast expansion was slowing down. It wasn't slowing down at all.

Zoom out far enough and galaxies stop being islands and become droplets on a web of filaments — the largest structure that exists.

Nothing is flying through space away from us. Space itself is stretching — like raisins drifting apart in rising bread.

It was not an explosion in space. It was space itself, everywhere at once, beginning to stretch — and the difference matters.

The oldest light there is, released 380,000 years after the beginning — and a sliver of old analog TV static was always part of it.

Where every atom in you came from, and what 13.8 billion years looks like squeezed into a single calendar year.

Thousands of confirmed planets around other stars, found by watching for a moth crossing a distant searchlight.

The Drake Equation isn't a calculator — it's a way of organizing our ignorance. And the silence it exposes is the strangest fact in science.

The Sun swells into a red giant, the last stars gutter out, and the lights go off one by one — told with wonder, not dread.

JWST, thirty-meter telescopes, gravitational-wave detectors — and a final zoom out to one pale blue dot, six billion kilometers away.

Writing, reading and multiplying enormous and tiny numbers without going cross-eyed.

Why gravity and brightness both fade as 1/distance², straight from the geometry of a sphere.

The rule period² = distance³, worked through — and how the same two numbers weigh the Sun and a black hole.

Where the Sun's staggering energy output actually comes from, step by step.

From the everyday Doppler ratio to cosmological redshift, recession speed and the Hubble constant.

Escape velocity from energy conservation, and the one-line leap to the Schwarzschild radius.

Parallax, Cepheids and Type Ia supernovae — and how each rung's error feeds the next.