Sixteen units, from the first tools to the frontier

The Astronomer's Toolkit

Numbers, units, angles and the physics that every later unit builds on

This unit collects the mathematics and physics that the rest of the program uses: powers of ten, astronomical units, angles on the sky, logarithms, scaling laws, Newtonian mechanics and the handling of uncertainty. Every tool is practiced on a real case, such as counting the galaxies in the universe from one Hubble image or weighing the density of a planet around another star.

Powers of ten and estimation · Astronomical units · Angles and the small-angle formula · Logarithms and scaling laws · Mechanics and gases · Uncertainty and significant figures

The Sky and Celestial Mechanics

How the sky turns, and the laws that govern every orbit

This unit starts with what anyone can see: the daily turning of the sky, the seasons, the phases of the Moon and eclipses. It then goes through Kepler's laws to Newton's gravitation and shows how a single force law lets us weigh the Sun and work out the speed a spacecraft needs to leave the solar system.

Celestial sphere and coordinates · Seasons, time and eclipses · Parallax and the parsec · Kepler's laws · Newtonian gravity and orbits · Tides and the Moon

Research spotlight Konstantin Batygin, California Institute of Technology

Light, Radiation and Spectra

How starlight is measured and what it says about its source

Almost all astronomical knowledge comes from light. This unit explains how brightness is measured and turned into distance, how the color of a star gives its temperature, and how the lines in a spectrum reveal composition, motion and physical conditions. With these tools a point of light becomes a star with a known size, temperature and velocity.

Waves and photons · Flux and magnitudes · Black body radiation · Hydrogen atom and spectral lines · Doppler shift and line profiles · Interstellar extinction

Research spotlight Ewine F. van Dishoeck, Leiden University

Telescopes and Observational Techniques

How light from the sky becomes a measurement

This unit explains how telescopes collect light and what limits the detail they can show: the aperture, diffraction, the atmosphere and the detector. Students work out the signal to noise ratio of a real observation and see how adaptive optics, interferometers and space observatories get around the limits of a single mirror on the ground.

Refractors and reflectors · Diffraction limit · Seeing and adaptive optics · Detectors and signal to noise · Radio interferometry · Space and survey telescopes

Research spotlight Marcia J. Rieke, University of Arizona

The Solar System

How a disk of gas and dust became eight planets, their moons and a trillion comets

This unit takes stock of the solar system and explains how it formed 4.567 billion years ago from a disk of gas and dust. You calculate planetary temperatures, the Roche limit and the positions of orbital resonances, and you meet the evidence that the giant planets changed their orbits early on and rearranged everything else.

Solar nebula and frost line · Radiometric dating · Planetary temperatures · Rings and ocean moons · Resonances and small bodies · Planetary migration

Research spotlight Alessandro Morbidelli, Collège de France and Observatoire de la Côte d'Azur

The Sun and the Properties of Stars

How the nearest star works and how all the others are measured

The unit takes the Sun apart layer by layer, explains its energy source, its magnetic cycle and the sound waves that reveal its interior, and then turns to the other stars. Students learn how distance, luminosity, temperature, radius and mass are measured, and how the Hertzsprung-Russell diagram and the mass-luminosity relation bring order to the results.

The Sun layer by layer · Solar cycle and helioseismology · Measuring stellar properties · Spectral classification · Hertzsprung-Russell diagram · Mass, luminosity and lifetime

Research spotlight Conny Aerts, KU Leuven

The Lives and Deaths of Stars

How a gas cloud becomes a star, and what the star leaves behind

This unit follows stars from cold molecular clouds through the main sequence to their ends as white dwarfs, neutron stars or black holes. You learn why mass decides the outcome, how supernovae work, where the chemical elements were made, and how a close companion can rewrite the whole story.

Molecular clouds and Jeans mass · Red giants and white dwarfs · Core collapse supernovae · Origin of the elements · Cluster ages · Interacting binaries

Research spotlight Alyssa A. Goodman, Harvard University, Center for Astrophysics | Harvard & Smithsonian

The Milky Way, Galaxies and the Expanding Universe

Our Galaxy, its neighbors and the evidence that the universe had a beginning

This unit maps the Milky Way and weighs it with the orbits of its stars, which leads to the case for dark matter. It then builds the distance ladder that reaches other galaxies, sorts those galaxies into types, and follows their redshifts to the Hubble-Lemaître law, the cosmic microwave background and an age of 13.8 billion years. It closes the Foundation part of the program with an inventory of what the universe contains.

Structure of the Milky Way · Rotation curves and dark matter · Cepheids and the distance ladder · Hubble sequence and clusters · Hubble-Lemaître law · Big Bang and microwave background

Research spotlight Chris Lintott, University of Oxford

Stellar Structure and Nuclear Fusion

What holds a star up and what keeps it hot

Four equations describe the inside of a star, and with a pocket calculator they give the pressure and temperature at the center of the Sun. You learn why protons fuse at a temperature a thousand times too low for classical physics, how the pp chain, the CNO cycle and the triple alpha process work, and how neutrino detectors deep underground checked all of it. The unit ends with the mass limits of stars and the origin of the elements beyond iron.

Equations of stellar structure · Virial theorem and timescales · Tunneling and the Gamow peak · pp chain, CNO cycle, triple alpha · Solar neutrinos · Mass limits and neutron capture

Research spotlight Anna Frebel, Massachusetts Institute of Technology

White Dwarfs, Neutron Stars and Black Holes

How dead stars resist gravity, and what happens when they cannot

When fusion ends, a star is held up by quantum mechanics or by nothing at all. The unit derives degeneracy pressure, the Chandrasekhar limit and the spin-down of pulsars, then turns to black holes: the Schwarzschild radius, time dilation near the horizon, and the ways in which accretion and orbital motion give dark objects away.

Degeneracy pressure · Chandrasekhar limit · Pulsars and magnetars · Schwarzschild black holes · Accretion and X-ray binaries · Finding stellar black holes

Research spotlight Jocelyn Bell Burnell, University of Oxford

Exoplanets and the Search for Life

How planets around other stars are found, weighed and examined for air

More than 6000 planets are known around other stars, almost all of them found without ever being seen. This unit explains the five detection methods with their formulas and blind spots, what the planet population looks like, how JWST reads the gases in a planet's atmosphere during a transit, and what would count as evidence of life.

Radial velocity and transit methods · Imaging, microlensing and astrometry · Planet population and radius valley · Atmospheres with JWST · Habitable zones and biosignatures

Research spotlight Didier Queloz, University of Cambridge and ETH Zurich

Active Galaxies and Supermassive Black Holes

How galaxies are weighed and what the black holes at their centers do

Stellar speeds weigh whole galaxies, and one star on a 16 year orbit weighs the four million solar mass black hole at the center of the Milky Way. The unit then explains how accreting black holes power Seyfert galaxies, quasars and jets, how the Event Horizon Telescope imaged two black hole shadows, and how black holes and clusters shape the galaxies around them.

Galaxy dynamics and virial masses · The Galactic center and S2 · Black hole scaling relations · Quasars and the unified model · Jets and the black hole shadow · Feedback and cluster lensing

Research spotlight Roger Blandford, Stanford University, Kavli Institute for Particle Astrophysics and Cosmology

Research spotlight Andrew C. Fabian, University of Cambridge, Institute of Astronomy

The Expanding Universe and Relativistic Cosmology

One equation for the age, the size and the history of the universe

This unit derives the Friedmann equation and uses it to compute how fast the universe expanded at any epoch, how old it is and how far away its galaxies are. Students learn why distance has several meanings at high redshift, what the horizons of the universe are, and how the Hubble constant is measured rung by rung with Cepheids, red giants and supernovae.

Scale factor and redshift · Friedmann equation · Critical density · Ages of model universes · Cosmological distances and horizons · Measuring the Hubble constant

Research spotlight Wendy L. Freedman, University of Chicago

The Hot Big Bang and the Cosmic Microwave Background

The thermal history of the universe and its oldest light

This unit follows the universe as it cools: the first second, the three minutes in which helium formed, and the moment 372,000 years later when atoms appeared and space became transparent. You learn to read the cosmic microwave background, whose acoustic peaks give the geometry and the contents of the universe to percent accuracy, and you meet the puzzles that led to the idea of inflation.

Thermal history · Primordial nucleosynthesis · Recombination and decoupling · CMB spectrum and anisotropies · Acoustic peaks · Inflation

Research spotlight Alan H. Guth, Massachusetts Institute of Technology

Research spotlight Hiranya V. Peiris, University of Cambridge

Dark Matter, Dark Energy and Cosmic Structure

What 95 percent of the universe is made of, and how we know

Ordinary matter makes up only 5 percent of the universe. This unit goes through the evidence for dark matter and the searches for it, shows how gravity built the cosmic web of galaxies, and explains why the expansion is accelerating. It ends at the present state of research: the DESI results on dark energy and the unresolved disagreement about the Hubble constant.

Evidence for dark matter · Dark matter searches · Growth of structure · Baryon acoustic oscillations · Dark energy and acceleration · The Hubble tension

Research spotlight Risa H. Wechsler, Stanford University, Kavli Institute for Particle Astrophysics and Cosmology

Research spotlight Adam G. Riess, Johns Hopkins University and Space Telescope Science Institute

Frontiers of Astrophysics

Gravitational waves, the first galaxies, radio bursts and neutrinos from deep space

The last unit covers measurements that became possible within the past ten years: gravitational waves from merging black holes and neutron stars, galaxies seen less than 300 million years after the Big Bang, millisecond radio bursts from other galaxies, and neutrinos from active galaxies. The student learns to read a chirp, to use a merger as a distance indicator and a radio burst as a probe of intergalactic gas, and finishes with open problems and practical advice on reading research papers.

Gravitational wave detectors · Chirp mass and mergers · Standard sirens · Pulsar timing and LISA · First galaxies with JWST · Fast radio bursts and neutrinos

Research spotlight Nergis Mavalvala, Massachusetts Institute of Technology

Research spotlight Roberto Maiolino, University of Cambridge