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Learn Astrophysics

Use ordinary physics to read starlight and explain how stars live and die.

Free to start. Built from a single prompt. No credit card.

Astrophysics applies the laws of physics to stars, galaxies and everything between them. Almost all of it rests on one source of evidence: light. From a star's spectrum you can read its temperature, composition and motion, and from gravity you can weigh it. A beginner starts with Kepler's laws and blackbody radiation, then builds up to stellar structure, nuclear fusion and the remnants stars leave behind. Learnr turns a single prompt into a personalized astrophysics course and teaches it in short, interactive lessons that adjust to how you are doing.

How to learn Astrophysics, step by step

Start with orbits and Newtonian gravity. Derive Kepler's third law from Newton's law of gravitation for a circular orbit, then use it to weigh the Sun from Earth's orbital period and distance. This one calculation shows how astrophysicists measure masses they can never touch.
Learn what light tells you. Study blackbody radiation, Wien's displacement law and the Stefan-Boltzmann law, then learn why atoms produce absorption and emission lines. Practice matching a stellar spectrum to its spectral class, O B A F G K M, from hottest to coolest.
Measure distance and luminosity. Work parallax problems in parsecs, then learn the inverse-square law that links apparent brightness to luminosity. The magnitude scale runs backwards and is logarithmic, so convert a few magnitude differences into brightness ratios until it stops feeling strange.
Build a star from physical balance. A star is a ball of gas where outward pressure balances inward gravity, called hydrostatic equilibrium. Add energy generation from fusion and energy transport by radiation and convection, and you can explain why massive stars burn out far faster than small ones.
Trace stellar lives on the H-R diagram. Follow a Sun-like star from the main sequence to red giant to white dwarf, then a massive star through supernova. Learn the Chandrasekhar limit, about 1.4 solar masses, and why it separates white dwarfs from neutron stars.
Practice calculations in small daily sets. Astrophysics mixes huge exponents and unfamiliar units like parsecs and solar masses. A few problems a day in a Learnr course keeps scientific notation and order-of-magnitude estimates sharp while new ideas pile on.

What you'll learn

Use Kepler's third law and Newtonian gravity to find the mass of a star or planet from an orbit
Estimate a star's surface temperature from its color with Wien's law
Read a Hertzsprung-Russell diagram and place a star on it by temperature and luminosity
Explain how hydrogen fusion powers the Sun through the proton-proton chain
Describe how a star's mass decides whether it ends as a white dwarf, neutron star or black hole

What's inside your Astrophysics course

A sample path Learnr builds for Astrophysics. Your real course adapts to your goal and level.

1. Gravity, orbits and Kepler's laws
2. Light, blackbody radiation and spectra
3. Measuring distance, brightness and luminosity
4. Stellar structure and nuclear fusion
5. Stellar evolution and the H-R diagram
6. White dwarfs, neutron stars and black holes

How long does it take to learn Astrophysics?

A conceptual introduction to stars and galaxies takes about 2 to 4 months at 3 to 5 hours a week. Working the physics quantitatively at a first undergraduate level usually takes a semester or two, and depends heavily on how comfortable you already are with calculus and introductory physics.

What you need before you start

Algebra, logarithms and scientific notation for the basics, plus introductory physics covering Newton's laws and energy. Calculus becomes necessary once you study stellar structure in detail.

Common mistakes to avoid

Treating the magnitude scale as linear. Each step of 5 magnitudes is a factor of 100 in brightness, and smaller numbers mean brighter objects.
Confusing apparent brightness with luminosity. A star can look faint because it is far away, so always account for distance before comparing stars.
Picturing a supernova as the fate of every star. Stars like the Sun shed their outer layers gently and leave a white dwarf instead.

How Learnr builds your course

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Astrophysics questions, answered

What is the difference between astronomy and astrophysics?

Astronomy is the broader study of objects in the sky, including observing and cataloging them. Astrophysics focuses on the physical explanation of what those objects are and why they behave as they do. Today the two overlap so much that the terms are often used interchangeably.

Do I need calculus for astrophysics?

Not to begin. Kepler's laws, Wien's law, parallax and luminosity problems need only algebra and logarithms. Calculus becomes important for stellar structure, orbital mechanics in depth and anything at the university level.

Can I learn astrophysics without a telescope?

Yes. Most of the subject is reasoning from data, and public archives of spectra and images are freely available. A small telescope or binoculars is a fun companion, but not a requirement.