A free, self-paced textbook in 7 chapters. Read a chapter, then drill it with the 213 companion flashcards using spaced repetition.
Astronomy is the scientific study of celestial objects, space, and the physical universe, while cosmology focuses specifically on the origin, evolution, and large-scale structure o...
A star is a massive, luminous sphere of plasma that generates energy by nuclear fusion, and most spend the majority of their lives on the main sequence, fusing hydrogen into helium...
A galaxy is a gravitationally bound system containing stars, gas, dust, and dark matter, often spanning tens of thousands of light-years. We live in the Milky Way, a barred spiral...
The leading theory of the universe's origin is the Big Bang, which holds that the universe began about 13.8 billion years ago from a hot, dense state and has been expanding ever si...
Modern astrophysics rests on Albert Einstein's twin theories of relativity. Special relativity describes physics at high velocities and is built on the postulate that the speed of...
Astronomers measure cosmic distances using a hierarchy of techniques known as the cosmic distance ladder, in which each method calibrates the next, extending reach from nearby star...
The space age began in 1957 with the launch of Sputnik 1, the first artificial satellite, by the Soviet Union. Cosmonaut Yuri Gagarin became the first human in space in 1961, follo...