Threshold 2 · Stars light up
Formation of Stars
Gravity gathers clouds of hydrogen into the first stars and galaxies, and the universe gets its first sources of light and heat.
Gravity is the weakest of the forces and the most patient. Given a slightly uneven fog of hydrogen and helium and a hundred million years, it pulls the denser patches together until they collapse, heat and ignite. The first stars are born, and with them the first light since the afterglow.
This chapter is about what a star is and why it works: the balance between gravity pulling in and the pressure of fusion pushing out, which lets a ball of gas shine steadily for millions or billions of years. It is also about how we know any of this from a distance we can never cross, by reading the light itself.
Stars gather into galaxies, galaxies into the cosmic web, and our own star, the Sun, turns out to be an ordinary member of an enormous family.
Lessons
- 2.1 Gravity versus pressureDraft
How a cloud of gas collapses, heats and ignites.
- 2.2 What a star isPlanned
Fusion in the core balanced against gravity: the equilibrium that lets a star shine for billions of years.
- 2.3 The lives of stars, sorted by massDraft
Why a star's mass decides everything: how long it lives, how it dies and what it leaves behind.
- 2.4 GalaxiesPlanned
Cities of stars, dark matter and the cosmic web.
- 2.5 Reading starlightPlanned
Spectra, distances and the cosmic distance ladder: how we measure things we can never visit.
- 2.6 The Sun, a typical starPlanned
Our own star as an ordinary star, with Aditya-L1 as India's eye on it.
- 2.7 One chart for every starPlanned
The Hertzsprung–Russell diagram, which organises every star by brightness and temperature.