Unit 19
Stars and the universe.
How we know stars we can never touch: their colour, spectra, lives and distances, and Hubble's law.
Before this unit
The unit, lesson by lesson
Lessons
5 lessons · 30 practice questions · about 2 hours
Starlight and temperature
A glowing body gives out a continuous spectrum that depends only on its temperature. Wien's law links the peak to the temperature, the Stefan–Boltzmann law the power, and together they read a star's temperature from its colour.
Spectral lines
A glowing gas gives out only certain wavelengths: jumps between the atom's energy levels. Each element gives a barcode of its own, and that is how we know what stars are made of, even from the dark lines in their light.
The life of a star
A star shines by nuclear fusion, and its mass decides everything: how bright it is, how long it lives, and how it ends, as a white dwarf, a neutron star or a black hole.
Measuring distances in space
The distance ladder: parallax for nearby stars, parsecs and light years, and the inverse-square law with standard candles, Cepheids and supernovae, for distant galaxies.
The expanding universe
Galaxies are moving apart, the distant ones faster: Hubble's law. The universe has no centre, space itself stretches and stretches light with it, and the Hubble time says roughly when it all began.