cosmic-distance-ladder

Variable stars change brightness over time, from gentle pulsations to eclipses and explosions. Cepheids follow a period-luminosity law found by Henrietta Swan Leavitt that turned them into distance markers, proved Andromeda lies outside the Milky Way, and still anchors measurements of the Hubble constant.

From parallax to supernovae, astronomers chain overlapping methods together to measure distances from the nearest stars to galaxies billions of light-years away. Here is how each rung of the cosmic distance ladder works, and why its top rung is now in dispute.