subject
Astronomy
How astronomers learn about the universe — what we measure, how we measure it, and what counts as evidence at scales no instrument can directly probe.
7 lessons in astronomy
How a Star Becomes a Black Hole
A star spends almost its entire life in a stalemate. Gravity pulls every atom toward the center; the heat of nuclear fusion in the core pushes back. As long as fusion keeps producing energy, the star …
4 min · deepening
How Stars Make Their Own Light
Hold your hand up to the Sun on a clear afternoon and you are catching photons that began their journey not eight minutes ago, as the textbooks sometimes suggest, but tens of thousands of years ago — possibly longer.
4 min · foundation
How We Know the Universe Is Expanding
In 1929, Edwin Hubble plotted the distances of galaxies against the speeds at which they appeared to be moving and saw something strange: the farther a galaxy was, the faster it was receding.
4 min · synthesis
Refractors and Reflectors: Two Telescope Designs
Point a long brass tube at Jupiter on a clear night and you are using, in essence, the instrument Galileo turned skyward in 1609.
4 min · comparison
What a Light-Year Actually Measures
The phrase sounds like a measure of time. A light-year — the very word ends in "year" — invites the ear to hear something temporal, as if it counted moments rather than miles. This is the first thing …
4 min · foundation
Why Galaxies Have Spiral Arms
If you photograph a spiral galaxy like M51, the arms look so solid you could imagine reaching out and touching them.
4 min · deepening
Why the Moon Always Shows Us the Same Face
Look up at a full Moon tonight, and then again next month, and again next year.
4 min · foundation