Follow your curiosity

What discovery has been shared with you?

FACTREE gives you something interesting to discover every day — and something worth talking about together.

Start with one fact. Explore it, go deeper, then follow whichever branch catches your imagination.

Choose subjects for a surprise

Exploring any topic

Begin your discovery

Your next discovery is one click away.

Choose one or more subjects above, or leave Any Topic selected and let curiosity decide.

Astronomy

Measuring the Rotation of Distant Galaxies

Quick fact

By analyzing the Doppler shift of spectral lines, astronomers can measure the rotation speed of galaxies that are millions of light-years away with remarkable precision, even without resolving individual stars.

Why this is interesting

Ever wonder how we know that galaxies spin? It's not by watching a time-lapse movie. We read the light itself to measure the motion of billions of stars.