sterrō

The Proto-Indo-European root *h₂stḗr gave birth to words for star across nearly every branch of the family. Sanskrit had tārā, Greek had astḗr, Latin had stella (from an earlier *sterla). The Germanic branch produced *sterrō, which became steorra in Old English. The word was already ancient when the first Germanic speakers looked up and used it.

Greek astḗr entered Latin scientific vocabulary and never left. Ptolemy's Almagest, written around 150 CE in Alexandria, used the Greek word throughout. When Arab astronomers translated Ptolemy in the ninth century, they kept Greek star-terminology alongside Arabic. When medieval Europeans retranslated from Arabic, astḗr came back into Western languages as the root of astronomy, astrology, and asterisk.

The Germanic word took a simpler path. Old English steorra became Middle English sterre, then star. No detours through Arabic, no scientific rebranding. The word sat in everyday speech for centuries while its Greek cousin accumulated prefixes and suffixes in university lecture halls. Star meant the point of light. Astḗr meant the object of study.

English ended up with both: star for the thing in the sky, and astro- for the science of it. The split is not an accident. It is the usual division between the word people actually say and the word scholars write. Star is Germanic. Astronomy is Greek. They refer to the same light, separated by register.