Propus — Double Star in Gemini

Propus, Eta Geminorum, is a red-giant semiregular variable of spectral type M2 IIIa paired with a close F-type companion in a 473-year orbit. Combined magnitude varies between 3.15 and 3.9 on a roughly 234-day pulsation, with a longer slow modulation. The system lies about 375 light-years away and its total luminosity approaches 2,500 Suns. The primary is about 300 times the Sun's radius and is slowly losing mass through a steady stellar wind.

Propus marks the western foot of Castor, the northern Gemini twin — it lies almost directly north of Betelgeuse in winter. Its deep orange-red color is lovely in binoculars. Propus sits less than a degree northwest of the Sharpless 2-249 / IC 443 Jellyfish Nebula supernova remnant region, and just 2 degrees west of M35 — making it a natural waypoint for winter deep-sky tours. Best observed December through April.

The name Propus is Greek, meaning "forward foot" — a fitting label for the star placed at Castor's leading foot. It was named in the 17th century through Latin translations of Greek astronomical texts. The variability was discovered by Julius Schmidt in 1865. The binary nature was established through speckle interferometry in the 1980s.