Castor
Where is Castor?
Look southeast, about 58° up at 05:17 in Gemini. It rises at 22:06 and climbs through the night.
- Rises
- 22:06
- Highest
- 07:37
- Sets
- 17:11
- Best after dark
- 05:17 · 58°
Can I see Castor tonight?
From London tonight, Castor is above the horizon after dark. It’s highest just before dawn, around 05:17, in the southeast at 58°.
At magnitude 1.6 it is one of the brighter points in the sky, easy to see by eye when it’s clear.
This is position and timing only: clouds, haze and light pollution aren’t included.
What does Castor look like?
Castor looks white to the eye. Its spectral type is A1.5 IV+: white subgiant (hotter than the Sun).
Eyes
A white star next to orange Pollux.
Binoculars
A single white point.
Telescope
At about 100× or more, Castor splits into two white stars very close together, a classic double star.
About Castor
Castor is the second-brightest star in Gemini, the Twins. To the eye it is a single white star, but it is one of the most remarkable multiple star systems known: six stars in three pairs, all bound together.
The system is about 51 light-years away. A small telescope splits the two brightest components, each of which is itself a close pair, and a faint red pair orbits farther out.
The two brightest stars of Castor take several centuries to orbit each other, and their separation as seen from Earth changes slowly over decades.
Castor C, the faint outer pair, consists of two red dwarfs that eclipse each other, so the pair dims briefly every 20 hours or so.
Although it is labeled Alpha Geminorum, Castor is slightly fainter than Pollux. In mythology the twins were brothers; Castor was mortal and Pollux immortal.
Castor in Gemini
Questions
How many stars are in Castor?
Can I split Castor with a small telescope?
Sources: SkyMate; Wikidata (CC0). Positions computed for your place and time with astronomy-engine. Data & Licences