Gemini
What is Gemini?
Constellations· Winter evenings (N) · two bright stars side by side · December Geminids. The brother stars Castor and Pollux stand side by side as the twins' heads. In mid-December the Geminids, one of the richest meteor showers of the year, radiate from here.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Pollux | 1.1 |
| Castor | 1.6 |
| Alhena | 1.9 |
The 3 brightest of the 8 stars used in the figure (7 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | winter |
|---|---|
| Stars | Castor & Pollux |
| Highlights | December Geminids |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Castor is six stars — the secret of one twin's head
Castor, one of the twins, is not a single star. A small telescope splits it into two, and those two take more than 450 years to orbit each other. Splitting their light into its colours reveals that each of them is a pair as well. A faint red star a little way off, Castor C, is also a pair of small red stars circling each other in less than a day. Six stars in all make up one system, 51 light-years away.
The star matters in the history of astronomy too. In 1803 William Herschel showed that Castor's two stars orbit each other, the first evidence that a double star could be truly bound by gravity. Pollux, the other twin, is an orange giant about 34 light-years away. The two heads are some 17 light-years apart and have nothing to do with each other.
The constellation in depth
Gemini is two parallel lines of stars, with Castor and Pollux at the top as the twins' heads. The two look alike at first, but their colours differ: Pollux is a little brighter and orange, Castor white. On winter evenings the brothers stand side by side to the upper left of Orion.
In mid-December the Geminid meteor shower brings more than 100 meteors an hour under a dark sky, making it the most reliably rich shower of the year. Most meteor showers come from debris left by comets, but the Geminids are unusual: their debris comes from an asteroid called Phaethon.
Gemini is also where new planets of the solar system have been found. In 1781 William Herschel discovered Uranus near the star Eta Geminorum, and in 1930 Clyde Tombaugh found Pluto on photographs centred near Delta Geminorum. The ecliptic, the path of the Sun and planets, runs through Gemini, which is why wandering worlds turn up here.
Frequently asked
- When can I see Gemini?
- Gemini is best seen in winter.
- What is the brightest star in Gemini?
- The brightest star in Gemini is Pollux, at magnitude 1.1.
- In one line, what is Gemini?
- Constellations — The brother stars Castor and Pollux stand side by side as the twins' heads.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.