Exploring Gemini

Everyone’s heard of it, and lots of people may think of it as ‘their’ constellation – Gemini, the Twins. It’s famous because it’s in the Zodiac, the path in the sky taken by the planets and indeed the Sun and Moon. In March 2022 there are no planets in Gemini, so its pattern of stars shows up clearly.

How to find the Twins. This shows the view from the UK in March. The Moon will be close to Castor and Pollux on 12 and 13 March 2022. Map adapted from Stellarium

The most distinctive feature of Gemini is the pair of stars, Castor and Pollux, named after the legendary Twins of Greek myth. Castor was mortal and Pollux was immortal. But the stars are certainly not identical twins. Find them for yourself, above and to the left of Orion by taking a line from Rigel through Betelgeuse. They are of similar brightness, but look closer and you’ll see that Pollux, the lower of the two, is noticeably brighter. And it is a yellow star, while Castor is white. The photo below was taken with a diffusing filter over the lens, which helps to emphasise the colour. 

Both stars are comparatively nearby, with Castor being about 50 light years away and Pollux about 33. Castor is actually intrinsically the brighter of the two, but its greater distance make it appear fainter as we see it. 

Castor abd Pollux in,closeup. Photo: Robin Scagell

How many stars?

To the naked eye, Castor is a single star. But view it through even a small telescope using a magnification of about 50 and you’ll find that it is a pair of stars, called Castor A and Castor B, separated by about six arc seconds. This is a genuine double star, and the two stars take some 460 years to make one full orbit. Surprisingly, even though this seems a long time, the stars have changed position in a human lifetime. Back in the 1970s, the stars were only about two arc seconds apart, having been at their closest in 1965. But since then they have been separating out, and have changed their position relative to each other. 

Nearby is another but fainter star, Castor C. You’d have to live a long time to see this move, because it has an orbital period of several thousand years.

But that’s not the end of the story. Each of these three stars is a close double or binary star, whose components orbit each other in a matter of days. The individual stars are all too close to be seen with telescopes, and we only know about them from spectroscopy (the analysis of the starlight). So one of the Twins has twins three times over! However, Pollux is a single star. 

At top, Castor A and B. At bottom, the fainter Castor C, which at magnitude 10 requires a medium-sized telescope to be seen well. Photo: Robin Scagell

Picking out the Twins

As well as the two main stars, the Twins in the sky are marked by patterns of stars that represent their bodies. So sometimes people might refer to ‘near Castor’s left foot’, for example. 

The stars that mark the twins are not particularly bright but you should be able to pick them out even from suburban skies. The only other fairly bright star in the constellation, other than Castor and Pollux, is Alhena, which is very slightly fainter than Castor, at magnitude 2. 

Incidentally, the circumstances (which we will not go into here) that resulted in one mythological Twin (Pollux) being immortal and one being mortal also produced two sisters, Helen and Clytemnestra, Helen likewise being immortal. Ian Ridpath’s Star Tales relates that Helen later became famous as Helen of Troy. However, there is no star in the sky named Helen. Apart, that is, from the innumerable ones that certain companies have charged people called Helen to ‘name’ a star, but which have absolutely no validity at all.

Credit: Robin and Sally Scagell

Gemini’s jewel box

Gemini lies in the Milky Way, which is the faint band of light that stretches around the heavens and is our own galaxy seen from the inside. ‘Faint’ is how most people in the UK would consider it, but those who live in dark-sky areas would say that it is really bright and obvious. This part of the Milky Way really is quite faint, however, as we are looking outwards where it is thinnest, rather than inwards towards the centre, which is what we see in late summer and autumn. 

Even so, there are many stars in the area and even in towns you can see plenty of them using binoculars. Where there are lots of stars there are usually star clusters, and there is one in Gemini that stands out. Again, town dwellers may struggle to see it, but from a reasonably dark sky the cluster M35 is a pretty sight. It’s easy to find, close to Castor’s left foot (see our previous sections) and the two stars Eta and Mu Geminorum act as a guide, as shown on the map. Both are reddish in colour, which binoculars show up well. 

Photography brings out the star colours in M35 well, as shown here. It also reveals that there is another, fainter star cluster nearby, NGC 2158. This is around 11,000 light years away, compared with M35’s 2800 light years, so it lies much farther out in the Galaxy.

Mu and Eta Geminorum, and the open cluster M35. Credit Robin Scagell
A closeup of M35 with NGC 2158. Credit Robin Scagell