Gemini – The Twins. The twins in question, Castor and Pollux of Greek Argonaut mythology, were actually half-brothers, born at the same time (hence twins) as a result of a dalliance by Zeus with Leda, the wife of Tyndareus, a Spartan king. As a result, Leda gave birth simultaneously to two sets of twins: Pollux and Helen who were children of Zeus and Castor and Klytemnestra, children of Tyndareus. We see the twins from Down Under as standing on their heads.
Pollux and Castor (the stars) represent those heads and are Gemini’s two main stars which stand out quite brightly at mag. 1.6 and 1.2, quite easily recognised this month almost directly north about 30o above the horizon. It is also useful to know that they are exactly 4.5o apart so you can use this to judge other angular distances in the sky.
While visible only as a single star in binoculars, Castor is in fact an amazing multiple star. Six stars in fact. Telescopes can reveal it to comprise two separate stars of magnitudes 1.9 and 3.0, orbiting each other every 470 years. However, both of these stars are themselves spectroscopic binaries (that is, so close together they can only be ‘split’ with their light spectrums, not visibly). That makes four stars. But telescopes will also reveal that Castor has a red dwarf companion which itself is an Algol type (eclipsing) binary star. That makes six stars in the Castor system in all. Amazing.
But back to the binoculars.
Find a (Castor) and b (Pollux), then from the map above locate the general shape of the constellation with its stars d, g, m and e. Gemini contains a number of optical doubles for binoculars. For example. just less than half way between d and g you will find a 4th mag. star, z (Zeta) Geminorum, an optical double, consisting of a yellow supergiant of mag. 3.7 to 4.2, and a wide (1.5' separation) mag. 7.6 companion. The yellow star’s magnitude is shown as a range because it is a member of a special kind of star – a Cepheid Variable. These stars vary in magnitude on a regular cyclic basis and astronomers use them to calculate distances to distant stars and even other galaxies.
Now find the 3rd mag. star m Gem, about 7o below and to the west of g. Just over 1/3rd the way from m to g you will find the 4th mag. star n (nu) Gem, another optical double, consisting of a hot blue-white star 500 light years away, with a wide but very faint 8th mag. companion just visible in binoculars.
But now for Gemini’s ‘star’ binocular object. Messier recorded this cluster in 1764 but it was certainly known before then, being visible to the naked eye as a cloudy nebula in clear dark skies. Starting from m Gem again, move just under 2o west to the 3rd magnitude star eta (h) Gem. Then move about 2.3° north-west of h to land on M35, a lovely open star cluster containing about 200 stars. (In fact, m, h and M35 should all fit comfortably within your binocular’s field of view.) As mentioned earlier, at 5th magnitude, M35 is visible in dark skies to the naked eye as a fuzzy patch about as big as the Moon. Binoculars will resolve it into 10 to 15 stars within a milky sprinkling of the less resolvable stars. With larger magnification binoculars you should be able to see a number of stars linking like looping chains. Look at it long enough and you will start to see all kinds of shapes and patterns. This cluster is 2,800 light years away.
In the image of M35 below (it’s that splatter of stars in the left-centre of image), you’ll notice a smaller more compact cluster near M35’s lower right boundary. That’s NGC2158 which is actually 10,000 light years beyond M35. You cannot see this with binoculars but is a nice target for telescopes of aperture 100mm or larger.
For a detailed list of more binocular objects in the southern sky, see my book “Heavens Above – A Binocular Guide to the Southern Skies”. A link is provided at the bottom of this website’s Home page.
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