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Double stars Tapping, Ken

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Publisher’s version / Version de l'éditeur: https://doi.org/10.4224/40000362 Skygazing: Astronomy through the seasons, 2019-01-29

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Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à [email protected]. Double stars are in fact double or multiple. Why this is takes us back to how stars form. Ken Tapping, 29th January, 2019 A cloud of gas and dust collapses into a disc. As it Ursa Major, “The Great Bear” is a big constellation. shrinks, it rotates faster and faster, just as a skater It fills a substantial chunk of the northern sky and spins faster when she pulls in her arms. The core looks like a large animal, maybe something like a of the cloud collapses to form a star, but the rest of bear. Most of the stars are faint, apart from seven the disc is spinning too fast to fall onto the new of them, with which we are very familiar; they star, so it forms a number of other lumps, all make up the shape of the “Big Dipper”, “The circling the star. The planets of the Solar System Plough”, “The Saucepan” or many other names, formed in this way. Whether a lump becomes a depending on where you’re from. planet or a star depends only on the size of the Starting from the end of the handle, the stars are lump. If the lump is large enough, the pressure and named Alkaid, Mizar, Alioth, and going temperature in the collapsing lump may become counterclockwise around the “bowl”, Megrez, high enough for nuclear fusion to start, in which Phad, Merak and Dubhe. These names are part of case we have a second star, or maybe a third or the enormous contribution to astronomy made by fourth. If the lump is too small, nuclear fusion does Arab scientists. Merak and Dubhe, marking the not start and we end up with a planet, asteroid or side of the bowl opposite the handle, are special. some other small rocky body. Many planetary They are known as “The Pointers” because they systems have some large planets, “gas giants”; show the direction to Polaris, the North, or Pole Jupiter, Saturn, Uranus and Neptune are examples Star, which is an important navigation reference, in our Solar System. If Jupiter, for example, were a although maybe, in these days of GPS, less so. few times larger, it would have become a red dwarf star. Luckily for us it didn’t. Look closely at Mizar. On a dark night, those of average sight will see a faint star close by. If One of the most memorable scenes in the first Star necessary, try using averted vision. Mizar has a Wars movie was Luke Skywalker looking wistfully companion, named Alcor. If you point a telescope at sunset on the desert planet of Tatooine, with at Mizar, you’ll see another companion star two suns heading below the horizon. This scene orbiting it. Alcor has a very close companion too. would be improbable in reality. It is likely that double-star systems have planets too. However, This time of year the constellation of Taurus, “The their orbits would be very complicated and the Bull” is prominent in the sky. Look right from Orion temperature variations on those planets as they or left from the Pleiades. Find Aldebaran, a bright, pass closer to one star or the other would make orange-red star. It lies at the top of a “V” of stars, them highly unpleasant places to live. So if we which represent the bull’s head. Aldebaran is his seek potentially life-bearing planets, double star angry red eye. Look halfway down that leg of the systems will not be the best places to look. “V” and you will see Theta Tauri (our modern star names are nothing like as romantic as the Arab, Mars, fading as it recedes, lies in the southwest Greek or Latin names). Each of the two stars is after dark. Venus and Jupiter lie close together in actually a close double star. the eastern sky before dawn. Venus is the brighter one. The Moon will be New on the 4th. There are many other double stars. Two of the prettiest and easiest to find are Albireo, a blue and Ken Tapping is an astronomer with the National orange pair in Cygnus, “The Swan” and Epsilon Research Council's Dominion Radio Astrophysical Lyrae, in Lyra , “The Lyre”. This is a quadruple Observatory, Penticton, BC, V2A 6J9. star. Both look wonderful through binoculars or a Tel (250) 497-2300, Fax (250) 497-2355 small telescope. Actually, a large fraction of stars E-mail: [email protected]