Dancing with the bear

Ursa Major, The Great Bear, is a big constellation.

It fills a substantial chunk of the northern sky and looks like a large animal, maybe something like a bear.

Most of the stars are faint, apart from seven of them, with which we are very familiar; they make up the shape of the Big Dipper, The Plough, The Saucepan or many other names, depending on where you're from.

Starting from the end of the handle, the stars are named Alkaid, Mizar, Alioth, and going counterclockwise around the "bowl", Megrez, Phad, Merak and Dubhe.

These names are part of the enormous contribution to astronomy made by Arab scientists. Merak and Dubhe, marking the side of the bowl opposite the handle, are special.

They are known as The Pointers because they show the direction to Polaris, the North, or Pole Star, which is an important navigation reference, although maybe, in these days of GPS, less so.

Look closely at Mizar. On a dark night those of average sight will see a faint star close by. If necessary, try using averted vision. Mizar has a companion, named Alcor. If you point a telescope at Mizar, you'll see another companion star orbiting it. Alcor has a very close companion too.

This time of year the constellation of Taurus, The Bull, is prominent in the sky. Look right from Orion or left from the Pleiades. Find Aldebaran, a bright, orange-red star. It lies at the top of a "V" of stars, which represent the bull's head.

Aldebaran is his angry red eye. Look halfway down that leg of the "V" and you will see Theta Tauri (our modern star names are nothing like as romantic as the Arab, Greek or Latin names). Each of the two stars is actually a close double star.

There are many other double stars. Two of the prettiest and easiest to find are Albireo, a blue and orange pair in Cygnus, The Swan and Epsilon Lyrae, in Lyra , The Lyre.

This is a quadruple star. Both look wonderful through binoculars or a small telescope. Actually, a large fraction of stars are in fact double or multiple. Why this is takes us back to how stars form.

A cloud of gas and dust collapses into a disc. As it shrinks, it rotates faster and faster, just as a skater spins faster when she pulls in her arms. The core of the cloud collapses to form a star, but the rest of the disc is spinning too fast to fall onto the new star, so it forms a number of other lumps, all circling the star.

The planets of the solar system formed in this way. Whether a lump becomes a planet or a star depends only on the size of the lump. If the lump is large enough, the pressure and temperature in the collapsing lump may become high enough for nuclear fusion to start, in which case we have a second star, or maybe a third or fourth.

If the lump is too small, nuclear fusion does not start and we end up with a planet, asteroid or some other small rocky body. Many planetary systems have some large planets, gas giants. Jupiter, Saturn, Uranus and Neptune are examples in our solar system.

If Jupiter, for example, were a few times larger, it would have become a red dwarf star. Luckily for us it didn't.

One of the most memorable scenes in the first Star Wars move was Luke Skywalker looking wistfully at sunset on the desert planet of Tatooine, with two suns heading below the horizon.

This scene would be improbable in reality. It is likely that double-star systems have planets too. However, their orbits would be very complicated and the temperature variations on those planets as they pass closer to one star or the other would make them highly unpleasant places to live.

If we seek potentially life-bearing planets, double star systems will not be the best places to look.

• Mars, fading as it recedes, lies in the southwest after dark.
• Venus and Jupiter lie close together in the eastern sky before dawn. Venus is the brighter one.
• The moon will be new on the 4th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.