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Science: All the best stars have a double: The Dog has a Pup. Heather Couper and Nigel Henbest look at binaries

There is more to some stars than meets the eye. Take Gemini's Castor, high in the south on February evenings. At first glance, it looks almost identical to its 'twin star' Pollux - only fainter and bluer.

But take a small telescope to Castor, and you find two stars instead of one. The pair are a double star, or 'binary', each in orbit about the other. They take 420 years to complete one circuit. When examined, each star of the pair turns out to be double. And there is another, fainter star in the system that is also a binary. What appeared to be one star is actually six.

Castor is not unusual. In fact, our Sun is a relative rarity in being single - more than half of all stars are double. Some astronomers believe this might limit the number of life-bearing planets in the Galaxy, because planets around double stars would have peculiar orbits.

The best-known double star in the sky is in the 'tail' of Ursa Major - Mizar and its companion Alcor. The two are easily visible to the naked eye and have many nicknames, such as 'the horse and rider'. They are not a real pair, being respectively 60 and 80 light years away. But Mizar does have another companion, which itself is double. Mizar, too, is double (and so is Alcor), making for another complicated system.

Some double stars are oriented in such a way that they appear to move behind and in front of each other. As a result, the light coming from the system seems to change. One such 'eclipsing binary' is Algol in Perseus. We owe most of our knowledge of Algol to the young deaf-mute English astronomer John Goodricke, who, in 1782, worked out why the star's brightness halved roughly every three days.

One of the most bizarre eclipsing binaries is Epsilon Aurigae. At the apex of the triangle of stars next to Capella, it stays constant in brightness for 27 years before undergoing a two-year eclipse. The mystery companion was once thought to be one of the biggest stars known: but the discovery that it is semi-transparent has led astronomers to believe that it is a huge disc of gas.

Double stars have been invaluable in helping scientists 'weigh' stars. There is no direct way of finding the masses of single stars. But by studying the orbits of double stars, you can discover how heavy each of them is - rather like balancing a dumbbell.

One of the heaviest pairs known is in the constellation of Monoceros (the Unicorn), a faint straggle of stars above Canis Major. Plaskett's Star turns out to be two stars very close together. Each component weighs in at 50 times the mass of the Sun - close to the limit at which a star would blow itself apart with the pressure of its own radiation.

At the other end of the scale is Sirius, the brightest star in the sky. The 'Dog Star' has a companion, nicknamed the 'Pup', which orbits Sirius every 50 years. If you weigh the Pup, it turns out to have the mass of a normal star. But it is incredibly faint: 10,000 times dimmer than Sirius. This all points to the Pup being very small - the size of Earth, but with the mass of the Sun.

The Pup is a star on the way out: a 'white dwarf' that has collapsed in on itself at the end of its life. Although a teaspoonful of its material would weigh a ton, the Pup has used up all its nuclear fuel, and can go no further. It will slowly cool to become an inert black cinder.

The Planets

This is not a good month for planet-watching. The only one worth trying for is Mercury, which sets almost two hours after the Sun on 4 February. But by mid-month, it will have dived behind the Sun again. Its neighbour, Venus, is also emerging into the evening twilight. You may just spot it setting an hour after the Sun by the end of February. Otherwise, the heavens are barren of planets until Jupiter rises at about midnight mid-month.

The Stars

Canis Major reaches its highest elevation in the evenings this month. Named after one of Orion's hunting dogs, it is an ancient constellation. It was important to the Egyptians, who based their calendar on the apparent motion of Sirius around the sky. They knew that when Sirius rose with the Sun in summer, the Nile floods were imminent. The Greeks also knew of Sirius's appearances in the parched months of summer, as commemorated in this poem by Alcaeus (7th-6th century BC):

'The cricket sounds sweetly from the leaves of the tree-top, and lo] the artichoke is blowing: now are women at their sauciest, but men leak and weak, because Sirius parches head and knees.'

Diary (all times GMT)

3 February: 8.06am Moon at last quarter.

4: Mercury at greatest eastern elongation.

10: 2.30pm new Moon.

18: 5.47pm Moon at first quarter.

19: 10.36pm minimum of Algol.

20: Mercury at inferior conjunction.

22: 7.24pm minimum of Algol.

26: 1.15am full Moon.

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