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.

(Graphic omitted)

Suggested Topics
Start your day with The Independent, sign up for daily news emails
ebooksA celebration of British elections
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Ashdown Group: Trainee Consultant - Surrey/ South West London

£22000 per annum + pension,bonus,career progression: Ashdown Group: An establi...

Ashdown Group: Trainee Consultant - Surrey / South West London

£22000 per annum + pension,bonus,career progression: Ashdown Group: An establi...

Ashdown Group: Recruitment Consultant / Account Manager - Surrey / SW London

£40000 per annum + realistic targets: Ashdown Group: A thriving recruitment co...

Ashdown Group: Part-time Payroll Officer - Yorkshire - Professional Services

£25000 per annum: Ashdown Group: A successful professional services firm is lo...

Day In a Page

Fishing for votes with Nigel Farage: The Ukip leader shows how he can work an audience as he casts his line to the disaffected of Grimsby

Fishing is on Nigel Farage's mind

Ukip leader casts a line to the disaffected
Who is bombing whom in the Middle East? It's amazing they don't all hit each other

Who is bombing whom in the Middle East?

Robert Fisk untangles the countries and factions
China's influence on fashion: At the top of the game both creatively and commercially

China's influence on fashion

At the top of the game both creatively and commercially
Lord O’Donnell: Former cabinet secretary on the election and life away from the levers of power

The man known as GOD has a reputation for getting the job done

Lord O'Donnell's three principles of rule
Rainbow shades: It's all bright on the night

Rainbow shades

It's all bright on the night
'It was first time I had ever tasted chocolate. I kept a piece, and when Amsterdam was liberated, I gave it to the first Allied soldier I saw'

Bread from heaven

Dutch survivors thank RAF for World War II drop that saved millions
Britain will be 'run for the wealthy and powerful' if Tories retain power - Labour

How 'the Axe' helped Labour

UK will be 'run for the wealthy and powerful' if Tories retain power
Rare and exclusive video shows the horrific price paid by activists for challenging the rule of jihadist extremists in Syria

The price to be paid for challenging the rule of extremists

A revolution now 'consuming its own children'
Welcome to the world of Megagames

Welcome to the world of Megagames

300 players take part in Watch the Skies! board game in London
'Nymphomaniac' actress reveals what it was really like to star in one of the most explicit films ever

Charlotte Gainsbourg on 'Nymphomaniac'

Starring in one of the most explicit films ever
Robert Fisk in Abu Dhabi: The Emirates' out-of-sight migrant workers helping to build the dream projects of its rulers

Robert Fisk in Abu Dhabi

The Emirates' out-of-sight migrant workers helping to build the dream projects of its rulers
Vince Cable interview: Charging fees for employment tribunals was 'a very bad move'

Vince Cable exclusive interview

Charging fees for employment tribunals was 'a very bad move'
Iwan Rheon interview: Game of Thrones star returns to his Welsh roots to record debut album

Iwan Rheon is returning to his Welsh roots

Rheon is best known for his role as the Bastard of Bolton. It's gruelling playing a sadistic torturer, he tells Craig McLean, but it hasn't stopped him recording an album of Welsh psychedelia
Morne Hardenberg interview: Cameraman for BBC's upcoming show Shark on filming the ocean's most dangerous predator

It's time for my close-up

Meet the man who films great whites for a living
Increasing numbers of homeless people in America keep their mobile phones on the streets

Homeless people keep mobile phones

A homeless person with a smartphone is a common sight in the US. And that's creating a network where the 'hobo' community can share information - and fight stigma - like never before