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
News
Bobbi Kristina Brown with her mother Whitney Houston in 2011
people
News
The actress Geraldine McEwan was perhaps best known for playing Agatha Christie's detective, Miss Marple (Rex)
peopleShe won a Bafta in 1991 for her role in Oranges Are Not The Only Fruit
News
peopleHere's what Stephen Fry would say
Sport
David Silva celebrates with Sergio Aguero after equalising against Chelsea
footballChelsea 1 Manchester City 1
PROMOTED VIDEO
ebooks
ebooksA special investigation by Andy McSmith
Arts and Entertainment
Caroline Proust as Captain Laure Berthaud in 'Spiral'
tvReview: Gritty, engaging and well-acted - it’s a wonder France’s biggest TV export isn’t broadcast on a more mainstream channel
Arts and Entertainment
music
Arts and Entertainment
Laura Carmichael in still from Madam Bovary trailer
film
News
i100
Sport
Serena Williams holds the Australian Open title
sportAustralia Open 2015 final report
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

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

iJobs Job Widget
iJobs General

Recruitment Genius: Online Media Sales Trainee

£15000 - £30000 per annum: Recruitment Genius: Now our rapidly expanding and A...

Recruitment Genius: Public House Manager / Management Couples

£15000 - £20000 per annum: Recruitment Genius: Are you passionate about great ...

Recruitment Genius: Production Planner

£20000 - £30000 per annum: Recruitment Genius: This fast growing reinforcing s...

Recruitment Genius: General Factory Operatives

£18000 - £35000 per annum: Recruitment Genius: This fast growing reinforcing s...

Day In a Page

As in 1942, Germany must show restraint over Greece

As in 1942, Germany must show restraint over Greece

Mussolini tried to warn his ally of the danger of bringing the country to its knees. So should we, says Patrick Cockburn
Britain's widening poverty gap should be causing outrage at the start of the election campaign

The short stroll that should be our walk of shame

Courting the global elite has failed to benefit Britain, as the vast disparity in wealth on display in the capital shows
Homeless Veterans appeal: The rise of the working poor: when having a job cannot prevent poverty

Homeless Veterans appeal

The rise of the working poor: when having a job cannot prevent poverty
Prince Charles the saviour of the nation? A new book highlights concerns about how political he will be when he eventually becomes king

Prince Charles the saviour of the nation?

A new book highlights concerns about how political he will be when he eventually becomes king
How books can defeat Isis: Patrick Cockburn was able to update his agenda-setting 'The Rise of Islamic State' while under attack in Baghdad

How books can defeat Isis

Patrick Cockburn was able to update his agenda-setting 'The Rise of Islamic State' while under attack in Baghdad
Judith Hackitt: The myths of elf 'n' safety

Judith Hackitt: The myths of elf 'n' safety

She may be in charge of minimising our risks of injury, but the chair of the Health and Safety Executive still wants children to be able to hurt themselves
The open loathing between Barack Obama and Benjamin Netanyahu just got worse

The open loathing between Obama and Netanyahu just got worse

The Israeli PM's relationship with the Obama has always been chilly, but going over the President's head on Iran will do him no favours, says Rupert Cornwell
French chefs get 'le huff' as nation slips down global cuisine rankings

French chefs get 'le huff' as nation slips down global cuisine rankings

Fury at British best restaurants survey sees French magazine produce a rival list
Star choreographer Matthew Bourne gives young carers a chance to perform at Sadler's Wells

Young carers to make dance debut

What happened when superstar choreographer Matthew Bourne encouraged 27 teenage carers to think about themselves for once?
Design Council's 70th anniversary: Four of the most intriguing prototypes from Ones to Watch

Design Council's 70th anniversary

Four of the most intriguing prototypes from Ones to Watch
Art should not be a slave to the ideas driving it

Art should not be a slave to the ideas driving it

Critics of Tom Stoppard's new play seem to agree that cerebral can never trump character, says DJ Taylor
Bill Granger recipes: Our chef's winter salads will make you feel energised through February

Bill Granger's winter salads

Salads aren't just a bit on the side, says our chef - their crunch, colour and natural goodness are perfect for a midwinter pick-me-up
England vs Wales: Cool head George Ford ready to put out dragon fire

George Ford: Cool head ready to put out dragon fire

No 10’s calmness under pressure will be key for England in Cardiff
Michael Calvin: Time for Old Firm to put aside bigotry and forge new links

Michael Calvin's Last Word

Time for Old Firm to put aside bigotry and forge new links