With the approach of winter, the constellation of Orion will begin to dominate the night sky. It can be located easily by anyone who cares to look for it, and it contains some fascinating astronomical objects - all of which can be seen with the naked eye. Orion is a splendid sight. It is large and beautifully proportioned, and it contains more bright stars than any other constellation. It is so prominent that it can be seen even from our most light-polluted cities.

Half an hour spent on the trail of this heavenly hunter will open your eyes to the wonder and majesty of the cosmos, and may lead to a life-long appreciation of the joys of star-gazing.

The group of stars we call Orion has been known for thousands of years. The Chaldeans knew it as Tammuz; the Syrians referred to it as Al Jabbar, the Giant; the ancient Egyptians associated it with the god Osiris; and the ancient Greeks named it after a mythological hunter (see box: The Orion myth).

However, the stars in Orion bear no real connection to one another. The familiar pattern we observe is dependent on our own particular vantage point in space. The nearest of Orion’s stars is about 20 light years away, while the farthest is separated from us by 2,000 light years. (A light year is the distance light travels in one year: roughly six million million miles.) If we were to travel toward Orion, the pattern would simply dissolve.

Although there is no real relationship between Orion’s stars (or between those of the other constellations for that matter), it is useful to group them together. Mentally arranging groups of stars into pictures and patterns helps us to navigate our way around the night sky.

If you look southward in winter and spot three bright stars close together in a line, you can be pretty sure you have found Orion’s belt. This in turn will help you find the reddish star Betelgeuse in Orion’s shoulder, the blue-white Rigel at his foot, and the ghostly Orion Nebula in his sword.

You can then use Orion’s belt to guide you downward towards Aldebaran in the constellation of Taurus, and upward towards Sirius in the constellation of Canis Major. By star-hopping like this you can quickly find your way around the entire night sky - an achievement that brings immense pleasure and satisfaction.

The constellation of Orion provides an easy introduction to the fascinating world of supergiants, variable stars, white dwarfs, nebulae and star clusters. And you can see it on any clear winter’s night, without the need for special equipment.

Orion is one of the chief attractions in the grandest and longest-running show on earth - the night sky. And you can see it for free (for observation tips, see box on page 10: Star party).

Betelgeuse

Most people assume that all stars are the same colour: a kind of silvery white. A quick look at Orion’s left shoulder will demonstrate that this is not the case. Betelgeuse (pronounced “bet’l-jooze”) is distinctly reddish.

The ancient Sumerians saw the stars that make up Orion not as a mighty hunter, but as a sheep. In their language Betelgeuse meant “armpit of the sheep”. But despite this unprepossessing name, Betelgeuse is an object that demands respect. It is the largest single thing that most of us will ever see.

Betelgeuse is an orange super-giant. Its size is staggering - almost 600 million miles across. That’s more than six times the distance between Earth and the Sun.

In his book Secrets of the Night Sky, Bob Berman helps to put this almost unimaginable vastness into context: “If that star were an empty jar and we could unscrew its lid to pour in balls the size of our planet at the rate of a hundred a second, we couldn’t fill Betelgeuse in 30,000 years.”

Betelgeuse is not just big. It is also more than 10,000 times more luminous than the Sun. So despite being more than 400 light years from Earth it is one of the brightest stars in our skies.

Its precise placement among the list of bright stars varies from month to month and from year to year. This is because Betelgeuse is a variable star.

At its brightest, it ranks sixth, behind Sirius, Canopus, Alpha Centauri, Arcturus and Capella. At its least bright level, it drops to 11th place.

Rigel

In the lower right-hand corner of Orion is another bright star, Rigel (pronounced “rye-jl”). Its name is derived from the Arabic word for foot.

In sharp contrast to the reddish colour of Betelgeuse, Rigel shines bluish-white. This variation in colour is due to the different surface temperatures of the two stars. Rigel has a surface temperature of about 12,000 degrees Celsius, while Betelgeuse is comparatively cooler at about 3,400 degrees Celsius. (The centre of a star is much hotter than its surface. Temperatures there are measured in millions rather than thousands of degrees.) Like Betelgeuse, Rigel is a super-giant. It is about 100 times the size of the Sun, though scarcely one-10th the size of Betelgeuse.

Rigel is among the most luminous stars visible to the naked eye. It has a luminosity about 50,000 times that of the Sun. It is situated a colossal 800 light years from Earth, but still ranks among the brightest stars in our sky.

The Great Orion Nebula

The Great Orion Nebula is one of the marvels of the night sky. A close inspection of the three stars in Orion’s sword reveals something unusual about the middle one. It appears as a blur rather than a pinpoint of light.

In fact, this fuzzy object is not a star at all, but a nebula - a luminous gas cloud measuring about 30 light years across, containing enough matter to form 10,000 suns.

The Orion Nebula can be seen with the naked eye. Binoculars depict it even better. A small telescope will reveal four bright blue stars inside it, which form a pattern known as the Trapezium. These are the latest in a series of infant stars formed inside the nebula. A larger telescope will reveal a further two fainter stars.

The Great Orion Nebula is much loved by astronomers, and is probably the most photographed object in the sky.

Orion’s Belt

Orion’s belt is the constellation’s most prominent feature. It is made from three bright stars: Alnitak, Alnilam and Mintaka.

The belt is probably the most recognised group of stars in the whole sky.

It even gets a mention in the Bible: “Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion?” (Job 38:31) Orion’s belt also acts as a convenient pointer to Sirius and Aldebaran, the brightest stars in the constellations Canis Major and Taurus.

Sirius

If you follow Orion’s belt downward in a straight line beyond the borders of the constellation you come to Sirius, the brightest star in the sky.

Since antiquity, Sirius has been known as the Dog Star, because it lies in the constellation of Canis Major, the Big Dog.

The Romans considered the Dog Star an omen of ill luck. But the ancient Egyptians greeted it with joy, as its first appearance in the late summer sky heralded the annual flooding of the Nile - an event that was essential to their agricultural prosperity.

Situated just 8.6 light years away form Earth, Sirius is one of the closest stars to earth. This proximity, coupled with its high luminosity (equivalent to about 33 suns), accounts for its exceptionally bright appearance.

Actually, Sirius is not one star but two: Sirius A, a main sequence star similar to our Sun; and Sirius B, a white dwarf. These are locked in orbit around each other, and so appear as one object unless viewed through a very powerful telescope.

A main sequence star is powered by nuclear reactions at its core, where hydrogen is fused into helium; whereas, in a white dwarf, this nuclear fire has gone out, causing the star to collapse in on itself.

Sirius A is thousands of times more luminous than its companion, and very much bigger. Because of this, Sirius B is often referred to as “the Pup”.

Although similar in size to Earth, the Pup is 35,000 times as heavy. A thimbleful of Sirius B material would outweigh a family car.

White dwarfs are quite common. Most stars, including our own Sun, will eventually share the fate of Sirius B.

Aldebaran

Follow Orion’s belt upward and you will come to Aldebaran, the brightest star in the constellation of Taurus, the bull. It is easy to spot Aldebaran because of its distinctly orange colour. Aldebaran is a red giant - a main sequence star that has run low on hydrogen. When this happens, the star expands and gets brighter and redder, and it begins to fuse helium into carbon. Aldebaran is 36 times the size of the Sun, and 100 times as luminous. It has been traditionally thought to represent the eye of the bull.

The Pleiades

Continuing to follow Orion’s belt upward past Aldebaran brings you to the Pleiades, also known as the Seven Sisters. These closely packed stars were all born in the same stellar nursery, and now travel together through space. They are one of the closest examples of a stellar cluster.

With the naked eye most people can make out six or seven stars in the Pleiades. With keen eyesight and good viewing conditions it is possible to make out nine. A telescope will reveal many more.

But the best way to observe the Pleiades is through binoculars. These will show up the entire cluster in all its glory - a sight well worth seeing.

The Orion myth

In Greek mythology Orion was a great hunter who boasted that he would rid the Earth of all wild animals and monsters. This angered Mother Earth, since all of these creatures are her offspring. So she sent a giant scorpion to kill him.

Orion fought bravely against the creature, but soon realised that no mortal weapon could penetrate its armour. Judging that discretion is the better part of valour, he jumped into the sea and began to out-swim the monster.

Unfortunately, Mother Earth was not the hunter’s only enemy. Orion’s close friendship with Artemis, goddess of the hunt, had aroused the indignation of her brother Apollo. Orion had a reputation for ill-treating women, and Apollo did not wish to risk seeing his sister disgraced.

As Orion swam across the sea, Apollo challenged his sister to try to shoot the “black object” hurtling through the waves. Artemis let fly an arrow, which pierced Orion through the head and killed him.

When she realised what she had done Artemis was distraught, and begged the renowned physician Asclepius to revive her friend. Asclepius consented, but Apollo persuaded Zeus to destroy him with a thunderbolt before he could complete the task.

With all hope of reviving Orion gone, Artemis preserved his memory by placing his image among the constellations, together with his two hunting dogs. She then placed the image of the scorpion at the opposite end of the sky. Now, night after night, the scorpion pursues Orion, but never gains on him. This is why the constellations of Orion and Scorpio are never seen together in the night sky.

STAR RESOURCES

Books

* Secrets of the Night Sky: The Most Amazing Things in the Universe You Can See with the Naked Eye

By Bob Berman

HarperCollins

Full of fascinating facts and figures.

* The Stars: A New Way to See Them

By Hans Augusto Rey

Sagebrush Educational Resources

The best, most accessible, introduction to the night sky.

* Find the Constellations

By Hans Augusto Rey Sagebrush Educational Resources.

Children’s version of Rey’s superb book.

(All books are available from Amazon) Software

* Starry Night EDU

By Imaginova Canada Ltd. Free seven-day trial version available from www.starrynight.com

A great tool for learning your way around the night sky.

Websites

* BBC Space at: www.bbc.co.uksciencespace

2Packed with high quality resources for the beginning astronomer.

STAR PARTY

Schoolchildren could be forgiven for thinking that learning astronomy involves only textbooks, videos and computer programs.

But the best way to learn about the stars is to go outside and look at them.

A “star party” gives pupils the opportunity to experience astronomy first-hand - it also gives them a chance to be out under the stars and have fun.

Organising the party is fairly straightforward. It takes a bit of effort, but this will be amply repaid in “oohs” and “ahs”.

First, decide on a date. If you wait until the New Year, you’ll be able to see Orion in all its splendour without having to stay out late.

To make it easier to see stars and other lesser objects, avoid nights when the Moon is full or nearly full.

You can get a preview of Orion during October, but you’ll need to get up at about 5am. If you do, be sure to check the planet Venus, which will be dazzlingly bright above the eastern horizon.

Hold your party at the darkest convenient site, somewhere where the horizon isn’t obstructed by buildings or trees. However, Orion itself is visible from even the most light-polluted locations.

Fill in risk-assessment forms and send out permission slips well in advance. Invite parents along (you may wish to make it compulsory for parents to accompany pupils). Inform the local press.

Make sure everyone dresses warmly - star-gazing can be a chilly business.

Warm food and drinks will help to keep bodies warm and spirits high.

Have a contingency plan in case of cloudy skies. The best thing is to agree an alternative date in advance.

If possible, begin the party indoors. Use charts to show the constellations you’ll be observing once you get outside. Start off by looking at the Plough (or Big Dipper), Cassiopeia, Auriga and Orion. If you make your demonstration charts using glow-in-the-dark stars, they’ll show up beautifully outside too (see Star resources box). If you’re not confident about finding the constellations, buy a copy of Hans Augusto Rey’s The Stars: A New Way to See Them. This superb book is easy to read and will have you star hopping with the best of them in just a few nights.

If you have access to an interactive whiteboard, download a free trial copy of Starry Night EDU from www.starrynight.com. Then you can show pupils an accurate, up-to-date simulation of the night sky you’re about to view, and you’ll also be able to switch constellation outlines on and off at the touch of a button.

Once you’re outside, allow a few minutes for everyone’s eyes to become adapted to the dark. More and more stars become visible as eyes adjust to the darkness.

The lenses of flashlights should be painted red with nail polish, as any bright light will dazzle the eyes.

At first, do all observing with the naked eye. Identify some constellations, bright stars and other objects.

Then try looking through binoculars. Many more stars will become visible.

Make sure you direct your binoculars at the Moon, the Great Orion Nebula and the Pleiades - they’re all splendid sights.

Telescopes are best avoided unless an adult is present who is experienced with using them. Most of the telescopes children buy to look at the night sky are completely unsuited to the task, even those that have exciting pictures of astronomical objects on the box.

Purpose-built astronomical telescopes are also useless without an experienced star-gazer to set them up and point them in the right direction.

Contact your local astronomy club, and ask if any members would be prepared to bring telescopes to your star party. They are usually only too happy to show you some amazing sights.

You are guaranteed gasps of delight and astonishment - and an evening that nobody present will ever forget.