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The Stars of Winter
The Stars of Winter

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Astronomer Will Gater delves into the incredible astrophysics behind some of the season's most famous luminaries

Betelgeuse by Reid H.

Betelgeuse by Reid H.


Of all the stars in the winter sky, Betelgeuse (Alpha Orionis) is arguably the one that prompts the most excitement and intrigue. To the eye it looks like a sparkling, orange-hued point of light, but decades of scientific study — some conducted using the most powerful astronomical facilities in existence today — have shown that a thrilling story is unfolding far away.

"Betelgeuse is a red supergiant with a radius in optical light of about 4.5 astronomical units — in other words, almost the size of the orbit of Jupiter," says Dr Anita Richards, who has studied the star as part of her research at the University of Manchester's Jodrell Bank Centre for Astrophysics.

Betelgeuse hasn't always been this bloated, ruddy leviathan however. It was once a hot O-type star, like Mintaka in Orion's Belt is. It would have had a blueish-white color and would have also been more massive than it is now — perhaps around 20 times the mass of the Sun.

"Such massive stars have much hotter cores than the Sun with faster nuclear fusion, using up most of their hydrogen [in] a few million years," explains Richards. "Fusion [of] heavier elements such as helium and carbon takes over, but the outer layers cool and expand; the increase in size means that the luminosity grows as the star becomes redder."

It's this process that has created the Betelgeuse we know today, but it's what will happen at the end of its life that excites many astronomers. "It will probably take at least a hundred thousand years for Betelgeuse to exhaust [its] fuel for nuclear fusion," says Richards. "Finally when it runs out, its inner layers are no longer supported by radiation pressure and collapse, releasing roughly as much energy in an instant as the Sun radiates in 8,000 million years — a supernova."

This violent detonation will be a truly breathtaking sight in our skies. "It will be brighter, as seen from Earth, than any other supernova in recorded history so far," says Richards, "as brilliant as the full Moon and visible in daylight."

Where to find it
Betelgeuse is easily visible on the left shoulder of Orion as you look at it with the naked eye. Key to finding it is identifying Orion itself, which is probably best achieved by locating the unmistakable trio of stars known as Orion's Belt. Betelgeuse is just under 10 to the north-northeast of any of them.


Like its bright companion Betelgeuse, the brilliant star Rigel (Beta Orionis) is a supergiant, nearly 80 times the size of our Sun. But even to the naked eye there's one striking difference between these two stellar behemoths: their colors. Betelgeuse is orange-white while Rigel sparkles with a blue tint. Why the difference? It all comes down to their temperatures. The hotter a star is the bluer it tends to shine, while cooler stars glow more red. And indeed Betelgeuse's surface temperature is about 3,300C while Rigel's is roughly 11,800C.

Where to find it
Rigel is one of the few stars in the winter sky that is so bright that it can be seen easily from heavily light-polluted city centres and suburban areas. From a dark-sky site it is a blazing point of light at the right foot of Orion.

W Orionis

Less famous than either Betelgeuse or Rigel, the scientific story behind the star known as W Orionis is no less intriguing. It has an atmosphere that swirls with large amounts of carbon.

"For a star to become carbon rich, something called the dredge-up needs to happen several times so that carbon from the inner parts of the star gets to the surface and [is] released to its atmosphere," explains Dr Lizette Guzman Ramirez, an ESO Fellow based at the Leiden Observatory in the Netherlands.

This churning has occurred within W Orionis as it has aged. The carbon can absorb blue wavelengths of light from the star; this, combined with its relatively cool temperature, means it has an exquisite red hue — something that's obvious through a telescope.

Where to find it
Although it's on the cusp of naked-eye visibility, it's easier to hunt down mag. +6.1 W Orionis with binoculars. One way of finding it is to imagine a rough equilateral triangle tilted on its side, the base of which is marked by Mintaka and Bellatrix (Delta and Gamma Orionis). W Orionis is at the apex.

Practical project
The deep red of W Orionis is a wonderful sight to see, but it's even clearer in photos. In this project we'll use a simple astrophotography technique to bring out the star's striking color and all you need is a DSLR, a lens with a focal length of 50mm or similar and a static photo tripod. First mount your camera on the tripod, check W Orionis is in the view and then focus the image. Then take four or five 30-second exposures and stack them together in software such as Startrails to create an image that shows the star field 'trailing' as the Earth rotates. By using a 50mm lens you should be able to capture some of Orion's other bright stars in the field of view and so when you compare their trails to that of W Orionis the remarkable ruddy hue of the latter should be very obvious.

The Trapezium Cluster

Cast your eyes towards the stars of Orion on a crisp winter's night and you may — if you're far enough away from the ravages of light pollution — be able to glimpse a fuzzy star at the heart of the Hunter's sword. What you are seeing is in fact not a star but the magnificent Orion Nebula, M42. This enormous, sprawling, mass of dust and gas clouds some 1,350 lightyears from us shines in our night skies due to a cluster of hot, young stars embedded within it, known as the Trapezium Cluster.

These infant stars are thought to have emerged from the nebula roughly one million years ago. Their story began as material in the nebula coalesced together to form dense clumps within the then cold, dark clouds. These clumps grew and grew until nuclear fusion reactions fired up in their cores and the stars within the cluster were 'born'.

As the stars started to shine they began to emit huge amounts of powerful radiation, which streamed out into the gas and dust around them. Slowly a vast cavern — whose sweeping walls glowed brightly due to this onslaught of intense ultraviolet radiation — was sculpted into their maternal nebula too. And that's what we see when we look at the Trapezium Cluster and the beautiful Orion Nebula around it today: an extraordinary tableau of star formation sketched in ethereal celestial light across the winter sky.

Where to find it
The Trapezium Cluster sits within the bright central part of the Orion Nebula, which is itself located within a pattern of stars often referred to as Orion's Sword. The easiest way to find M42 is to scan your telescope south from the central star in Orion's Belt, called Alnilam (Epsilon Orionis), by a little over 4 until you come across the nebula and the embedded cluster.

Practical project
Few celestial objects are as captivating as the Orion Nebula seen from a dark-sky site, but for keen stargazers just starting out in astronomy spying the four most prominent stars of the Trapezium Cluster, within M42, is a rite of passage; so in this project we're going to cover a few additional tips for tracking them down. Assuming you've managed to locate the Orion Nebula in your telescope using our tips above, the first thing to note is that the Trapezium itself is much smaller in angular diameter than you might think — you'll need to use a magnification of at least 75-100x to get a pleasing view of it. As we've already mentioned, the cluster resides in the brightest part of the nebula, but if you need another signpost to it, look for the nearby 'dark' region of nebulosity that 'points' the way to it.


Compare Aldebaran (Alpha Tauri) to Betelgeuse and you'd be forgiven for thinking that the two are very similar stars — they're alike in color and not very different in brightness. Both are swollen, ageing stars in fact, but Betelgeuse is much more massive. "Aldebaran is only about 1.3 times the mass of the Sun," says Dr Anita Richards. This means that Aldebaran's eventual demise will be very different from Betelgeuse's. Instead of creating a supernova it will slowly shed its outer layers to form a beautiful glowing planetary nebula with a white dwarf at its centre.

Where to find it
At the start of January Aldebaran is high in the south at around 21:45 UT. The V of the Hyades star cluster is a helpful signpost to the star, but if you have trouble finding that use an imaginary line extending northwest from Orion's Belt to point you in the direction of the stars of Taurus, and thus the Hyades.


No discussion of the science of the winter stars would be complete without mentioning dazzling Sirius, the alpha star of Canis Major. There's no other star that rivals it in the heavens at this time of year, and it's the brightest star in Earth's night sky full stop. So why does Sirius appear so impressive in our skies? Well, it's a relatively bright star in itself but it's also very close to us too at a distance of 8.6 lightyears. To put that in perspective, brilliant Rigel in nearby Orion is over 100 times farther away!

Where to find it
Though Sirius may be bright, if you're new to astronomy finding which one of the dazzling stars in the winter sky it actually is can still be a challenge. Thankfully there's a little trick you can use. If you can find the much more recognisable Orion's Belt, it actually 'points' in the direction of Sirius, if you follow the line of the belt down from right to left.

The Pleiades

If the Trapezium Cluster in the Orion Nebula is a vision of the birth of stars, then the magnificent Pleiades, or M45, in the constellation of Taurus shows what happens as these glittering collections of stars age and evolve. After open star clusters emerge from their maternal nebulae they drive away the gas and dust around them before slowly scattering into the surrounding Galaxy.

That's precisely what we're seeing when we look at the many members of the Pleiades, which are thought to be 125 million years old — we're looking at a grouping of young stars that are no longer swathed in the dense, often glowing, nebulosity associated with their formation. Over time the stars within the Pleiades will likely disperse further.

In fact it's thought that our very own star, the Sun, may have once belonged to a star cluster like M45. Astronomers believe they've even been able to track down one of the Sun's siblings, a star within the constellation of Hercules known as HD 162826. Its composition and orbital history within the Milky Way matches the Sun's, yet it is now 110 lightyears from us.

Where to find them
The Pleiades sit about 14 to the northwest of the bright star Aldebaran. At the end of January you'll find the cluster high in the southwest sky around 21:15 UT. If you can't spot it with the naked eye try scanning along a line roughly northwest from the Hyades star cluster with a good pair of binoculars.

About The Writer
Will Gater is an astronomy writer and journalist. Visit his website willgater.com and follow him on Twitter at @willgater.

Copyright Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Date Taken: 02/06/2017
Author: Will Gater, BBC Sky at Night Magazine
Category: Astronomy

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