What’s the best-known constellation? The Plough (or Big Dipper)*? The Southern Cross? Actually, it has to be Orion, because unlike the other two it’s visible the whole world over, it’s very easily recognised and it has more bright stars than any other star pattern.
This is a good time to look for it. Just look due south on a clear night and you’ll see it, with three stars in an almost straight line, flanked by a quadrilateral of other stars. It’s with us in the evening skies from December to April, so it’s the classic winter constellation. Check back here from day to day to learn more about its stars and other features.
* OK, we know that the Plough isn’t an actual constellation, but simply the brightest seven stars of Ursa Major. Technically is it an asterism and not a constellation.
These details refer to observing from the northern hemisphere only.
The line of three stars that make Orion so distinctive are known as Orion’s Belt. This comes from the classical figure of Orion as a Hunter, which is a link dating back from well before the Ancient Greek times. He is often depicted with his right arm raised holding a club, and his left arm holding out a shield to defend himself from Taurus, the Bull, which he faces to the west.
The bright star to the top left of the quadrilateral of Orion is Betelgeuse, marking his right shoulder, and the one to the lower right is Rigel, marking his left knee. The other two stars of the quadrilateral are Bellatrix, at top right, and Saiph at lower left. Where have you come across Bellatrix before? Probably from Bellatrix Lestrange, in the Harry Potter books. The name comes from the Latin for ‘female warrior’.
About Orion’s Belt
The three stars of Orion’s Belt, from bottom right to top left, are named as Alnilam, Alnitak and Mintaka. The stars are of almost equal brightness and are pretty well evenly spaced. It’s a coincidence that they form a line as seen from our viewpoint. They are all at somewhat different distances from us, with Alnilam at about 2000 light years, Alnilam at 1250 light years and Mintaka at about 1200 light years. Actually, we don’t have very good measurements for their distances. It’s not as though we can travel there! Astronomers have to look for tiny shifts in their position as the Earth orbits the Sun (the parallax method) and also compare what we know about the properties of similar stars. So each measurement could be wrong by 20%.
But these stars are so far away that although stars do move over time, even our most distant human ancestors would have seen Orion and Orion’s Belt looking very much as it does now.
Finding the Orion Nebula
The three stars of Orion’s Belt, plus Saiph and Rigel, are a good guide to locating the Orion Nebula, one of the showpieces of the sky. You need to look right in the middle of the patch of sky surrounded by these stars. What you see will depend on how dark your sky is. From a dark country sky you should be able to see three fairly faint stars in the middle of this space in a north–south line and probably a few more besides. From the suburbs, you’ll be lucky to see anything. If this is the case, use binoculars. The middle one of the line of three is actually the location of the Orion Nebula.
You may have to use the astronomer’s trick known as averted vision – that is, not looking directly at the object but slightly to one side. This is because your centre of vision isn’t as sensitive to light as the region around it.
The Orion Nebula’s colours
With binoculars or a small telescope, the Orion Nebula is just a pale colourless misty patch, even on a dark, clear night in the country. So what about all those colours you can see in photos – the reds and the blues and sometimes the greens? What about finding a really big telescope to look through?
You’d be disappointed. Not even the largest telescope will show those colours to the human eye. A big telescope will gather more light, it’s true, but it’s a sad fact of physics that although it will show more detail and reveal fainter stars, it can’t make the nebula bright enough that your eye can see the colours. It just looks larger and more detailed, but still no colours.
What your eye can’t see in a fraction of a second it will never see, but a camera can go on building up light and show the colours that are actually there. They are real colours, caused by the gases in the nebula, but the only way you can ever see them is to take a photo.
Starbirth in the Orion Nebula
The Orion Nebula is known as a starbirth nebula. Places like this are where stars are born. It’s a gigantic cloud of gas and what astronomers call dust, which is really ice grains mixed with solid particles such as carbon. This material is normally dark and cold, and there is lots of it about, but only where it is particularly dense does it start to gather and condense down through the force of gravity to form blobs just a light year or so across.
Once a blob forms, it just keeps on getting smaller and smaller, and increasingly hot, as its own gravity makes it form into what eventually becomes a star. Eventually, after tens of thousands of years, the star gets so hot that it starts to shine, and this makes the gas surrounding it glow. It’s a bit like a streetlight on a foggy night. The fog obscures the surroundings, but you can see the gas around the streetlight.
Dust and gas occupies a large part of the constellation of Orion but we only see it where the stars are being born, which is the Orion Nebula.
Bright young things
The main stars of Orion, with one exception (Betelgeuse), are all extremely bright and young stars. Young, for a star, means maybe a few million years old. For comparison, the Sun is about 4½ billion years old. But these are no ordinary stars. They are extremely massive and bright stars, not at all like the placid old Sun, which (fortunately for us) is pretty average in mass and brightness. If the Sun were in the Orion Nebula you’d need a moderately large telescope to see it at all.
Because they are using up their hydrogen fuel so rapidly, even the most massive stars won’t last very long. They might last just ten million years.
Some of the youngest stars are right in the centre of the Orion Nebula, where there’s a group of four stars known as the Trapezium (guess why). They are each some 1½ light years apart, and this cluster is probably only a few tens of thousands of years old. You can see the Trapezium with a small telescope so look for it if you get the chance. Although the stars are not bright in our sky they are brilliant in reality.
Star colours in Orion
Are all stars white? You might think so, but even with the naked eye Orion shows that they are not. Compare Rigel with Betelgeuse. The difference between them is obvious to most people with normal colour vision. Rigel is blue-white while Betelgeuse is more orange in colour. Neither of the colours are deep, and the same is true of most star colours, but when you see a contrast they become more obvious.
Rigel is classified as a blue supergiant star, while Betelgeuse is a red supergiant. Okay, ‘red’ is a bit of an overstatement. It is orange, really, and even the reddest stars are not as red as traffic lights, say. The colours are more obvious when you look at the stars with binoculars or a telescope, and some stars do indeed resemble glowing coals. Actually, the colours are the same as those of any heated object. Heat a metal wire to the same temperature of 3500 K and it will have the same colour as Betelgeuse. White incandescent light bulbs are about this temperature. To get an incandescent light bulb to the same temperature as Rigel it would need to get up to 12,000 K, and it would quickly burn out. Some welding arcs are the same colour as Rigel. To our eyes, Rigel appears more white than blue, but it does have a bluish tinge that can show up in photos.
The elusive Horsehead
Read any astronomy book or magazine and you’re very likely to come across a picture of the Horsehead Nebula in Orion. It’s one of those subjects that’s a natural choice – colourful and curious. Of course, it’s actually a dark cloud of gas and dust silhouetted against a brighter gas cloud, but the equine resemblance is striking. So you’d want to see it with your new telescope. But don’t bother. Very few stargazers have ever actually seen it with their own eyes other than in books.
This is because the bright nebula shines mostly with the red light from hydrogen, which is right at the far end of the spectrum of light we can see, so our eyes are very insensitive to it although camera sensors can pick it up. Even then, you need a long exposure to reveal it well. Observers with dark and crystal-clear skies can spot it with large telescopes that allow them to magnify the image sufficiently to spot the dark blob, but under typical conditions it remains invisible. So the Horsehead remains a photographic target for most of us.