Once you’ve seen Orion, you’ll never forget it. It’s one of the most recognisable constellations, with its three bright stars in an almost regular line, and four stars surrounding them. It’s also the brightest of all constellations, in terms of the number of bright stars it contains within its area.
In star lore, which dates from thousands of years ago, Orion represents a giant, with the three stars being his belt from which hangs a sword. He faces Taurus, the Bull, whose head is shown by a V-shape of stars known as the Hyades cluster. Orion is holding up a shield which is marked by a curved line of stars, and holds a club above his head, about to strike the charging bull.
Having all these bright stars together is not a matter of chance, as happens in the case of some other constellations – the five stars of the W of Cassiopeia, for example, are at distances between about 66 light years and over 610 light years, so there’s no particular connection between them. But many of the stars of Orion, particularly those south of the Belt stars, are between 700 and 800 light years.
They are especially bright stars, so despite being quite distant they still outshine most other stars in the sky which are much closer. Look at Procyon, over to the left of the picture, in the constellation of Canis Minor, the Lesser Dog. That appears about as bright as some of the stars in Orion but it’s a much less luminous star only 11 light years away. Procyon is about seven times as bright as the Sun, while Rigel in Orion is about 120,000 time brighter.
And while we’re talking dogs, take a look at the star down to the lower left of Orion, which the Belt stars point to down to the south-west. This known as the Dog Star because it’s in the constellation of Canis Major, the Greater Dog. It does outshine any of the stars in Orion – in fact it’s the brightest star in the night sky – but then it’s only eight light years away, one of the nearest stars. (The Sun is the nearest star in the sky, of course!)
And no page about Orion would be complete without mentioning the Orion Nebula. You can find it with just binoculars by looking below the Belt stars, although a telescope brings it out better. Time exposures with cameras show the most beautiful detail and colours – but sadly, you can’t see them with the eye even with the largest telescope as our eyes just aren’t sensitive enough.
Fun facts about Orion and the Dog Star
The Orion Nebula is a birthplace of stars – although the stars take tens of thousands of years to emerge from the gas.
A faint semicircle of glowing gas surrounds Orion – the remains of a star that exploded about two million years ago.
The brightest stars in Orion will live for only a few million years as they use up their fuel at a prodigious rate.
Sirius has a small white dwarf star orbiting it, appropriately dubbed The Pup. This very dense star has about the same mass as the Sun but is only about the diameter of the Earth.