Orion is probably the most obvious and striking constellation, and it is a great starting point for learning your way around the stars.

You can see that he is raising a club or a spear above his head, and holds a bow - Orion is the great hunter, centrepiece of the winter sky. He is perpetually locked in combat with Taurus, the Bull. Below his belt hangs what we politely refer to as his ‘sword’, though other cultures have identified it with a different item of gentleman’s equipment.

The sword hides a secret. Its middle star is not a star at all, but a vast glowing cloud of gas and dust, the Great Nebula in Orion. This cloud is about 30 light years across, big enough to encompass our Sun and its nearest twenty or thirty neighbours. Inside, new stars are forming with what seem to be the precursors of planets. The Nebula appears unremarkable in visible light, but if we could see in the infrared, it would be a great blazing splash across the sky, four times the size of the full Moon:

(Image from seds.org. This is on approximately the same scale as the picture above.)


If you take a line through the stars of Orion’s belt and follow it down and to the left you will come to Sirius, the brightest star of all. Sirius is called the Dog Star because it is in the constellation of Canis Major, the larger of Orion’s two hunting dogs. Sirius is a southerly star, so for northern hemisphere observers it generally appears low on the horizon, like here:

Sirius over Mill Hill village


If you look up from Sirius you will see another bright star at the left of Orion, sort of on its own. This is Procyon in Canis Minor, the lesser dog. We’re always hearing a lot about Sirius, but you don’t often see press releases about what Procyon’s up to. It must get a bit frustrating sometimes always being the Number Two. I like Procyon the best.

If you follow a line up from Sirius through Procyon and a bit to the left, hovering over Orion’s shoulder you will see Gemini, the Twins, which we covered in a previous lesson (pay attention at the back).

Aldebaran and the Pleiades

Now go back to Orion and extend the line between Orion and Sirius in the opposite direction. You will come to a bright red star, Aldebaran, the glowing eye of the Bull which always faces Orion. If you keep going you will see a fuzzy patch of middling bright stars, the Pleiades.

Aldebaran and the Pleiades

Happily, Mars wanted to be in this picture too, so it sidled in at the last minute.

Looking more closely at the Pleiades in this 100% crop we can see the Seven Sisters (Alcyone, Merope, Maia, Electra, Taygeta, Celaeno and Asterope) and their parents Atlas and Plione. Alcyone is the eldest sister so she is always in charge if Atlas and Plione go out, which sometimes leads to friction with little Asterope wanting to stay up late and watch TV.

In fact although you can see six or seven stars with the naked eye, there are about 3,000 stars in the Pleiades cluster. That might seem a lot until you realise there about four hundred billion stars in the Galaxy. The Galaxy is so vast that it beggars comprehension. It is about 100,000 light years across, which means absolutely nothing to us humans, but if the whole Solar System were shrunk to the size of a golf ball, the Galaxy would be about the size of North America.

For a minute the Galaxy almost seems impressively large, but it is just one of hundreds of billions of galaxies. There are probably as many galaxies in the Universe as there are stars in our Galaxy. If you start thinking about this type of thing it tends to make your head go a bit woozy and you have to have a sit down. The numbers involved are so much bigger than anything our brains evolved to cope with, we can’t even get a vague sense of the size of the Universe. Whenever you think about it there is a feeling of standing dizzily at the edge of some appalling abyss. Every time you look up into the night sky you are literally staring into infinity.

And we are journeying through that infinite cold, dark void, in a tiny blue spaceship. Unfortunately we are not carrying any spares in the boot so if anything goes wrong with the spaceship, we are pretty much screwed. It is enough to make you drive a bit more carefully.

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