Orion’s belt-buckle

When I was about fourteen, I decided to adopt a star. I was standing on my parents’ patio on a glorious chilly evening. The stars were out in relative abundance for the suburbs. I thought I should pick a star so, wherever I saw it in future, I would be able to relate to it; to imagine it twinkling above my home.

I knew how to find the North Star, but that seemed too obvious a choice, so I sought out the short line of three evenly spaced stars I’d noticed many times before. I found them to the south-west, hanging in the darkness above the garage, and, reckoning it would be the easiest to remember, chose the middle star to be my star. I didn’t know at the time that the three stars in question comprised Orion’s Belt; and I’ve only just found out from Wikipedia that my star goes by the Arabic-derived name Alnilam, meaning string of pearls. Alnilam, it turns out, is a blue-white supergiant: the 29th brightest star in the night sky. Not bad for a humble belt-buckle.

The constellation of Orion photographed from Anglesey. Alnilam is shown dead centre.

Although the stars of Orion’s Belt look to be in a straight line, that’s only because we see them from our particular vantage point in the galaxy. In reality, Alnilam is roughly half as far away again from us as its two apparent companions, Alnitak and Mintaka: very roughly 1,340 light-years to their 800.

Just think about that for a second. In fact, actually count out one second. Go on, I mean it, I’ll wait…

In the time it takes to count a second, a beam of starlight travels 300,000 kilometres through space (that’s 186,000 miles in old money). That’s slightly farther than the most reliable car I ever owned travelled during its entire lifetime. Or, to put it another way, if you could somehow steer its course, a beam of light could travel around the earth seven-and-a-half times per second. Yet the light of Alnilam, striking our retinas when we happen to glance up at it on a chill winter evening, left that star shortly after the Romans abandoned Britain, several hundred years before the Norman Conquest, in an era we are no longer supposed to refer to as the Dark Ages. How unimaginably vast is our Milky Way Galaxy? Yet it is only one of an estimated two-trillion galaxies in the universe. Two trillion. That’s a two followed by twelve zeros: 2,000,000,000,000. How unimaginably insignificant are we in the vast non-scheme of things?

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With hindsight, I suppose I could have chosen my star more carefully: the constellation of Orion isn’t visible from the UK for three months of the year, from May to July. It’s still up there, of course, but at that time of year Orion rises and sets during the hours of daylight, so is banished from our skies by the brightness of the sun.

But Orion’s absence during the northern hemisphere’s summer months makes its reappearance in our night skies in late summer and early autumn a cause for celebration. Indeed, for many years, I’ve marked my ‘official’ start of autumn by the first pre-dawn sighting of Orion over the gate at the end of our driveway. There he stands, just like last year, facing west, club raised aloft: just as he will have stood, long before the story of Orion the hunter was invented, as the Bronze Age inhabitants of the Moor above my house gazed out on the same pre-dawn sky; just as he stood almost four decades ago, that starlit evening above my parents’ garage; just as he will stand, for many years to come, when I am long gone.

Yet perhaps I shouldn’t be so sanguine about Orion’s endurance. Like everything else, the configuration of the night sky evolves with time. Constellations change shape; new stars are born; old ones die. Ultimately—very soon by astronomical timescales—my adopted star, Alnilam, is likely to go out in a blaze of glory: a massive supernova. Indeed, perhaps it already has. With the speed of light as the limiting factor for spreading news throughout the universe, Alnilam might have exploded a thousand years ago, and we still wouldn’t know a thing about it for another 300 years or so.

So maybe I should enjoy Alnilam’s pin-prick of Dark Age light in the night sky while I still can.

That’s certainly my intention.

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