The Night Sky

Twinkle, Twinkle, Bang.

A roaming group of hunters, the direct ancestors of modern humans, gaze at a new star, a piercing pinpoint of bright, white light that shines in the late afternoon sky. As evening draws in, the light reveals its source; the dazzling glint of a sword, the blade of another hunter who has emerged from the night – the constellation of Orion. A full moon rises and its shadows compete with those of the new star; except that the star is only new to Homo heidelbergensis and its glare, which outshines an entire galaxy for months on end, is the last that anyone will see of it for around 340,000 years.

Our ancestors have witnessed not a new star, but the death of an old one; a spectacular supernova at relatively close quarters, the swan song of a sun that was at least eight times as large as our own and an explosion so large that its remains are still hurtling across the sky at 15,000 miles an hour. Today, we know the remnants of this star as Geminga.

It’s well named. In 1972, Italian astrophysicists discovered a powerful source of radiation coming from an apparently empty patch of sky in the constellation of Gemini. Geminga, it turns out, is not only a handy contraction of “Gemini gamma radiation source”, but is also pronounced exactly the same as a word in the Milanese dialect that means “it isn’t there”. It was there, however, it just took twenty years of observations through the most powerful telescopes on earth to see what’s left of it; a rapidly spinning, super-dense radioactive ballbearing just 12 kilometres in diameter which emits a faint glow a million times dimmer than even the least enthusiastic star that can be seen by the naked eye.

It’s ironic that Geminga is only barely visible, because the manner of its disintegration blew away a local patch of gas and dust that normally fills the space between the stars – the interstellar medium – and left us an exceptionally bright night sky as a consequence. Scientists call this hole in the cosmic cobwebs that normally obscure our view the ‘Local Bubble’ and its effect on our outlook is surely staggering. To see why, you only need to imagine the night sky without its canopy of stars. In the 1950s, science fiction authors Robert Silverberg and Randall Garrett invented a planet that was permanently sheathed in clouds – Nidor – where the human-like occupants had no concept of a universe beyond the clouds. Perhaps because of the lack of any external stimulation, Silverberg and Garrett painted Nidorian culture and technology stuck in an agrarian phase not unlike early Renaissance Europe.

Similarly, in Douglas Adams’ Life, the Universe and Everything, the planet of Krikkit exists in a dust cloud, blissfully unaware of the existence of the rest of the universe. Having built a spaceship that could take them outside of the dust cloud and thereby witnessing the wonder and magnificence of the universe before them, they casually declare war against all of it. Although these are only works of speculative fiction, the mind can only boggle at how different our civilisation might have turned out without such a rich and opulent night sky, a backdrop that has, perhaps, given all of our dealings on Earth something of a sense of perspective.

Without the Local Bubble, which is less like a bubble and more like an open cylinder, there would simply not be as many stars to see in the sky and, consequently, it would be far less exciting because of it. Even the stars that would be visible would appear much redder and dimmer, for the same reasons that a hot, dry day in high summer produces aesthetically appealing sunsets - we’re looking at the sun through a thick layer of dust. And it’s not just a clearer view of our galaxy, where the majority of objects we see in the night sky reside, but also other galaxies – as luck would have it, the Local Bubble is oriented at right angles to the disk of the Milky Way, offering us a window of clear space to observe the rest of the Universe through.

But even now, when Geminga has done its cosmic housekeeping, set the default setting for the brightness of the stars and long hurtled away from the scene, the constellation from which it was borne – Orion – is still captivating. Gazing up on a moonless winter’s night, Orion draws the eye and dominates the midwinter sky as he strides across the heavens with the kind of chutzpah we demand from a Greek hero. It isn’t one of those fiddly zodiac signs that look nothing like their designation, either; it’s a constellation, along with the Big Dipper or Plough, that does exactly what it says in the star catalogue. You want a big saucepan in the sky? No problem. How about a large man with a club, wearing a belt and dagger? Here’s Orion.

Like the Plough, Orion is a constellation that a lot of us know from a very early age. Perhaps these easy-to-identify groupings of stars call on our innate ability to recognize patterns in a jumble of visual data. That intuitive sense that once alerted us to the presence of partially obscured man-eating tigers in the undergrowth is still alive, and now that there is less call for that kind of thing in the suburbs of Godalming and Splott, it assists us instead, in the solution of sky-based dot-to-dot puzzles. Many of those patterns are purely an outcome of our position in space; the stars themselves can be at wildly different distances from us and the constellations would look very different from another part of the galaxy. Most of the stars of Orion, however, are moving through space together in a loose association and the constellation’s shape is, by and large, not merely an accident of our line of sight. Either way, as a shape in the sky, Orion is so ingrained in our consciousness, it would be difficult to combine its stars into any other shape and that, it appears, is how it has been with almost every cosmologically-aware culture since the Babylonian star catalogues of the Bronze Age, in which Orion is the Loyal Shepherd of Heaven. The Babylonians inherited the constellation from the Sumerians who saw their own hero – Gilgamesh – in the pattern of stars. The Bible, too, mentions the same configuration of stars (naming Orion as ‘Kesil’) three times, while the ancient Egyptians called him the Soul of Osiris. He appears in Homer’s Odyssey and Iliad as both a legendary hunter and as a constellation; in short, wherever you look in history, Orion is there in some form or another.

For those reasons alone it would be a terrible shame if one of its number – like Geminga before it – went bang tonight. That is, it would be a shame were it not for the spectacular surprise that the star that marks the left shoulder of Orion, Betelgeuse (there are various pronunciations, but most people opt for beetlejuice) appears to be preparing for us.

Betelgeuse is a red supergiant - not only one of the brightest, but also one of the biggest stars in the sky - a relatively cool, low density sun, around twenty times the mass and over one and a half billion times the volume of our own but little more than half as hot. Red supergiants have brief, glorious lives; Betelgeuse is under nine million years old and will be lucky to last another million before it goes, in the words of one astrophysicist, ‘ker-blooey’. By way of comparison, our dependable, gracefully-aging sun is five billion years old and will be around for a long while yet. Betelgeuse’s life span, however, is so short that while it first shined on a relatively modern world - albeit one that predates the earliest species of humans, even Heidelbergensis – its disintegration will be witnessed by man, possibly tonight, maybe in a 1,000 years or more but certainly, in terms of deep time, very soon (just as long as man doesn’t go ‘ker-blooey’ first).