The colours of the night

When I first started to venture to dark spots on the north coast as I began my journey into astro photography, I well remember the spine tingling feeling. Standing on the ancient basaltic stones of the Giant's Causeway, below the imposing basaltic cliffs that seemed to soak in all available light, in a darkness so thick and palplable that you felt you could grab it if you closed your fist around it. Those are times that will cause a man to confront all his childhood 'things that go bump in the night' fears!

And yet, the lure of the stunning starry skies above kept drawing me back. I may well have been spooked; but the delights of seeing the skies adorned with the canopy of stars the like of which you can only see in such dark locations called me back, time and time again.

Over time, the fears slowly faded away. If you're down on the stones in the dark enough and no freaky ghostly appirition appears, I guess you start to feel safe. But the wonder of the starry skies above has anything but vanished. If anything, as I find out more and more about what is above and around me, I am even more amazed.

And so it was, last Saturday night, that I found myself back down on at the Causeway after dark. The skies were very clear. The stars were out. The darkness was thick. And I was in my element!

There was not much chance of aurora then, but that's not to say that there was nothing on view. I was going to content myself with photographing the Milky Way. But I was about to discover that the skies were about to reveal the most glorious light and colour show above me.


I had first noticed a green glow in the sky as I called in to Dunluce Castle on the way by. Thankfully the scaffolding around one of the towers has finally been removed, making this a wonderful photogenic location once more. Whilst there, long exposures on the camera revealed a green glow along the sky. But not aurora. This was another astronomical phenonema altogether: airglow.

Airglow appearing behind Dunluce Castle. The light streak behind the castle is an iridium flare, the sun glinting off a passing satellite.

Airglow is produced at very high altitudes, around 80km above the ground. Here, in the thermosphere, radiation from the sun energises the atoms here, especially oxygen. Into the night, these oxygen atoms eventually decay and release photons of light, as well as colliding with ions in the upper atmosphere (releasing light by a process that glories in the name chemiluminescence!). The wavelength of light emitted by the oxygen is mostly green, hence the colour of the airglow.

Although nothing can quite beat seeing a well structured aurora in the sky, a diffuse airglow can still be an amazing sight. And, as I saw it in the sky at Dunluce, I thought I'd head down the to the stones of the Causeway to see it in all its glory there.

North America Nebula

The pink glow of the North America Nebula hangs above the Causeway stones.

So, out onto the stones I ventured in the pitch dark, now very much at home in my nocturnal habitat. The waves were pounding the columns, and the tide was in. This would give me great foreground interest in the 30 second exposures I was going to be taking. With that, the air glow, at part of the Milky Way hanging in the sky to the north, it was with great anticipation that I set up my camera and started shooting away.

I previewed my image: there indeed was the misty ocean and the green hue of the airglow. But I noticed some other colour towards the bottom tip of the Milky Way. A distinct patch of reddish/pink. In fact, I had first noticed this a few weeks ago when I was at Glencoe, Scotland over Christmas. There, well away from light pollution, I captured a great image of the Milky Way in a small gap inthe cloud (that sadly only lasted about 15 minutes). And there was that reddish glow shining through - and now, here it was again.

The reddish/pink glow of the North America Nebula, as seen over Glencoe, Scotland. Denub, the bright star to its right, is the 19th brightest star in the sky, and has been estimated to be up to 200,000 times brighter than our sun!

But what was causing this colour? A quick message the next day to Paul Evans of the Irish Astronomical Society revealed it all: I had managed to capture the colour of the North America Nebula. So called because its shape bears an uncanny resemblence to the map of North America, this vast cloud of interstellar gases is thought to measure a staggering 100 light years across. In our sky, it covers an area four times the size of the full moon. The gases are energised by photons from a nearby star. In the case of the North America Nebula, it is hydrogen atoms that are most dominant, and the light they release is of a redder wave length. 

Despite its collasal size, the nebual is not very bright in our sky. With the long exposure on the camera, however, not only does it become brighter, but also the colours are drawn out, giving a hint of the majesty of this faint interstellar gas, 1800 light years away from where I was standing.

The airglow was 80km high, at the very edge of what could properly be called our atmosphere, and I was seeing the greens a fraction of a second after it was released. The photons of red light from the nebula that were falling onto my camera sensor had travelled 9,400,000,000,000,000 km, and left a staggering 1800 years ago. And I happened to be standing on my own, in the dark at the Causeway just when those particular photons of light arrived.

So, the Causeway may be dark. So it might have been a bit scary in the past. But when you're staring at the glorious majesty of such a night sky display, presented in a wonderful kaleidiscope of colour for those who had eyes (or camera sensors!) to see, there are few other places you'd rather be!

Many thanks to Paul Evans for cluing me in as to what that strange pinkish glow was in my photo!