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Ever-increasing activity on the sun means there has been a lot of auroral action in the past few weeks. As a result, I have spent far too many hours perched beside Hoopers Inlet on Otago Peninsula, gawping in wonder at inspirational displays of the southern lights. One of the many reasons I love stargazing at Hoopers is that, on calm nights, the inlet’s shallow water perfectly mirrors the sky. This allows amateur photographers like me to capture memorable images like the one accompanying this article.
The picture was taken about 11pm, when the Southern Cross and pointer stars (Alpha and Beta Centauri) were at their lowest point. They were also due south in the sky. The low altitude of the stars made it easy to capture both them and their reflections in the same image. I attached a wide-angle lens to my favourite camera. After mounting it on a sturdy tripod, I opened the shutter for two minutes. In that time, the rotation of the Earth caused the star images to trail. Slow undulations in water level caused by wind have also slightly blurred the reflections of the stars, making them larger and easier to see.
While the auroral colours you can see are of interest, what I find absolutely fascinating about this picture is the range of colour which is apparent in the stellar reflections on show. Astronomers love seeing colour in the sky because it provides information which can be used to discover more about the universe.
A star’s colour tells us its surface temperature. Blue stars are hot, with temperatures generally more than 10,000degK. Red stars are much cooler; their temperatures can be as low as 3000degK.
Gamma Crucis is the coolest star of the five comprising the Southern Cross. The star appears distinctly orange in the picture. Astronomers have measured Gamma’s surface temperature to be just under 3500degK. Alpha Crucis is the hottest star in the Southern Cross; its surface temperature is more than 30,000degK which is why its reflection is so intensely blue.