Herschel’s Garnet Star
Weather forecasting skills are almost as important as knowledge of the night sky when trying to practise astronomy in the UK. So it proved on Friday evening with our scheduled monthly observing evening on Blackheath. Despite Met Office claims that the skies would remain cloudy all evening, a study of the satellite data showed that we were due a decent period of cloudless skies. We deferred the “go/no-go” decision until nearly 6pm to be certain, but ultimately decided to take the risk and run the event.
It turned out to be a good decision in the end, though the heavy dew and moisture in the air made for difficult observing conditions. We still managed to see some spectacular sights, regardless of these difficulties.
First up for viewing was the Pleiades cluster (M45) in Taurus. With the naked eye, it was difficult to pick out more than 5 or 6 stars in this famous cluster. Training a telescope with a wide-angle low-power eyepiece on the cluster reveals more than a hundred stars, a quite beautiful sight. Many of the stars in the cluster are a similar colour, blue-white stars, which formed perhaps only 100 million years ago. Through my refractor, the stars were pinpoint sharp and you had a real sense of floating through space when viewing them at a wide-angle.
With Mars having set already, Jupiter not rising until 11pm and Saturn hiding near the Sun, we were restricted in what solar system objects we could target. However, Uranus and Neptune are well placed for viewing. Though nowhere near as spectacular to view through an eyepiece in comparison to closer planets, it’s still great to be able to show visitors these ice giants in the sky, particularly when you consider that they are 1.7 and 2.7 billion miles away at present!
Uranus was in a rather hazy part of the sky when we first targeted it, meaning that it was difficult to pick up any colour. A little later however, using the trick of de-focussing slightly, we could see a slight green-blue colour. With Neptune, though further away, a pale blue disk was clearly discernible.
Next up was Albireo (Beta Cygni), the glorious double star in Cygnus. The colour difference between the two stars is stunning. Most people see a blue star and a yellow star, but take some time at the eyepiece and you realise that the blue star is a much deeper blue, almost indigo, and the yellow star is slightly more orange, almost gold. It’s always fun to ask visitors which of the two they think is the hottest star. Invariably they make the mistake of saying the yellow star. It is amazing how our perception of colour differs so completely to the reality of our universe. One for the anthropologists I suspect. The blue star is over 7,000 degrees hotter.
We then moved on to one of my favourite objects in the sky, and one that is often overlooked, the Garnet star (or Mu Cephei, made famous by Herschel who noted it’s deep garnet colour). It’s very easy to find at this time of year, almost overhead in the constellation of Cepheus. In smaller telescopes, you often see the star as a deep red, but the larger the scope, the more orange-yellow it becomes. Through my 4-inch refractor, it was a deep orange colour and a really striking object to view – again, de-focussing slightly will enhance the colour. The Garnet star is a red supergiant, one of the largest and most luminous in the Milky Way. It was interesting to contrast the colour with Betelgeuse, another red supergiant, which looks rather pale in comparison.
If we placed the Garnet star in our solar system, instead of our own Sun, it would extend midway between the orbits of Jupiter and Saturn. An object this huge is destined to go supernova one day, perhaps within the next million years or so. So massive is the star that the remaining vast hydrogen cloud will probably be centred on a black hole.
By this time, the constellation of Orion was starting to rise above the eastern horizon. Telescopes were immediately trained on the Orion Nebula (M42). Despite being low down in the sky, the faint nebulosity was visible, with the trapezium cluster easy to see. I placed an O-III filter on my eyepiece to try to bring out some detail in the nebula, and was very gratified to see clear structure in the emission nebula, with a semi-circle of gas and dust visible to the eye. A great way to finish the evening.
We’d been out on a Blackheath far longer than expected, about 3 hours in all, but the equipment was getting thoroughly soaked by the heavy dew, and a high haze was starting to move in from the east. A good time to start packing up.
We didn’t have a huge number of visitors during the evening, as I suspect people had been put off by the weather forecast. My tip, for what it’s worth… do your own forecasting… use your eyes! Still, we had a steady stream of people popping along, so it was definitely worthwhile running an event.
Thanks to all who attended, in particular to Nick and Tej. Thanks for your help guys. Without you, we couldn’t have run the event.
Hope to see more people at our next Blackheath observing session on Friday 19 December!