Double Cluster

Transcription

Welcome to Sirius Stargazing. I'm TK and in this episode we're going to look at two clusters, together known as the Double Cluster in Perseus. This really is a great target for your binoculars; two very fine bundles of stars in just one field of view, and I think once you've found it, this is something you're going to come back to again and again, challenging yourself to make out more stars every time when the conditions seem a little bit better. Although it's not the only instance where two clusters appear close together, the Perseus Double Cluster is really unrivalled in its brilliance and I think it rightly deserves to be called simply 'The Double Cluster'. It's certainly punchier than using the two clusters' catalogue numbers, which are NGC 869 and 884. NGC stands for 'New General Catalogue', a really extensive list of very varied objects that was completed in 1888 by the Danish-born astronomer, John Dreyer. Because Charles Messier didn't include the clusters together or even separately in his catalogue, which is a popular checklist for amateur astronomers, the English astronomer and broadcaster Sir Patrick Moore labelled them as number 14 in the Caldwell catalogue, which I think many of us fondly consider to be something of a sequel to Messier's. Over 7,200 light years away, these two clusters formed very recently from the same molecular cloud, or nebula, and all 600 or so of these stars are super young - somewhere around 10 million years old or less. That's at least five hundred times younger than our Sun. They're also supergiants, many thousands of times brighter than the Sun. So in terms of colour, they generally range from blue through to white, but dotted among them are a few prominent Red Giants. These particular stars are already approaching the end of their lives and they will surely explode in spectacular supernovas. Even though both clusters are quite easy to separate as fuzzy white blobs with the naked eye, by using your binoculars in very good conditions, you'll almost certainly see differences in the colours of some of the stars, but I think the most rewarding thing of all is seeing the true extent of the two clusters, not just the bright cores, and to help you with this I suggest using Averted Vision. Now this is a technique astronomers use to get a better view of fainter objects, essentially by not looking at them. When you find what you're looking for with binoculars or a telescope, it's instinctive to just stare at it intently, almost willing your eyes to adjust to it and make out more detail, but unfortunately when you do this, you're only making things more difficult for yourself. You see, your eyes have two vision systems, involving different types of photosensitive cells on the retina. In well-lit conditions, you use cones, which are sensitive to colour, but not useful for detecting dim sources of light. When your eyes dark adapt and your pupils dilate, you begin to use rods as well, which give you better night vision, but only allow you to see in black and white. Right in the centre of the retina for each eye is a small area called the 'fovea' where there are no rods - only cones. So this creates a blind spot at the very centre of your vision in dark conditions. To make the most of those rods, you're going to have to train yourself to look slightly away from what you want to see. Now, don't fret if you have trouble with this, its something that takes a lot of practise to really master, but I'm still pretty confident that you'll be impressed by the difference it makes even for the first time that you try it. By the way, in my experience its marginally better to look above the object than below it. Looking to one side would be optimal, but only if you're using one eye through a telescope. By using binoculars, you're already getting the benefit of using both your eyes, and this actually gives your brain twice the information with which to separate the light from the dark. Now, finding the Double Cluster isn't too tough, as you can see it with the naked eye once you know where to look. The easiest method is to take these two stars in the 'W' of Cassiopeia and to use them as pointers. Just draw an imaginary line through them and continue with it in the direction of Perseus, and I'm sure you won't miss the two distinct groups of stars like sprinklings of diamonds against the backdrop of the galaxy. So get practising with Averted Vision and see how many of these gems you can unlock. Until next time, clear skies.