Sky Diary

As the sky darkens in late July evenings, several bright 'marker' stars can be easily seen around the sky. At its lowest annual point in the sky, Capella can be seen quite low in the north. Towards the south-west, and at a higher elevation than Capella is Arcturus, brightest star of Boötes, which the handle of the Plough points towards. The three bright stars towards the south-east form the Summer Triangle, the brightest of which is Vega of the constellation Lyra, together with Deneb of Cygnus and below these two, Altair, brightest star of Aquila. Due south and high in the sky is the less bright constellation of Hercules, a distinctive shape one can almost imagine as the mythological figure, kneeling with a raised club in his hand. Hercules is an ancient constellation but was seen originally as being inverted. More recent imagination sees Hercules kneeling upright in the sky with the upper 'keystone' shape of stars forming his chest and on the right hand side of the keystone we can find the Great Globular Cluster of Hercules, Messier 13. This is one of more than 150 such clusters which orbit our Milky Way galaxy. A tight cluster of stars, it is 22,000 light years distant but is so large that, even at that distance, it can be made out as a fuzzy extended star in binoculars. With a moderate sized telescope it can be seen to be made up of many stars, probably around half a million.

Just west of Hercules is the faint constellation Corona Borealis. This is a small constellation, the stars of which form a semicircle and look rather like a crown. It is distinctive in shape although not particularly spectacular, but there are two stars of special interest within it. One star, called R Coronae Borealis, near the centre of the semicircle of stars, varies considerably in brightness, fading irregularly over many months then returning to its former brightness. It is believed that this is caused by clouds of carbon developing in the star's atmosphere and dimming its brightness. The second interesting star is known as T Coronae Borealis. This is just outside the semicircle of stars and is usually too faint to be seen with the unaided eye. At around 80 year intervals, this star suddenly brightens and becomes as bright as the stars of the Plough. The reason for the outburst is that it is a double star, one of which 'feeds' off the other until it flares brightly. T Corona Borealis is predicted to have such a flare this year. This has not yet occurred so it will be worth keeping an eye on as it will be very obvious among its neighbouring fainter stars.

It is worth continuing to look for noctilucent clouds towards the north throughout July. These delicate white or slightly blue clouds at very high altitude will gradually become less frequent towards the end of July. The first sighting this year was on the night of the 2nd / 3rd June and further displays may be seen until mid-August.