Summer Triangle

Ken Kennedy, of Dundee Astronomical Society, tells us what we can expect to see in the July sky. “The bright nights of July wash out all but the brightest stars at our latitude,” he says, “but the three stars which form the Summer Triangle will stand out in the south-east.

“The two topmost stars of the triangle, Vega and Deneb, will be high in the sky by 10pm with Vega probably being the first star to be seen in the bright evening sky. Following Vega, Deneb is less bright but, as the sky darkens towards midnight, it should be possible to see the cross which marks Cygnus, the swan, with Deneb at the tail-most end while the swan’s neck points downwards towards the south and the centre of the triangle.

“The lowest star is Altair, brightest star of the constellation Aquila. All three stars are hotter and younger than our sun. Vega is the brightest and is at a distance of 25 light years while Altair is only 16.7 light years away. The odd star out is Deneb, slightly less bright than the other two but at the great distance of around 1,600 light years.

“This means that to appear so bright it is much more massive than our sun and also much younger with an estimated age of around 10 million years compared to the Sun’s 4.5 billion years. Massive stars don’t live long and it is likely that Deneb will explode as a supernova in only a few million years, so have a look at it while you can!

“Vega is the brightest star of the small constellation of Lyra which represents the lyre of Orpheus which, according to Greek myth, was placed in the sky by Zeus. A few degrees south of Vega can be found the Ring Nebula which was designated as number 57 in Charles Messier’s catalogue of 1781. A medium sized telescope will show this nebula as a small, slightly oval shaped ring, almost like a smoke ring. It is an expanding sphere of gas thrown out in the death throes of a sun-like star some 1,500 years ago. It will eventually disperse leaving a white dwarf star as the ashes of the original star.

“Last month I mentioned the Great Globular Cluster in Hercules, and Lyra also contains one of these distant star clusters known by Messier’s number 56. It is not far away from the Ring Nebula and is not as bright as the Hercules cluster but is worth a look with any telescope which can show the Ring Nebula. Messier 56 is 33,000 light years distant and is believed to be more than 13 billion years old.

“There will be a total solar eclipse on July 2 but it will only be seen in the South Pacific Ocean, ending in Chile and Argentina. A partial eclipse of the Moon takes place on the night of the July 16/17 and will be seen from UK. Mid-eclipse will be at 10.30pm but the moon will be very low in the south-east at this time.

“Mercury, Venus and Mars all close in on the sun during July and it will not be possible to see them. Jupiter is well placed in the south at around 10.30pm but, although bright, will be quite low in the sky. Saturn will be in the south south-east at about the same time and elevation but will be much less bright than Jupiter.

“The moon is new on the 2nd, at first quarter on the 9th, full on the 16th and at last quarter on July 25.”