Friday, 26 August 2016

The Autumn Sky | Northern hemisphere

0h 15 Sept

With the autumn equinox on 22 September the nights are now getting longer, and the autumn constellations are coming into view. Andromeda the chained maiden, Pegasus, the winged horse and Perseus the hero are all there, and so are Andromeda’s parents Cassiopeia, the queen, and Cepheus the King, all of which represent the famous legend of the stars.

According to the Greek legend, Andromeda was the beautiful daughter of Cepheus the King of Ethiopia. Her mother, Queen Cassiopeia, enraged the sea nymphs by boasting of her beauty. Neptune, in order to punish her for vanity and conceit that her cherished daughter should be chained to a rock by the seashore, where she would become pray of a terrible sea monster that was ravaging the coast.

The unfortunate Andromeda was forced to submit to this barbarous treatment, and in chains awaited her fate. Luckily, just as the monster Arrived on time, just as the monster was about to seize her, Perseus the champion flew down to her rescue bearing with him the hideous head of the Medusa. He held this up before the monster’s eyes and straight away the creature was turned into stone, and Perseus gallantly released Andromeda, and as a just reward he married her.

Low in the northern sky the seven bright stars marking the plough are low down, the two forward stars of the bowl, Marek and Dubhe, are known as the pointers because they lead the way to Polaris the Pole star which is high up. Continue the imaginary line further and you arrive at the doll’s house pattern of stars marking Cepheus with constellation of Cassiopeia close by. The five bright stars are a prominent so cannot be mistaken.

Cassiopeia lies in the Milky Way so this area of sky is rich in open star clusters all visible in a pair of 10x50 binoculars. Two of the stars point down into the NE along the Milky Way into the nearby constellation of Perseus. Midway between the two constellations is the lovely double star cluster NGC 869 which is wonderful to see in binoculars. A little further down is the relatively bright star Beta Persei, otherwise known as Algol, which represents the eye of the Gorgon, Medusa. It is a variable star that is usually of magnitude 2.3, but regularly at intervals of 2 days and 20 hours its brightness decreases. In 4½ hours in wanes to Mag 3.5, then after 20 minutes it brightens again to Mag 2.3.

Returning to the stars of Cassiopeia the opposite two stars lead the way onto the SSE where the constellation of Andromeda is on view high up. The 4 bright stars of Andromeda form a line a short distance across the sky originating in one corner of the great square of Pegasus. That star is named Alpheratz, next in line is Beta Andromedae, followed by Beta (Mirach), and finally Gamma (Alamak) which is a lovely double star; through a small telescope the two stars have a contrast of yellow & blue. The most celebrated object in constellation is the great Andromeda Galaxy M31, which can be found at the top of a line of 2 stars above Mirach the mid star of the figure.

The Andromeda galaxy is visible to the unaided-eye on crisp clear evenings, binoculars show it really well. M31 lies at a distance of 2.2 million light years and is a true spiral galaxy like our own Milky Way Galaxy. Its splendid appearance is clearly shown in deep sky photographs that can be captured by amateur astronomers with the right equipment; this image was taken by my friend Terry Hancock.

Another reasonably bright galaxy M33 lies directly below Mirach in the constellation of Triangulum.

The constellation low down in the SE is Cetus that represents the celestial monster in the Perseus & Andromeda legend.

Reasonably high up in the western sky are the 3 bright stars that make up the Summer triangle, Deneb in Cygnus the celestial swan, Vega in Lyrae, and Aquila the Eagle. Cygnus lies in the rich star field of the Milky Way and so binoculars will reveal a number of star clusters and nebula, which are colourful gas clouds in space.

Vega (α Lyrae) is a bright blue star, and close by is the lovely double star epsilon Lyrae, which is a multiple star system also visible with binoculars.

So there is much to see in the autumn sky, and a pair of binoculars is all you will need to see many of the lovely star clusters, nebulae and galaxies on show.

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