A useful star

Ken Kennedy of Dundee Astronomical Society tells us what we can expect to see in the September sky. “The familiar star arrangement which makes up the Plough is low in the north during late evenings at this time of year,” he says.

“Following an imaginary line projected upwards from the two ‘pointers’ of the bowl of the Plough, the next fairly bright star encountered is Polaris, the Pole Star.

“This star is about five times the mass of the sun and a bit hotter but doesn’t seem all that bright because it lies at a distance of 433 light years. Polaris is a useful star as it is within one degree of the North Celestial Pole, the point in the sky round which the sky seems to turn.

“It has been used for centuries by sailors who can find their latitude by measuring its elevation. Balancing the Plough on the other side of Polaris is the distinct but flattened W shape of Cassiopeia. Cassiopeia sits very high and is next to Cepheus, a constellation which some say looks a bit like a house depending on how it is viewed. Cepheus is much less obvious than is Cassiopeia as the stars forming it are less bright.

“If you can find Cepheus, look towards the peak of the ‘roof of the house’ in the vicinity of Polaris. In that area lies a very active galaxy known as a quasar which has a nucleus containing one of the most massive black holes known.

“This supermassive black hole is around 10,000 times the mass of the black hole at the centre of our own Milky Way galaxy. You will not be able to see this monster as it is 12.1 billion light years away. This, however, may be fortunate as if it were at ‘only’ 280 light years distant it would provide earth with as much energy as our sun does and would certainly cause the earth to overheat.

“Looking southwards from Cepheus you can’t miss the cross formed by Cygnus, the swan flying along the Milky Way and to the east of Cygnus the square of stars which forms the body of Pegasus, the flying horse. When you realise that Pegasus is inverted it becomes easy to trace his neck and head stretching out westwards from the bottom right hand star of the square.

“Then look at the upper two stars and you will find the legs. Once you see these, the constellation Pegasus makes a lot more sense. Continue downwards towards the southern horizon and you will see Mars shining brightly in an area which has no bright stars.

“Mercury may be seen low towards the east at around 5am during the first week in September rising about an hour before the sun. Venus sets about half an hour after the sun and will not be visible.

“Mars remains bright but is gradually dimming. Because of its orange colour it will be obvious towards the south at around 9pm. It is still very low in the sky in the northern hemisphere.

“Jupiter sets an hour and a half after the sun and may be seen with some difficulty very low in the south-west at around 7.30pm. Saturn precedes Mars along the southern horizon and is at about the same elevation. It is much less striking than Mars and sets at about 10.30pm.

“The moon is at last quarter on the 3rd, new on the 9th, first quarter on the 16th and full on September 25.”