With the darkening skies of September the autumn constellations are displaying themselves in the south and towards the south-east. The large cross of stars which forms Cygnus, the Swan, is high in the sky due south with the brightest star, Deneb, at the upper end of the cross. The name Deneb comes from the Arab word for tail, so it is appropriate that it is at the end of the tail of the Swan. Deneb is a blue supergiant star with a luminosity some 100,000 times that of our Sun. Distance estimates have been somewhat uncertain until recently when it was placed at about 1,500 light years. Deneb’s age is only about 11 million years which is far younger than our Sun which is 4,600 million years old.
The easily recognisable shape of Cygnus has given rise to many myths in lands across the world. It is one of the 48 constellations listed by Ptolemy in the 2nd century and is the 16th largest of the 88 currently recognised constellations. Within this relatively large constellation there are a number of interesting astronomical objects. The area of Cygnus contains anumber of star forming regions which can be seen on long photographic exposures. One in particular, NGC 7000, is known as the North America Nebula because of its striking resemblance to that continent. It is situated just eastwards of Deneb and, in the best of dark conditions, can just be made out as a faint misty patch by the unaided eye. Moving along the left branch of the cross from the central star to the most easterly star, just below this is a supernova remnant of a very large star which exploded about 15,000 years ago. It is known as the Veil Nebula and was discovered by William Herschel in 1784. It is 2,400 light years distant and the entire nebula has a diameter of 130 light years. If it had been seen at the time when it appeared in our sky about 12,000 years ago it would have appeared as bright as Venus. Once again, in 1904, Dundee’s own Williamina Fleming discovered part of this nebula known as Pickering’s Triangle, but credit for the discovery was given to Edward Pickering as women were not recognised in the field of astronomy in these days.
I always feel that summer is slipping away when the autumnal equinox comes along. This year it occurs at five minutes past midnight on September 23rd when the Sun slips southwards over the equator. At this time the Earth’s axis is neither tilted towards or away from the Sun but the Sun’s north pole is tilted slightly towards Earth. This allows enhanced solar wind to reach Earth and we are more likely to see auroral activity for a few weeks before and after the equinox. At the equinox the length of day and night are approximately the same and in our area we get properly dark skies from about 9pm.
Sky Notes courtesy of Ken Kennedy of Dundee Astronomical Association
The Planets
Mercury and Venus
During September Mercury and Venus set at about the same time as the Sun so will not be visible.
Mars
Mars rises at 12.30am and will fade into the brightening dawn at around 6am when it will have reached an elevation of 40 degrees towards the east-south-east.
Jupiter
Jupiter rises after Mars at 3am and at 6am will still be quite bright in the east at an elevation of 20 degrees.
Saturn
will be at its highest point, due south, at 2.30am when it will reach an elevation of 36 degrees, much higher than it has been for a number of years. From being almost edge on to us a year ago, Saturn’s rings will have opened to an angle of 7 degrees and will be easily visible with a small telescope.
The Moon
The Moon is at last quarter on the 4th, new on the 11th, at first quarter on the 18th and full on the 26th September
