Essential Information

Royal Observatory

02 Sep 2007

This month we can see the planets Jupiter in the early evening and Mars just after midnight, and plenty of interesting objects in the Milky Way arcing overhead. The word planet originates from the Greek meaning ‘wanderer’. As the planets orbit around the Sun, they slowly wander across the starry background. During September, Jupiter wanders slightly closer to the Sun, being visible in the west for 3 hours after sunset at the start of the month, but only for 2 hours by the month's end. Looking at Jupiter through binoculars, you can see up to four moons that orbit the planet. Mars is also a wanderer, rising in the east earlier and earlier each day. During September, Mars is rising around midnight (BST). The bright red-giant star Aldebaran to the right of Mars, and the bright yellow star Capella above Mars, act as useful guides. At this time of year, the Earth is on the side of the Sun which allows us to see constellations such as Hercules, Cygnus and Lyra, as well as the Summer Triangle directly overhead (see our star map). In Cygnus, the star that makes up the head of the swan is called Alberio. Although it looks like a lone star to the unaided eye, a small telescope (or binoculars) reveals that this is actually a double star. It is the stark difference in colours that makes this double star such a delight! Although there has been no sign of any orbital motion in the last hundred years of observation, calculations suggest that they orbit around each other once every 70,000 years. Close to Alberio, is the Dumbbell nebula – the remains of a dead star. At the centre of the Dumbbell is a faint white dwarf star, the left over core of a star that puffed out its outer layers to form the nebula. From observing the speed of expansion, the nebula is estimated to be about 11,000 years old. It is visible with a small telescope, some 1,300 light-years away. In Lyra, another nebulae is visible through a small telescope – the ring nebula. Looking like a ring of smoke, it lies 1500 light years away. At the centre is a very hot white dwarf star emitting intense ultraviolet radiation, causing the ring to glow. Visible towards the North West this month is the great globular cluster in the constellation Hercules. Through even a small telescope the sight is stunning. This cluster contains several hundred thousand stars! The cluster is about 25,000 light-years away, on the very outskirts of our own Milky Way galaxy Look towards the North, and you can find the pattern of stars called the Plough (the tail and body of the great Bear, Ursa Major). The handle of the Plough points towards Arcturus, which can be remembered by the ditty “Follow the arc to Arcturus!”. Arcturus is a bright red-giant star, 16 times wider than our own Sun, and over 100 times brighter. Happy observing!