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Astronomy and Space articles by Martin George of the Launceston Planetarium 22 February 2014 The Brightest Star in the Night Sky Since writing recently about the first star to become visible in the evenings, which at this time of the year is Sirius, I have had a few questions about that star, and why it is so bright. Sirius is a brilliant star, visible high in our northern evening sky. It is quite easily identified by first looking for the three stars in a row in the constellation of Orion. These form Orion's belt, but they are also well known as the base of 'The Saucepan', formed from some of the stars of Orion. If you extend a line from the belt stars upwards and to the right, you will come across Sirius. Of course, Sirius is not the brightest object in the current evening sky. When the Moon is there, of course, it far outshines Sirius. Currently the giant planet Jupiter is visible much lower down in the northern sky, and is also clearly brighter than the star. Of course, Jupiter is a planet, not a star. The name Sirius comes from the Greek Seirios, meaning 'scorching'. The name is thought to have been applied not only because of Sirius' brightness, but also because of it being in roughly the same direction as the Sun in the northern hemisphere summer. Its prominence within the constellation of Canis Major, The Great Dog, led to it being called the Dog Star, and this then led to the term dog days for midsummer days. As with any star, Sirius' apparent brightness arises from a combination of its true brightness and its distance. Stars really do have different brightnesses, and of course they have a wide range of different distances from us. To begin with, Sirius is more than twice as massive as the Sun, and is 26 times as bright. It is also relatively nearby, being only 8.6 light years from us. This is only about twice as far as Alpha Centauri, the closest star system to the Sun, which is the brighter of the two 'pointer' sitars to the Southern Cross. The effect is easy to understand if you imagine holding a torch very close to your eyes. It could be quite dazzling, but if the torch were held hundreds of metres away it would look far, far less bright, and there would be a distance at which you would not be able to see the light of the torch at all. However, a very bright torch at that great distance may still be visible. Astronomy and Space articles by Martin George of the Launceston Planetarium 22 February 2014 Apart from its brilliance, Sirius is also famous for its companion star in orbit around it. It is known as Sirius B, or The Pup. When spotted by Alvan Clark in January 1862, it became the first white dwarf star to be discovered. Earlier observations of a 'wobble' in Sirius' path across the sky had suggested the presence of an unseen companion but, up to that point, it had eluded astronomers because of the brilliance of Sirius itself. White dwarf stars represent one of the possible endpoints in the normal lifetime of a star; indeed our Sun will become a white dwarf billions of years from now. They are very small, but very dense, objects. A teaspoonful of material from Sirius B would have a mass of several tonnes! Article by Martin George, Launceston Planetarium, QVMAG. Reproduced with permission of the Mercury newspaper.