The centre of the Milky Way
The Age
Thursday July 23, 2009
A teapot, clusters and more are within the constellation of the archer. By Perry Vlahos.BELOW the scorpion's tail lies Sagittarius the archer, one of the earliestdocumented constellations.Traditionally, it depicts a centaur preparing to release his arrow at Scorpius in order to avenge the death of Orion ... but that's another story.Through the passage of time, exactly how the ancient Greeks saw all that in this group of stars has been lost. Most astronomers now see the brightest stars of Sagittarius as a teapot. In the early evenings at this time of year it is balanced on its handle halfway up the eastern sky.None of its stars make the list of 20 brightest but the distant suns of the "teapot" asterism are bright enough to stand out easily.The Milky Way is also most prominent in Sagittarius and may even be glimpsed from the outer suburbs.Kaus Australis is the brightest star in the constellation and its name traditionally means "southern bow". It's a young giant star with a luminosity about 200 times that of the sun, whitish-blue in colour and some 145 light years from our solar system.Almost as bright to the naked eye is Nunki. Latest research puts Nunki at a distance of 220 light years. It also has an incredible temperature of 22,000 kelvins, compared with the sun's 6000, and it shines with a total luminosity of 3300 times that of our local star.Sagittarius has many amazing clusters and nebulae within its borders, perhaps the easiest to view being the Lagoon Nebula and M22.The Lagoon Nebula is a gigantic interstellar cloud of mainly hydrogen gas and dust.From a dark sky, it can be made out with the naked eye as a glowing patch imbedded in the Milky Way. Its length runs to 110 light years and it lies some 4100 light years from us.Binoculars or a small telescope show a number of stars encircled by misty haze. The nebula has given birth to this young cluster of stars and is creating more.M22, on the other hand, is a globular cluster of some 700,000 old stars, 10,600 light years away.Globulars get their name from their spherical shape. They all lie outside our galaxy and orbit the central bulge, somewhat resembling a swarm of bees around a hive. A good pair of binoculars will show it as a soft, round glow.From a rural sky, a small telescope may display some of the brighter individual stars.When looking at the Milky Way in Sagittarius, we are viewing the centre of our galaxy, but we need to see past the stars of the teapot by another 25,000 light years. The exact centre of the Milky Way galaxy is marked on our charts, but with the naked eye there is nothing remarkable to see. However, radio telescopes and infrared optical telescopes have helped us understand that there is an enormous monster that lies hidden there in the form of a super-massive black hole.Its huge central mass drives the spiral motion of our galaxy and helps keep the 300 billion stars of the Milky Way together.The hole has grown this large by gorging itself on enough gas, dust, stars and planets to equal the mass of 4 million suns.Jenny Craig, Gloria Marshall and Weight Watchers combined can't quell this sort of appetite.To commemorate the 400th anniversary of the first recorded observation of the moon in July, 1609, by Englishman Thomas Harriot, the Astronomical Society of Victoria is hosting free public viewings at Melbourne Observatory on July 28-30, from 7pm. No bookings required. Information at www.asv.org.au
© 2009 The Age
Share This