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20020717 Summer Sky Objects (Ehrmin).doc Page 1 of 4 From the Everett and Seattle Astronomical Societies, this is IT'S OVER YOUR HEAD for the week beginning July 17th, a look at what's up in the sky over Puget Sound. The glory of the summer sky is highlighted by the Milky Way, a giant spiral galaxy, which stretches from the northern horizon in Perseus, through the cross-shaped constellation Cygnus and down to Sagittarius in the south. The Milky Way is packed with riches such as star clusters, nebulae, double stars and variable stars. The disk of the galaxy is a flattened, rotating system which contains our Sun and other intermediate-to-young stars. The Sun sits about 2/3 of the way from the center to the edge of the disk, about 25,000 light years. The Sun revolves around the center of the galaxy once every 250 million years. To see the Milky Way in all its beauty, you need to travel to a location far away from the glow of city lights. One of the highlights of the Milky Way is the Big Dipper, our stellar signpost. The Big Dipper now lies in the northwest with its handle pointing toward Arcturus. The Big Dipper is not a constellation but an asterism or distinctive group of stars. It consists of 7 bright stars forming a dipper, a small pot with a long handle. In England it is often called 'the plough'. Fugitive slaves before the Civil War knew it as 'the drinking gourd', a signpost in the sky pointing the way north to safety in Canada, where slavery was outlawed. The Big Dipper and the north star, Polaris, are featured on the Alaskan state flag. 20020717 Summer Sky Objects (Ehrmin).doc Page 2 of 4 High overhead and the first star to appear after sunset is Vega, in Lyra the Harp. Vega, along with Altair and Deneb, form the summer triangle, a familiar asterism of three stars. Vega is also one of three brilliant stars that divide the northern heavens into thirds, the others being Arcturus and Capella. At magnitude zero, Vega is the sky's fifth brightest star. Epsilon Lyrae, one of the most observed multiple star systems, is easy to find in the constellation of Lyra. Epsilon Lyrae splits into two pairs of stars, Epsilon 1 and Epsilon 2. Epsilon 1 is two stars of magnitude 4.7 and 6.2, physically connected and taking about 1200 years to complete one orbit. Epsilon 2 consists of 5.1 and 5.5 magnitude stars. They are also physically connected and orbit once every 585 years. Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars. They populate the halo or bulge of the Milky Way. Our galaxy has about 200 globular clusters. Open or galatic clusters are loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and are believed to originate from large cosmic gas and dust clouds. Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster due to velocity changes in mutual closer encounters and encounters with field stars and interstellar clouds crossing their path. 20020717 Summer Sky Objects (Ehrmin).doc Page 3 of 4 There are several types of nebulae. Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states. Emission nebulae are sites of recent and ongoing star formation. The Orion nebulae, also know as M42, is a good example of an emission nebulae. Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. Reflection nebulae are clouds of dust which are reflecting the light of a nearby star or stars. Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebulae in about 5 billion years. They have nothing to do with planets. The name came about because they often look like planets in small telescopes. Variable stars are stars that change in brightness. These brightness changes can range from a few hundredths to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star. Stars change in brightness when they are very young, or when they are very old. Over 30,000 variable stars are known and catalogued, and many thousands more are suspected to be variable. There are a number of reasons why variable stars change their brightness. Pulsating variables, for example, swell and shrink due to internal forces. An eclipsing binary will dim when it is eclipsed by a faint companion and then brightens when the occulting star moves out of the way. 20020717 Summer Sky Objects (Ehrmin).doc Page 4 of 4 For more on this week's topic, or to find out about local astronomy groups and activities, set your browser to “www.itsoveryourhead.org”. Our broadcast was written by Jim Ehrmin and engineered by Greg Donohue. Until next time, this is Jim Ehrmin and ________ __________, wishing you clear skies and good viewing.