Wednesday, April 16, 2008

The Milky Way and the Galaxies

Star chart above showing the Milky Way in the region of Scorpius and Sagittarius. Visible nebulae are indicated by the position of the yellow NGC numbers. Star spectral colors of stars, such as red for Antares, are also indicated.

On the Frontiers of the Cosmos: The Milky Way and the Galaxies
One of the most imposing celestial sights is the Milky Way, a band of light produced by billions of stars. The single stars, if they each stood isolated in the sky, would be too faint to be seen, but all of them together combine into a band of light. Interspersed among these billions of stars are clouds of dark dust that in places block the light from reaching Earth and give the Milky Way an uneven, ragged look. The universe includes more than the Milky Way, there are other, similar formations in space that are made up of stars and dust and gasses, each formation is a "galaxy" like our Milky Way. ("Galaxy" and "Milky Way" have the same meaning since gala is Greek for "milk. ")

One of these galaxies is the Large Magellanic Cloud in the southern sky, visible to the naked eye even at full moon as a large, misty area. When viewed through a telescope it turns out to be a huge aggregation of stars. The Magellanic Clouds (there is a small as well as a large one) are named after the navigator Magellan. The Large Magellanic Cloud is at a distance of about 179,000 or about 180,000 light ­years. Astronomers estimate that it contains approximately 15 to 30 billion stars. It also includes many nebulae, star clusters, and other obiects that show up in varied colors in photographs of the clouds.

All the galaxies together, many of them millions or billions of light years away make up the universe, which today's astronomers can explore with a vast array of instruments: radio telescopes like the 300-foot (100-m) antenna in Effelsberg/Eifel, sophisticated mirror telescopes like the one at the European Observatory in Chile that can cancel out air disturbances automatically, and the Hubble space telescope, which is able to photograph distant celestial bodies with previously unattainable accuracy.

The Hubble space telescope and other instruments have recently made possible the discovery of planets outside our solar system. By the middle of 2006, about 170 such planets were found. More sophisticated instruments are scanning the skies and more, smaller "extra-solar" planets are being dis­covered. This increases the likelihood of discovering intelligent life on another world. Such a discovery would be the most profound in the history of our planet.