Carl Sagan: The Milky Way Galaxy
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Our voyage has taken us 200 million light-years to the local group dominated by two great spiral galaxies. Beyond M31 is another very similar galaxy, its spiral arms slowly turning once every quarter billion years. This is our own Milky Way seen from the outside. This is the home galaxy of the human species. In the obscure backwaters of the Carina-Sagittarius spiral arm we humans have evolved the consciousness and some measure of understanding. Concentrated in its brilliant core and spewed along its spiral arms are four hundred billion suns. It takes light a hundred housand years to travel from one end of the galaxy to the other. Within this galaxy are stars, worlds and it may be an enormous diversity of living things and intelligent beings and space-faring civilizations. Scattered among the stars of the Milky Way are supernova remnants, each one the remains of a colossal stellar explosion. These filaments of glowing gas are the outer layers of a star which has recently destroyed itself. The gas is unraveling returning stars stuff back into space. And at its heart are the remains of the original star a dense shrunken stellar fragment called a pulsar, a natural light-house blinking and hissing, a sun that spins twice each second. Pulsars keep such perfect time that the first one discovered was thought to be a sign of an extraterrestrial intelligence, perhaps a navigational beacon for great ships that travel across the light-years and between the stars. There may be such intelligences and such starships, but pulsars are not their signiture. Instead they are the doleful reminders that nothing lasts forever, that stars also die. We continue to plummet falling thousands of light years towards the plain of the galaxy. This is the Milky Way, our galaxy seen edge on, billions of nuclear furnaces converting matter into starlight. Among the many glowing clouds of interstellar gas is one called the Orion Nebula, only 1500 light years from Earth. These three bright stars are seen by earthlings as the belt in the familiar constelation of Orion the Hunter. The nebula appears from Earth as a patch of light, the middle star in Orion'd sword. But it is not a star. It is another thing entirely. A cloud that veils one of nature's secret places. This is a stellar nursery, a place where stars are born. They condense by gravity from gas and dust until their temperature's become so high that they begin to shine. Such clouds mark the birth of stars as others bear witness to their deaths. And after stars condense in the hidden interiors of interstellar clouds what happens to them? The Pleiades are a loose cluster of young stars only 50 million years old. These fledgling stars are just being let out into the galaxy still surrounded by wisps of nebulosity the gas and dust from which they formed. There are clouds that hang like inkblots between the stars. They are made of fine rocky dust, organic matter, and ice. Inside, a few stars begin to turn on, nearby worlds of ice evaporate and form long comet-like tails driven back by the stellar winds. Some stars are flimsy as a soap bubble, others are a hundred trillion times denser than led. The hottest stars are destined to die young. But red giants are mostly elderly. Such stars are unlikely to have inhabited planets. But yellow dwarf stars like the Sun are middle aged and they are far more common. These stars may have planatary systems.
Duration: 8 minutes and 7 seconds
Country: United States
Views: 177 (4 embedded)
Posted by: rianaa on Apr 2, 2011
Carl Sagan in his deep godlike voice tells us yet another fascinating story about our wondrous Universe.
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