Wednesday, June 04, 2008
Summer vacation at an island universe
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Blog
A clear, warm late-spring night is special. It entices you to sit outside under the stars and look into the depths of the universe. It invites you to think about the cosmic picture.
All the stars you see, without exception, are members of our disk-shaped Milky Way galaxy. It extends 100,000 light-years across and, in our neighborhood, is 3,000 light-years thick. Our sun sits 26,000 light-years from the galaxy's center.
When we look at the Milky Way's glowing band, we are viewing no more than 6,000 light-years into the disk edgewise. Unfortunately, intervening gas and dust prevent us from seeing its massive star-packed center. Tonight, when we look straight up, we are gazing directly out of the disk's plane, into the incredible vastness of intergalactic space. The distances to the other 150 billion galaxies range from a few million light-years to several billion. This is our current conceptual model of the universe and it is confirmed by extensive observational evidence.
One hundred years ago, the accepted view of the universe was much different. Many astronomers believed it to contain only one galaxy -- our own Milky Way. In other words, the Milky Way was the universe. Its diameter was thought to be 30,000 light-years with the sun positioned near the center. That model, called "Kapteyn's Universe" named after its originator Dutch astronomer Jacobus Kapteyn, was about to be discarded.
In December 1908, a new telescope near Los Angeles began operating, easily becoming the world's largest. The Mount Wilson 60-inch telescope was powerful enough and sensitive enough to shed light on the mysterious "spiral nebulae" that populated the heavens. Most astronomers regarded them as systems of swirling gas and dust caught in the process of condensing into new stars complete with planets. That is exactly the impression that they gave in the best astrophotographs of the day.
Because of the observations that the 60-inch telescope was soon to provide, astronomers were forced to abandon Kapteyn's Universe model of the Milky Way galaxy's being the entire universe. This and other soon-to-be-built telescopes undeniably showed that the spiral nebulae, in fact, were galaxies each consisting of billions of stars. Moreover, it was found that their distances were millions to billions of light-years from our Milky Way galaxy. Accordingly, each galaxy was thought of as being a separate "island universe."
This new model enlarged the volume of Kapteyn's Universe by more than 100 trillion times. This nearly incomprehensible jump was disconcerting to many people; some refused to believe it. If a model or theory is maintained without modification, in spite of the preponderance of incontrovertible evidence showing otherwise, then it is not science. It is more along the lines of wishful thinking. In science, the model must fit the observational evidence. Kapteyn's Universe did not.
While you're pondering the merits of the Island Universe model, take time to look closer to home. Enjoy some of the fascinating celestial sights found in the skies of Southwest Virginia.
About 10 p.m. Saturday in our western sky, the thin crescent moon glows just below Mars, and, 24 hours later, a slightly thicker crescent moon moves near the star Regulus and planet Saturn.
For the rest of the month, keep a close eye on Mars as it slides nightly toward Regulus and Saturn. As you do, watch the Milky Way rise higher in the east and think about the cosmic picture.
