Sunday, April 26, 2009
Big Dipper became portal to galaxies far, far away
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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Its seven stars aren't even among the brightest in the sky, yet they form the best known star pattern. Evenings in May, when it lies nearly overhead, is the best time of year to gaze at its distinctive bowl and handle configuration. Look up at the Big Dipper.
In the late 19th century, astronomers looked closely at these stars and measured their distances. What they found surprised them. Five of the Dipper's stars, along with a handful of other, mostly dimmer ones in roughly the same area of the sky, all stood about the same distance from us -- 80 light-years. Furthermore, they discovered that these stars act as a cluster moving in the same direction through three-dimensional space. This cluster is now termed the "Ursa Major Moving Cluster." However, don't expect to see them change position from one year to the next; you must be patient. In 15,000 years, the cluster will move eastward on the starry dome about the apparent diameter of the full moon.
The Ursa Major Moving Cluster is the closest stellar association. It is closer than Hyades and Pleiades star clusters in the winter sky, and closer than the less-known cluster in the May sky, the 280 light-year distant Coma Berenices cluster (aka, the Coma Cluster). The map indicates their specific distances in light-years.
Thirteen years ago, astronomers again turned their attention to the area near the Big Dipper. This time, they weren't interested in the Dipper's stars. In fact, they didn't want any stars in our galaxy, the Milky Way, to interfere with their quarry.
In December 1995, a team of scientists focused the Hubble Space Telescope for 10 days on a small spot -- smaller than 1 percent of the visual sky area covered by the moon -- of apparently empty space near the Big Dipper. The image that they captured astounded the scientific community and changed our understanding of the cosmos.
That image, known as the "Hubble Deep Field," revealed more than 3,000 galaxies, many lying more than 13 billion light-years away. In effect, this was a snapshot of how the universe appeared only a few hundred million years after the "Big Bang" and nearly 9 billion years before the Earth was formed. It was like looking back in a person's life to when they were only a few months old. Take a good look for yourself at this incredible picture: apod.nasa.gov/apod/ap980607.html. The Big Dipper may never appear the same again.
Not all the celestial attractions of May take place many light-years away as our morning sky offers early risers some captivating planetary sights. Beginning May 17, the moon slides above bright Jupiter and four mornings later it passes Venus. To the lower left of brilliant Venus, shines the much dimmer Mars.
With a pair of binoculars, look at Jupiter in the southeast at 4 a.m. after May 21. You will see a star (Mu Capricorni) directly next to the glaring Jupiter.
Not far from the upper left of that star lurks a dimmer object, our solar system's most distant world, Neptune. Look carefully because it is very faint, which explains why it wasn't recognized until 1846. This "conjunction" must be viewed from a location away from bright lights and before the morning twilight begins.
May's celestial sights illustrate that stargazing is a fascinating hobby. If you would like to learn more about it, visit the telescope exhibit celebrating National Astronomy Day at Community School's Strawberry Festival in Elmwood Park on May 2. In the evening, there will be free observing of the moon and Saturn in the Roanoke City Market, weather permitting.
John Goss writes a monthly astronomy column for The Roanoke Times.
