by Dr. Whitney Shane, MIRA’s Charles Hitchcock Adams Fellow |
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Fixed Stars With summer upon us, we will see more of the Milky Way each night, starting with the region toward the galactic center in Sagittarius in the south through Cygnus and on to Perseus in the north east. We know that this great band of stars, most of them too faint to be seen individually, is the disk of the Galaxy in which we are located. We also know that disk galaxies generally show spiral structure, and we would like to see signs of this structure in the distribution of stars. Looking across a spiral arm, we would expect to see the stars belonging to this arm. But looking along the length of the arm, as we do where it curves around the galactic center, passing from the near side to the far side of the Galaxy, we should see a concentration of stars, because our line of sight passes through a great length of arm. Thus we might expect to see periodic concentrations of light as we scan along the Milky Way from south to north. In fact, we see nothing of the sort. The irregular light distribution which we see is due to relatively nearby star clouds and dust lanes. This same dust obscures virtually all of the light from the distances at which we would look along the spiral arms. Fortunately the light emitted by the neutral hydrogen atom, radio waves with a wavelength of 21 centimeters, is not absorbed in this way, so we can locate the principal inner arms by looking for the 21 centimeter peaks. The peak of the innermost, or 3 kiloparsec, arm is located in the small constellation Scutum. The next, so-called Scutum Arm would actually be seen in the south west corner of Aquila, and the last, the Sagittarius Arm, named for the prominent star clouds seen where this arm passes closest to the Sun, would be seen near the northern edge of Aquila. This whole region, extending clear up into Cygnus is heavily obscured by a nearby dust lane, so that there is no possibility of seeing any of this in visual light. Planets Mercury was visible in the morning sky for about two weeks around the first of August. Venus will become visible very low in the south-western evening sky toward the end of September. Mars, which was in conjunction with the Sun on July 1, is visible in the eastern morning sky during August, and its visibility will improve as the quarter progresses. Jupiter was low in the north-eastern morning sky during July, but it rises earlier as the quarter progresses and by September will be easily observable during the morning hours. Saturn is now a little west of Jupiter and thus more easily observable during the morning hours. It rises in the late evening during September and is stationary on the 12th. Uranus and Neptune, both in the constellation Capricornus, passed opposition during the summer. Neptune on July 27 (when an observer on Neptune would have the rare opportunity of seeing the Earth transit the Sun!) and Uranus on August 11. Meteor Showers The traditional high point of the meteor year, the Perseids, around August 12, is spoiled this year by a nearly full moon. Eclipses There were three eclipses during July, but all of them will have passed before this brief report reaches our readers. A partial solar eclipse on July 1 was visible from only far in the southern hemisphere. A total lunar eclipse on the early morning of July 16 was one of the deepest and longest on record, but in our region the Moon set just as totality began. Finally, on the evening of July 30 we could observe just the beginning of another partial solar eclipse, this one being best observable from far northern latitudes. |
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 Go back to Summer 2000 Newsletter index | MIRA home mira@mira.org © 2000, 2001 MIRA Last Updated: January 18, 2001 by et Last updated 1/18/01 et |
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