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| This month’s star chart
shows the northeastern sky about 10:00 PM, July 14, 2002, at the time of
our star party. This month we feature Cygnus, the home of the two
finest double stars in the sky. Albireo, at the head of the Swan,
consists of a golden yellow star, about mag 3, and a blue companion, mag
5. The stars are about 34 arcsecs apart, too close to resolve with
the naked eye. But with a small scope, or even binoculars in a steady
hand, the stars are easily resolved. It is unclear whether the stars
are an “optical double,” merely appearing close together by chance, or
whether they are gravitationally bound and orbiting each other. If
the latter, then they have a very long period.
61 Cygni is the other fine double in Cygnus, a matched pair of orange stars, mag 5.3 and 5.9. These stars orbit their common center of gravity in about 650 years. The stars are now about 30 arcsecs apart. Because of its rapid proper motion, its movement in relation to the other stars, Bessel chose 61 Cygni in 1838 for the first successful measurement of the distance to a star, about 10 light years. The Great Cluster in Hercules is one of the finest globulars. The Ring Nebula in Lyra and the Dumbell Nebula in Vulpecula are fine examples of planetary nebulae - which of course have nothing to do with planets. Someday, astronomers should reform our terminology! |
You can print a copy of this star chart so you can take it outside.
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