Around the Sky in 28 Days – Day 9
September 17, 2010
For the past 2 days the Moon was traversing Sagittarius, which is a Centaur archer or a teapot depending upon how you see it. For the next few days the Moon will travel among the stars of Capricornus, a half fish half goat but really just looks like a sack of potatoes to those lucky enough to observe from skies dark enough to even see the constellation.
Still for a faint constellation it has a lot going for it and a few faint naked eye (or bright binocular) stars are within a star hoop from the Moon. Alpha and Beta Capricorni make a wide double star just to the upper left of the Moon. A closer examination of both stars find that each is also part of an even closer double pair.
Beta Cap (also called Dabih) is magnitude +3.1 with a much fainter magnitude +6.1 companion. Both are easily seen and separated with a pair of binoculars. The two stars are located at a distance of 330 light-years from us and 21,000 AU from each other. The fainter star, Dabih Minor, is a class B giant with a too-close-to-be-seen F dwarf 30 AU away. The brighter star, Dahib Major, is probably a K giant with a less massive class B companion. There is even evidence of other unseen stars in the system!
Alpha Cap (also called Algedi) consists of 2 stars that can be resolved with the unaided eye. The stars are ~7 arc minutes apart, or ~1/5 the span of the Moon. Though each star is very similar in type (both G), they are not related to each other and it is just coincidence that they appear so close together on the sky. The brighter star is 109 light-years away and is 43 times more luminous than the Sun while the fainter is 6 times further away at 690 light-years and over 900 times more luminous than the Sun. For more, on Alpha Cap see the STARS site, for more on Beta Cap go here.
For tomorrow – Day 10 – I’m actually not sure yet…