Observing Project 12A The Showcase Doubles

After years of careful study, astronomers have come to realize that most of the stars in the local area do not circle the galaxy in isolation but have at least one partner. In our first observing project, we'll look up close at some of the most beautiful and unique in our sky. Let's take a look around the sky beginning with the summer sky and going around the sky season by season.

Let's start in the northern hemisphere summer. Earlier in this chapter we highlighted Albireo, which is at the foot of the Northern Cross. This pair is almost universally recognized as the most beautiful in the sky. The two components of Albireo are topaz orange and sapphire blue in color creating a stunning contrast. In addition, the background Milky Way stars have been described as a field of diamond dust creating a stunning setting for our multicolored jewel. The two components are both reasonably bright at magnitude +3.0 and +4.6 respectively. The star is also remarkably easy to view because its components are separated widely enough to make them easy to view in the smallest of telescopes with the lowest of magnification. The two components are spaced 34 arc seconds apart. Further, the since the eye is only capable of discerning so many different colors, the result of having two stars with such different colors is such close proximity is that the eye exaggerates the contrast between the two stars.

Another very easy double to split is at its best about at the same time as is Albireo, in late summer. The alpha star of Libra, Zubenelgenubi, is most prominent about the same time as Albireo is. This star is famous first and foremost for its tongue-twister name. Zubenelgenubi along with nearby Zubeneschamali were in antiquity the northern and southern claws of Scorpio. Libra was once a part of Scorpio (and the only constellation of the zodiac that is not emblematic of a living thing) many thousands of years ago. Zubenelgenubi is also a widely spaced double consisting of a magnitude +3.0 white spectral type "A" star and a fainter magnitude +4.8 spectral class "F" star. The two components are separated by almost three arc minutes so they are extremely easy to split and can be split in fact with the unaided eye. Unlike Albireo, the stars of Zubenelgenubi are similar in color. Can you tell the difference in color between the two?

As summer turns to fall, the string of beautiful stars rises high overhead in the constellation Andromeda. The last of stars in the beautiful line of stars is Almach (Gamma Andromedae). The name is Arabic like most star names and refers to a type of wildcat indigenous to the Middle East region. Almach is another multicolored extraordinary beauty. The two components are separated by ten arc seconds making them very easy to see. Gamma-1 is a magnitude +2 golden spectral type K giant in the process of evolving off the main sequence into a giant. Its partner, Gamma-2 is blue-green in color and shines at fifth magnitude. The companion is in itself double, though far tougher to see. Gamma-2 is composed of two fifth-magnitude spectral type "A" stars, white-blue in color circling each other every sixty years. Though the two stars are currently near maximum separation, they are still less than half an arc second apart. A 12-inch telescope at minimum is needed to split Gamma-2. But there is still more. The brighter of the Gamma-2 pair is also double, requiring a spectroscope to split. All three stars are about the same in color, so the point of light we call Almach is actually a quad.

Rising to zenith as autumn fades to winter is the brilliant Castor. Because of its affiliation with Pollux, Castor is commonly placed among the ranks of first-magnitude stars when in fact it is the brightest of the second-magnitude stars. Castor is also one of the heavens' most famous multiple stars. A telescope of 3 inches or larger splits Castor in two white components split by about two arc seconds. One component, Castor A is magnitude +1.9 and spectral-type A1; while the other, Castor B is magnitude +2.9 and spectral-type A8. Castor A and Castor B travel around each other in an elliptical orbit requiring about 400 years to complete. Currently Castor A and B are near their minimum separation so high power is required to split them. Lying about a minute to the south is a third component, a magnitude +9.0 M-class star called Castor C. Castor C's orbital period around the bright pair is unknown because we have not been able to observe it long enough to get a good look at its motion. Castor C is more than 1,000AU from the barycenter of the bright pair. All three of the stars are visible in any telescope with adequately dark skies. But when we look even closer with a spectrograph we find that each of the three components is also double. Castor A is two identical A1 stars that orbit each other each 9.2 days at a distance less than one-tenth the distance from the Sun to Mercury. Castor B's twin stars circle even faster completing a joint circuit each 2.9 days. Castor C is comprised of a pair of nearly identical spectral-type M1 red dwarfs. The two stars each have about 0.6 solar masses and are separated by less than two million miles. These two stars race around each other in only twenty hours and either one or both of these stars are UV Ceti type variables (flare stars). Splitting the A and B pairs is relatively easy, but still a bit of a challenge. Use all the power you can and wait for a moment of still air. The C pair is far enough away to be easily found, but are your observing skills good enough to identify it? Study the system indoors first so when you go outside, you will know what it is you are looking for. A few minutes of preparation can make a tough task easy but a lack of any preparation makes easy tasks very hard.

During the later half of winter and early spring, Leo the Lion rides prominent overhead. In the Sickle of Leo, north of Regulus is the bright star Algeiba. This beautiful double, illustrated earlier, consists of a pair of stars, one yellow magnitude +3.0 and one orange magnitude +4.0. The stars are close, not so close as Castor though not nearly so easy as Albireo and Almach. Like with Zubenelgenubi, the stars are similar in color, but not identical. Zubenelgenubi is a bit easier because of the distance between the components, while those of Algeiba are split by only six arc seconds.

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