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ight Sky stars

Save our skies

Be a considerate neighbor:

Reduce nighttime glare.
Shield all your outside lights downward
(or turn them off completely)
and enjoy the beautiful, starry night sky.

August 2006 Night Sky


There is a type of star, called variables, which changes brightness over time. The brightness changes of these stars can range from a thousandth of a magnitude to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star. Over thirty thousand variable stars are known, and many thousands more are suspected to be variable. There are a number of reasons why variable stars change their brightness. Pulsating variables, for example, swell and shrink due to internal forces, while an eclipsing binary will dim when it is eclipsed by a faint companion, and then brightens when the occulting star moves out of the way.

So, get out your binoculars and let's play some stellar hide-and-seek. You will definitely need binoculars to observe these stars.

Z Ursa Majoris is located in the constellation Ursa Major and within the bowl of the Big Dipper. Z UMa is easy to find, in a friendly constellation. Its semi-regular periodicity is exciting to follow and results from several different pulsation periods within the star. This variable is perfect for beginning observers to locate and monitor, and it is bright enough to be followed with small telescopes or binoculars.

The star changes between a magnitude of 6.4 and 9.3 over a period of 196 days.

R Coronae Borealis is one of the most interesting and most peculiar of all variables. It has been a favorite with observers ever since its discovery nearly two hundred years ago. Located inside the bright circlet of stars that form the Northern Crown, R CrB is usually easy to find with binoculars at sixth magnitude. R CrB type stars stay at maximum and then intermittently experience fluctuations because dust cloud periodically obscures the star. R CrB, when exposed, is a star that usually shines around sixth magnitude, and it is during this time that the star is at maximum. At highly irregular time intervals that are unpredictable, the star enters a deep minimum caused by a dust cloud. There are no good explanations for the why or how dust clouds form. Eventually, the dust cloud moves out of the way, reexposing the photosphere of the star. This star varies between a magnitude of 5.9 and a very dim 14.4.

RY Sagittarii is located in the constellation of Sagittarius, just a few degrees north of the boundary shared with Corona Australius. In fact, it can be found just between the handle of the tea-kettle asterism in Sagittarius and the crown of Corona Australius. RY Sgr is another variable belonging to the R Coronae Borealis-class (RCB) of stars. With just under fifty known RCB variable stars, it appears to belong to an elite gang of rebellious variables. While astronomers have been trained to think that activity of a star generally occurs as the star brightens, RCB stars prefer to do just the opposite. While spending the majority of their time at maximum brightness, this odd lot of variable stars are seen to be in an active phase as they fade to fainter magnitudes. RY Sgr varies between 6.0 and 13.3 magnitude irregularly because of the dust clouds it generates.

R Cyg can be found at the northwestern wing of the swan constellation of Cygnus. Nestled between the bright stars of iota and delta Cyg, R Cyg is relatively easy to find. R Cyg has an average magnitude of 7.5 at maximum and 13.9 at minimum, with a mean period of 429.2 days between bright and dim.

Anyone can be a variable-star observer. All you really need to begin observing are your eyes, a pair of binoculars, and a little patience. For more information about variable stars and how to observe them, see the American Association of Variable Star Observers Web site,


• Mercury and Venus can be found side-by-side very low in the east just before sunrise the first week of August.
• Mars this month will be very low in the west after sunset the first half of the month.
• Jupiter is high in the sky after sunset, shining as the brightest “star” in the sky.
• Saturn will rise with the Sun this month and will not be visible.
• Look for the full moon on August 9 and a new moon on August 23.

If you have a question, comment, or suggestion for Charlie, e-mail him at:


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