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Starry Night's Fall Sky Tour: Easy Targets in the Western Sky
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
30 September 2002
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Western Sky / Monday SeptThe western sky is a good place to begin stargazing, because you don't even need a compass to orient yourself. Just go out in the early evening and find the dimming afterglow of the Sun. That's west. As the sky darkens, a few bright stars will appear. | Your Tour Guide | | Maps and images made with the new version of Starry Night Pro software. | | Click to enlarge  Easy-to-find stars in the western sky this week mapped at 9 p.m. from mid-northern latitudes.
|  Constellations in the west, as seen at the same time of night. |  Printable all-sky map gives an overview of the fall sky. | | The west is dominated by Arcturus, which shines at a magnitude of about -0.07 on a scale used by astronomers to measure the apparent brightness of objects. The lower the number, the brighter the object. The brightest stars are zero or first magnitude. Negative numbers are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6. Even city dwellers can see Arcturus. The bright star sits in the constellation Botes (Boo-OH-tes) the Herdsman. It is the fourth brightest star in the sky and can sometimes appear yellowish in color. Arcturus is about 37 light-years away, which means the light you see tonight took 37 years to get here. [More about Arcturus] Tracing a line up from Arcturus, you can spot Izar, also in Botes, just to the right of your imaginary line. About 170 years ago, telescopic observations revealed that Izar is actually two stars rotating about each other. There are lots of these so-called binary stars, and Izar is unremarkable in most other ways, too. Higher up and slightly left of the line you've drawn in the sky is Gemma, also known as Alphecca and sometimes spelled Alphekka (astronomers aren't always precise!). Whatever it's called this big burning ball of gas is some 60 times brighter than the Sun, but it's 75 light-years away, so of course it appears much like other bright stars from our point of view. To the trained constellation spotter, this star is one in a semi-circle that form the so-called Northern Crown, also known as the constellation Corona Borealis. Now drop back down to the elevation of Izar, and swing your eyes to the right. In the northwest sky you'll see three bright stars near one another: Alkaid, Mizar and Alioth. These make up the handle of the Big Dipper, in Ursa Major. But we won't go there because that puts us into the northern sky -- the topic for more Easy Targets tomorrow as our Fall Sky Tour continues. Meanwhile, learn more about the autumn sky in last Friday's Spacewatch column: The Celestial Sea. Fall Sky Tour Home | Spacewatch 101: Tips and Terms | Sky Calendar & Moon Phases | About Your Tour Guide
Starry Night software maps the sky from your location. In this video, learn what one noted astronomer thinks of it. 
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