By Deborah Byrd
EarthSky Communications

Thursday, Oct. 1, 2009
    
The very bright object in the southeast to southern sky on these October 2009 evenings is the planet Jupiter. But many have asked about a bright star –– not as bright as Jupiter, but still very bright and twinkling very actively –– in the west after sunset. Jim wrote, “When I looked at it through my binoculars it flashed brilliant red, brilliant blue and bright white. I live in Indiana and called my cousin in Wisconsin, who also could see it. It was quite beautiful. Do you know what it was that I was looking at?”
    
Jim, every year at this time, we get questions about three different stars that are flashing different colors. One is Capella in the constellation Auriga the Charioteer, which is now in the northeast in mid-evening. One is Sirius in the constellation Canis Major the Greater Dog, which is now in the south before dawn.
    
And the third is the one you’re asking about, Jim, in the west to northwest after sunset. This star is Arcturus in the constellation Bootes the Herdsman. Its flashing colors for the same reason Sirius and Capella are … because all three of these stars are bright and, at this time of year, noticeably low in the sky. When you see an object low in the sky, you’re seeing it through a greater thickness of atmosphere than when it’s overhead. The atmosphere refracts or splits the stars’ light to cause the colorful flashing you observed, Jim.