That bright star in the west in the evening in October – flashing in colors – is Arcturus. Once it gets good and dark, and you live in the Northern Hemisphere, you can verify that this star is Arcturus by using the Big Dipper asterism. The arc of the Big Dipper handle extended outward always points to Arcturus.
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 shining in the west to northwest after sunset. This star is Arcturus in the constellation Bootes the Herdsman. It’s 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 these stars to flash in the colors of the rainbow.
At mid-northern latitudes, scintillating Arcturus adorns the western evening sky all through October.
Tonight for binoculars: Mars and Regulus before dawn
Although you can see the close conjunction of the planet Mars and the star Regulus with the unaided eye in the predawn hours, binoculars help to bring out the colors of ruddy Mars and sparkling blue-white Regulus. Mars and Regulus, the brightest star in the constellation Leo the Lion, will occupy the same binocular field of view for at least another week.