Five hours later Algol will reach its minimum brightness while overhead in the eastern sky. At that time it will be climbing the northeastern sky, approximately above and between the bright star Capella and Jupiter. EDT (or 03:23 GMT on September 14), Algol will begin its drop in brightness. But when fully dimmed, Algol's brightness of magnitude 3.4 is almost identical to Rho Persei (or Gorgonea Tertia or ρ Per), the star sitting just two finger widths to Algol's lower right (or 2.25 degrees to the celestial south). This happens when a fainter companion star with an orbit nearly edge-on to Earth crosses in front of its much brighter primary, reducing the total light output we perceive.Īlgol normally shines at magnitude 2.1, similar to the nearby star Almach (aka Gamma Andromedae). During a ten-hour period that repeats every 2 days, 20 hours, and 49 minutes, Algol's visual brightness dims and re-brightens by about a third. (Image credit: Chris Vaughan/Starry Night)Īlgol, also designated Beta Persei, is among the most accessible variable stars for skywatchers. Look carefully for two dim stars offset to the upper right from the center of the square. Using unaided eyes only, from the suburbs, the Great Square appears empty. The Anishinaabe of the Great Lakes region views the square as the torso of Mooz, the Moose. The pattern might remind you of a baseball diamond when you see it because it's often tilted with one corner downwards.įor the Lakota people, the square represented the great shell of Keya, the Turtle. The square's edges are about 1.6 fist diameters (or 16 degrees) in length, and it spans two fist-widths (about 20 degrees) measured corner to corner. Pegasus, which climbs the Eastern sky during September evenings, contains one of the most recognizable asterisms in the sky, a giant square of four similarly bright stars called the Great Square. (Image credit: Chris Vaughan/Starry Night) 3: Gibbous moon joins bright Jupiter (all night) For telescopes, use low magnification and look for M31's two smaller companion galaxies, the foreground Messier 32 and more distant Messier 110 (inset) Sunday, Sept. The three highest (westernmost) stars of Cassiopeia, Caph, Shedar, and Navi (Gamma Cas), also conveniently form a triangle that points towards M31.īinoculars (orange circle) will show the galaxy best. Under dark skies, the galaxy can be seen with unaided eyes as a faint smudge located 1.4 fist diameters to the left (or 14 degrees to the celestial northeast) of Alpheratz, the star that forms the left-hand (northwestern) corner of the square of Pegasus. It covers an area of the sky measuring 3 by 1 degrees (or six by two full moon diameters). This large spiral galaxy, also designated Messier 31 (or M31) and NGC 224, is the closest large galaxy to us - at a distance of "only" 2.5 million light years. September evenings feature the Andromeda Galaxy, which is already climbing the northeastern sky after dusk.
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