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Getting to Know the Great Dog

The Dog Star’s neighbor once shined almost as bright as Venus

The Globe at Night campaign continues through the end of the month, so you still have a chance to contribute to this stellar effort. “Citizen scientists” — that’s you and me — are asked to study the constellation Orion and upload your sightings to the organization’s web site. Find details at www.globeatnight.org.
    The hunter’s neatly aligned belt stars point almost straight down to the brightest star in the heavens, Sirius in Canis Major, the Great Dog. A Greek-rooted word, Sirius means sparkling and scorching. The ancient Egyptians worshiped this star as their god, the dog-headed Anubis.
    Marking the dog’s forward paw is the constellation’s third-brightest star Mirzam, meaning the announcer in Arabic, as this star rises just before Sirius.
    The second-brightest star in Canis Major is Adhara, marking the great dog’s loin. Five million years ago, Adhara was a mere 34 light years away, and it blazed at –4 magnitude — nearly as bright as Venus! Seen from earth, no other star has ever shone so bright except our own sun. Since then, Adhara has raced from us and is now some 430 light years away and shines at a still-respectable magnitude 1.5.
    Less than five degrees directly below Sirius is the open star cluster M41, named after the 18th century French astronomer Charles Messier, who catalogued the night sky’s brightest objects. With today’s light-drenched skies, M41 appears as a single point of light. But that wasn’t always so. Around 325 BC, Aristotle wrote: “one of the stars in the thigh of the Dog had a tail, though a dim one: if you looked hard at it the light used to become dim, but to less intent glance it was brighter.”
    Staring at a point in the darkened sky, you’ll see what the Greek philosopher meant. Gaze straight at a star, and it dims. Look at it out of the corner of your eye, however, and the same star appears noticeably brighter. This is because our eyes have two distinct light-recognizing cells: Rods and cones. When we stare intently at something, the cone cells to the center of our retina react, providing us detail and color. However, in dim lighting, the rods at the retina’s outer edge respond.
    Today astronomers know that M41 is in fact hundreds of stars, but if you hope to see more than one point of light in this cluster, you’ll need binoculars or a modest telescope.