About 20 million years ago, in a region of the sky now known as the constellation Taurus, a vast cloud of cosmic dust and gas collapsed and gave birth to a swarm of about 500 stars. Today, that group of distant suns has special status as the most prominent star cluster visible from Earth: the Pleiades.
To the unaided eye, the Pleiades is a delicate, compact arrangement of six stars in a little dipper configuration (the real Little Dipper is actually much less conspicuous). Binoculars reveal another 30 to 50 stars in the cluster, and telescopes show the remainder.
The Pleiades is easily found using Orion's prominent three-star belt, which points up and to the right directly toward the cluster.
According to Greek mythology, the seven brightest stars of the Pleiades are the seven daughters of Atlas and Pleione. A soap-opera yarn typical of the myths tells how Orion the Hunter burst into the girls' bedroom. Venus saved the seven sisters from whatever was on the nimrod's mind by placing them in the heavens just out of Orion's reach.
Records from antiquity always assign seven stars to the compact cluster of stars, but excellent vision and dark skies are required to see all seven. I can usually detect five or six, but the seventh is tough. It lies right beside the handle star.
The fact that all mythology, without exception, refers to seven stars, not six, has led some astronomers to speculate that the seventh star has faded over the centuries. And, sure enough, number seven, Pleione, has a peculiar spectral signature and slightly variable light output. Today, it is only half as bright as Taygeta, number six. Maybe it was brighter in antiquity, nobody can say for sure.
One thing is clear, though. Some people can see more than seven stars in the cluster. The best I have ever done is nine, but some of my colleagues with exceptional vision (better than 2 0/2 0) can see more.
I remember one night in 1961 I had my telescope set up in my back yard in suburban Toronto when I was joined by Robert MacQuarrie, my next-door neighbor. It was a beautiful October night (the sky over the suburbs was black in those days), and the Pleiades were high in the east. I mentioned the Seven Sisters mythology, and he said, "But I see at least 10." Somewhat skeptical, I handed him my notebook and asked him to draw the cluster.
MacQuarrie quickly sketched the positions of 12 stars. All 12 were plotted correctly according to my star atlas. I was astounded. Unimpressed with his obvious visual acuity, MacQuarrie shrugged, took a peek through the telescope and went home to bed. Since I knew he had no particular interest in astronomy, this was a clear case of remarkable vision.
Nobody needs above-average vision to see Mars this month. The planet looms in the eastern sky all evening, its brilliant orange glow making it unmistakable. Mars was closest to Earth in late November, but it is still near minimum distance.
As Earth passes Mars, the planet moves relative to the background stars. By coincidence, Mars is now heading toward the Pleiades. It will appear closest to the cluster -- just three degrees away, or twice the width of your thumb at arm's length -- in early January.