Stars over Sisters

December is an ideal month in which to view one of the brightest and nearest star clusters in the entire sky. The Pleiades, also known as the Seven Sisters, is located in the constellation Taurus the Bull.

To find the cluster, start at Orion, draw a line through the three stars in his belt and extend the line northward and westward until you encounter a V-shaped pattern of stars in Taurus. Beyond the V-shape, and generally up and to the right of Orion, you will see a blur of stars. That is the Pleiades, and it represents the shoulder of the Bull. To some, the cluster takes on the shape of a tiny dipper.

Although the Pleiades are comprised of hundreds of stars, only about seven of the brightest stars in the cluster are visible to the naked eye. The stars are considered sibling stars scientifically, meaning that they were born from the same cloud of gas and dust around 100 million years ago.

The Pleiades are also siblings in the many myths about the star cluster. While the Seven Sisters-Alcyone, Celaeno, Electra, Maia, Merope, Sterope, and Taygeta-are denoted by some of the brightest stars in the cluster, their parents, Atlas and Pleione, are also found here. According to one Greek fable, after Atlas was forced to carry the heavens on his shoulders, Orion began to pursue each of the Pleiades. As a comfort to their father, Zeus decided to put the sisters into the sky. Orion is seen still following the sisters across the night sky.

In some photographs, the cluster appears embedded in a nebulous cloud of gas and dust. Astronomers formerly thought that this material was left over from the cluster's formation, but it is now believed that the Pleiades are simply passing through a particularly dusty region of the interstellar medium. The cluster is virtually our next-door neighbor, lying at a distance of just 444 light-years.

Shooting stars fly from the constellation of Gemini this month. The Geminids meteor shower peaks between December 12 and 14, when 50 meteors per hour or more can be seen under ideal conditions. Gemini will hover near the eastern horizon early in the evening, but be nearly overhead just after midnight.

Most meteor showers are associated with comets, but the Geminids are caused by debris shed from asteroid 3200 Phaethon. This is a rich shower, rivaled only by the Perseids meteors in August. This year, however, a last quarter moon will wash out some of the early morning streaks of light.

The winter solstice arrives at 3:03 p.m. PST on December 21 when the sun reaches its lowest noon-time altitude above the southern horizon on its annual journey across the sky. At that time, the northern hemisphere will officially be plunged into winter. The shortest daylight hours also occur at this time.

That bright object ablaze in the eastern sky in the late evenings and early mornings is the planet Jupiter. Mars and Venus can be viewed low in the west for a short time after sunset. The two planets are moving in opposite directions. Venus will rise higher in the evening sky in the coming months while the sun will soon catch Mars, hiding it from view for an extended period of time. Mercury will join these two planets low in the west toward month's end. Saturn is a morning object and will rise two hours before the sun by mid-December.

The month begins with a waxing gibbous moon that becomes full by December 6. The meteor-spoiling last-quarter moon will occur on December 14. On December 21 the moon goes dark (becomes new) then recovers to first quarter by December 28.