Mars, Orion, and the Pleiades Star Cluster

Published 6:45 pm Tuesday, December 15, 2009

During winter, stars seem brighter than at any other time of year. In reality, more stars are visible in summer than in winter, but in summer they are immersed in a brighter Milky Way and thereby lose some of their individual prominence.

In winter, the night-facing side of earth is turned outward, away from the center of the Milky Way, so winters brightest stars stand out against a blacker background and are more noticeable as a result.

Six of the nine brightest stars visible from our latitude, including Sirius the brightest of all, are arranged in a circular pattern high in the eastern sky during winter early-evenings. This circle of six stars is appropriately named the Winter Hexagon. Once you see the stars of the Winter Hexagon you will not easily forget them.

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Adding to the interest and wonderment of this years winter starshow is the re-appearance of orange-red Mars which makes a close approach to earth every 26 months.

During December, Mars can be found in the constellation Leo, the lion. On the date of the winter solstice, December 21, Mars will turn and begin to move slowly backward (retrograde) into Cancer, the crab.

The back and forth motion of Mars was a mystery until the German astronomer Johannes Kepler explained it almost exactly 400 years ago. Kepler showed that it was the motion of the earth relative to the other planets that caused this bizarre and unsettling behavior, and not a peculiarity of Mars itself.

On the night of the winter solstice Mars rises about 8 p.m. and remains visible until dawn. Trace a path from Mars to where the sun set two hours earlier through the Winter Hexagon to a point just north of the orange star Aldebaran in Taurus, the bull. There you will find a tightly bound star grouping called the Pleiades Star Cluster.

When Galileo first focused his homemade telescope on the Pleiades he counted 25 stars in the cluster, but the actual number is in the hundreds. Because the cluster is so distant (400 light years), only the very brightest of its members can be seen with the naked eye from earth-in fact seven can be seen by a person with perfect vision. For this reason, the cluster has a second very pleasing name-the seven sisters.

Because empty spaces appear between the stars of the Pleiades (or Seven Sisters if you prefer), it is called an open star cluster.

Hundreds of such open clusters can be easily found and resolved by small telescopes during winter or summer in and around the band of the Milky Way, but none as lovely and eye-catching as the Pleiades. Each naked-eye star in the Pleiades Star Cluster is a sun like our own, but in reality much brighter. If the sun were at the distance of the Pleiades it would be invisible without optical aid. The unseen stars inside the Pleiades (those that are just visible in binoculars) are approximately equal to the sun in intrinsic brightness. This fact serves as a graphic illustration of the vastness of our universe.

When stars are connected by imaginary lines to represent a real or mythological figure, the result is a constellation (literally, with stars). Constellations have been invented by every known culture, often for the purpose of defining the boundaries of various parts of the sky so as to make sense of it all. To the south of the Pleiades it would be impossible not to notice the most famous constellation of all, mighty Orion, the hunter.

The most obvious feature of Orion is his belt, a trio of equally spaced bright stars in an almost straight line. Such a sparkling alignment occurs nowhere else in the entire sky, so Orions identity is unambiguous.

Below the belt is Orions sword, which also consists of three stars, though not nearly as bright as are those of the belt. The sword lies at the angle to the belt that you might expect a sword to be, and the bottom star in the sword was named by Medieval Arab astronomers Nair al Saif which means the bright one of the sword.

On close inspection of the swords middle star you will notice that you cant quite make it out as a distinct object, but as a kind of blurred image or cloudy patch. Here is where a telescope makes all the difference.

Even the smallest telescope reveals the blurred star to be a vast white nebula containing a dazzling open star cluster of four stars arranged in the shape of a trapezoid. These four stars illuminate a cloud of hydrogen large enough to hold 20,000 solar systems lined up side by side. Yet you can cover the entire area with the tip of a pencil.

The Orion Nebula is a stellar nursery where stars are being born at this very moment, and it is the nearest and brightest of such nebulas.

While the stars of the Pleiades are at a distance of 400 light years the light you see from the Orion Nebula began its journey to your eyes almost 1,600 years ago, about the time ancient astronomers were inventing the constellations and naming the stars. At that immense distance, even the brightest of the Pleiades stars would be invisible from earth without optical aid, and our sun could be seen only in a professional observatory.

Why not venture out after sunset on Saturday, December 19, to FENCE where telescopes will be set up and astronomers will be ready to point out these and many other celestial wonders free of charge? The only thing you need to bring is your interest, warm clothes, and (if you have them) binoculars.

We hope you will join us! Merry Christmas!~ Skywatch written by Jim Cooper