Spot a Star Cluster

The color-composite image of the Pleiades, or Seven Sisters, was taken by the Palomar 48-inch Schmidt telescope and is part of the Digitized Sky Survey. CREDIT: NASA, ESA and AURA/Caltech

With the waxing Moon not overly bright this week, it's an ideal time to break out your binoculars and explore the profusion of open or galactic star clusters now evident in our evening sky. Such clusters represent aggregations of young, recently born stars.  They are in our galaxy's local spiral arm, into whose interior we are looking at this time of year. These stars condensed out of the interstellar gas in this part of our Milky Way system.

Two of the very best are high in our evening sky and toward the south at around 7 p.m. local time.  You can easily locate them by using the famous three-star belt of Orion. 

Extending an imaginary line from the belt, upwards and to the right will take you to the clusters of the Hyades and Pleiades of Taurus, the Bull.  Interestingly, Australian aborigines believed that the three belt stars of Orion were three young men dancing to the music played by nearby maidens (the Pleiades).  Indeed, the most beautiful and famous star cluster in the night sky is the Pleiades.  

On the evening of Mar. 3, the fat crescent Moon will be situated near the Pleiades.  Western parts of the U.S. and Canada and all of Alaska will see the Moon cross in front of the Pleiades (an occultation).  Go here for more details.  On the following night, the first quarter Moon will appear to stand well above the Hyades. 

The Seven Sisters

Few star figures are as familiar as the Pleiades, known also as the Seven Sisters.  Anyone having difficulty in recognizing various stars and constellations should start with the Pleiades, because there is nothing else like them in the sky and nobody can look very long at the heavens on a winter night without noticing them and wondering what they are.

The Pleiades have always been held in high esteem through the ages.  Ancient Grecian temples faced the Pleiades at their rising.  One passage leading out from the Great Pyramid at Cheops in Egypt was directed toward the Pleiades when they passed overhead.  The Japanese Feast of the Lanterns are the remnants of ancient Tauric rites in honor of the Pleiades. 

And do you drive a Subaru?  Back in 1953, five Japanese companies merged to form Fuji an automobile corporation which adopted the "Subaru" cluster of stars as its official logo for its line of automobiles.  The Subaru cluster is none other than the Pleiades.

To the average eye, this group looks at first like a shimmering little cloud of light.  But further examination, aided by good eyesight, will reveal a tight knot of tiny stars.

About 250 stars have been identified as members of this cluster.  Gaze at them through binoculars: the brightest stars glitter like an array of icy blue diamonds on black velvet.  Or as Tennyson wrote, they "Glitter like a swarm of fireflies tangled in a silver braid."

Several stars in the cluster seem to be enveloped in clouds of dust, perhaps left over from the stuff of which they were formed.  About 440 light years away and some 20 light-years across, the group is dominated by hot blue stars that have probably formed within the last 100 million years.  Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.

Interestingly, widely separated and totally different cultures have always described the Pleiades as the "Seven Sisters," "Seven Maidens," or "Seven Little Girls."  Yet, only six stars are readily visible to most.  Those with more acute eyes can glimpse up to 12.  But why this cluster has been cited by more than one early people as having seven members remains a mystery. 

The Hyades and "The Follower"

The Pleiades cluster is in the Bull's shoulder.  The Bull's face is plainly marked by the fine V-shaped cluster of the Hyades.  Notice the bright orange star at the end of the lower arm of the V, which represents the Bull's angry, fiery eye. That's Aldebaran, "The Follower"; it rises soon after the Pleiades and pursues them across the sky.  The Hyades are among the nearest of the star clusters, which explains why so many of the separate stars can be readily seen.  At a distance of 130 light years, the Hyades members travel through space like a flock of geese, their paths ultimately converging toward a between the stars Betelgeuse in Orion and Procyon in Canis Minor, while receding from us at the rate of 100,000 miles per hour. 

Aldebaran on the other hand, is merely an innocent bystander that does not belong to the Hyades at all and is moving toward the south almost at right angles to the cluster's motion and twice as fast.  Taurus's V-shaped head is thus literally going to pieces.  For 25,000 years or more it will pass for a V, but after 50,000 years it will be quite out of shape. 

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.