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The Alpha Persei cluster

January 11, 2011
By Aileen O’Donoghue

As winter settles in on the Adirondacks, a cosmic parade of brilliant constellations rises to illuminate the night. The Alpha Persei (PER-see-eye) Cluster, glittering around Mirfak (aka ? Persei) begins the parade as it rises in the early evenings of September, followed closely by Capella (ca-PELL-uh) in Auriga (OHR-ree-gah) and the Pleiades (PLEE-uh-dees) in October.

This tight cluster of stars is mistaken by some as the Little Dipper, which is actually attached to Polaris, the north star. The Pleiades, also known as the seven sisters, are called Subaru in Japanese explaining the logo seen on the Outbacks and Foresters cruising about.

Astronomers have determined that there are upwards of 1,000 gravitationally bound stars about 440 light years away, making the Pleiades a wonderful target for binoculars and small telescopes. Most of us can pick out the brightest six stars: Alcyone, Atlas, Electra, Maia, Merope and Taygeta, all daughters of Atlas and Pleione in Greek mythology (along with fainter Sterope and Celaeno). It's fitting to have the stars named after sisters since this is a birth cluster of stars a cosmic litter!

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Stars are formed by the gravitational contraction of vast clouds of hydrogen. During collapse, they fragment into thousands of globules that get dense enough and hot enough to form stars. Thus stars form in clusters and tightly bound clusters like the Pleiades indicate young stars that have not yet drifted away from each other in the gravitational currents of the Milky Way. The Pleiades are thought to have formed about 115 million years ago. Compare this to our sun's mature age of 5 billion years! When the Pleiades were forming, so was the modern Atlantic ocean as Africa separated from the Americas, and flowering plants were beginning to spread across the globe.

Another cosmic litter rises into the evening skies in November, forming a V with the bright red star, Aldebaran (al-DEB-uh-ron). These are the Hyades and are much closer (150 light years) and older (625 million years) than the Pleiades, with about half as many stars. Aldebaran is about halfway between us and the Hyades, a red giant star 50 times larger than the sun, 65 light years away. Its name translates from Arabic to "the follower" because it follows the Pleiades in the winter parade.

Both the Hyades and the Pleiades are in the constellation of Taurus, the bull. The Hyades form the bull's nose with Aldebaran as his eye and his horns extending toward the east. I actually see the legs of the bull as indicated by the small stars in the diagram. These are not part of any standard stick-figure for Taurus and some of the stars actually belong to other constellations. However, they lead me to see the bull coming out of the sky with his body facing slightly to our left, but his head, having caught sight of the Pleiades, is turned to our right as he pivots on his left hoof to follow the sisters.

As Taurus turns to face the Pleiades, he also catches site of the constellations of Aries and Pisces, other signs of the Zodiac. The astronomical Zodiac is the ring of constellations found along the Ecliptic, the path of the sun across the sky, the plane of Earth's orbit. The moon and all the planets travel near this line since all orbit in about the same plane. Pluto's orbit is tilted 17 degrees from the other planets', which was one of the reasons it was reclassified as a sub-planet in 2006. This week, you will be able to see the moon move along the Ecliptic from tonight's first quarter in Pisces through the thickening waxing gibbous in Aries, Taurus and Gemini, to the full moon in Cancer next Wednesday.

Another important plane projected onto the sky is that of Earth's Equator, known as the Celestial Equator. Mintaka (min-TALK-uh), the right-most star of Orion's belt lies just below this line and Jupiter, still very bright, is close to the point where it crosses the Ecliptic, the Vernal Equinox. As you look at the sky in the early evening, the sun is off to the right, having set a few hours earlier. (The diagram is for 7 p.m. tonight, and sunset will be at 4:40 p.m.) Thus, the sun is on the part of the Ecliptic south of the Celestial Equator in the constellation of Sagittarius. As Earth orbits the sun, the sun appears to move about one degree per day along the Ecliptic. On March 20 at 6:21 p.m., the sun will pass this point in the sky to move north of the Celestial Equator and bring us spring. From the perspective of an observer at sunset, the Vernal Equinox will appear to move toward the western horizon, finally meeting the sun there on March 20. So if you're anxious for spring watch this spot!


If you have any questions about astronomy, please visit the Adirondack Public Observatory website at or e-mail Aileen at



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