Monday, January 01, 2007

Dr. Ken Rumstay's SPIRAL CORNER

OBSERVER'S NOTEBOOK

Greetings all!

Winter 2006 officially began on December 21st at 7:23 pm EST in the U.S. I love to stargaze this time of year; it seems as though the winter evening sky has more than its share of bright stars.


Image: courtesy Roger Smith/NOAO/AURA/NSF

Orion, the most prominent constellation in the sky is high in the southeast. Look for three fairly bright stars in a straight line; this is the belt of mighty Orion! Every primitive culture has imagined a human figure in this pattern of stars, and Orion is usually portrayed as a warrior or a hunter. I prefer the latter.

Surrounding the three stars of Orion’s belt is a large rectangle formed by four stars, two of them exceptionally bright. At the upper-left corner is Betelgeuse, an orange supergiant star located some 520 light years from Earth. Most of our star names are of Arabic origin, though some are Greek and others Latin, and the names usually have some meaning. Translated from the Arabic, Betelgeuse means “armpit of the giant,” and in artistic renderings of Orion, this star is invariably located at the base of his right arm. Some things are better left untranslated!

Betelgeuse (at left) is the tenth brightest star in the sky, while Rigel in Orion’s left leg, is number seven. These two stars make a remarkable contrast. Both are tens of thousands of times more luminous than our own Sun, but for somewhat different reasons.

Image: HST/NASA

Rigel’s tremendous power output results largely from its high surface temperature; this blue-white star is twice as hot as our Sun, and about fifty times larger in diameter. Orange Betelgeuse is cooler than our Sun, but its huge size more than compensates for this. In fact, if Betelgeuse were transposed into the center of our solar system, this orange supergiant, with a diameter 700 times that of the Sun, would stretch more than halfway to Jupiter!

Orion contains numerous fainter stars, of course, and a chart will help you pick out his sword and shield. By picturing Orion as a hunter, we can construct a little tableau around him using the surrounding stars. A hunter has to have hunting dogs, right? And Orion has two. Below Orion and to his left lies the “Dog Star” Sirius, the very brightest star in our night sky. This star is not nearly as luminous as Betelgeuse or Rigel, but it is much closer to us, and lies only nine light years away. To visualize Canis Major (the greater dog), picture Sirius in its head. Below Sirius lies a triangle of three faint stars; remember, the dog is standing on his hind legs, but with one paw thrust towards Orion’s feet.

Sirius (left) in images taken by the Chandra X-ray Observatory, clearly shows its white dwarf companion. A white dwarf is a hyperdense star of compacted atomic matter that manifests itself when certain normal hydrogen-burning stars exhaust their thermonuclear fuel.

Image: NASA/CXC/SAO


The lesser dog, Canis Minor, requires a stretch of the imagination. Above and to the left of Sirius lies the bright star of Procyon. It is in fact the eighth brightest star, falling between Rigel and Betelgeuse. Like Sirius, its brightness stems from its proximity, only eleven light years away. Canis Minor is a long, skinny dog (a dachshund?) composed of just two stars: Procyon and Gomeisa.

Now, what prey is Orion hunting? Above Orion, and a bit to the right, is another bright orange star. Aldebaran, the eye of Taurus the Bull, ranks number thirteen in our list of brightest stars. Located 65 light years away, it is slightly hotter than Betelgeuse, but not nearly so large (“only” 25 times the Sun’s diameter!).

Aldebaran lies at one corner of a “V” of faint stars; these form the Hyades, the star cluster closest to Earth. Above Aldebaran lies a more distant, but much more familiar cluster, the Pleiades. Look for a group of five or six faint stars, forming a tiny box with a handle about the size of the of the Full Moon. Many people mistake this for the Little Dipper, but that recognizable collection of stars is much larger, and is always found in the northern sky. In Arabic, Aldebaran means “the follower”: for as the Earth turns Aldebaran follows the Pleiades as they move from east to west.

Finally, to the left of Procyon and slightly above it is the constellation Gemini, the twin brothers of ancient mythology watching the hunt. Look for two bright stars close together. The upper star is Castor; his twin Pollux lies just below.

If you own a telescope or a pair of binoculars, there are two easily-observed objects to look for in the winter sky. The first is of course the Pleiades (at left) in Taurus.




Easily visible to the naked eye, a telescope reveals more than a hundred stars in this fine cluster. The second is the Great Nebula in Orion.

Image: NASA

Below the central star of his belt (at about half the distance to Rigel) appears a tight grouping of faint stars; this is Orion’s sword. If you have sharp eyesight, you will notice that one of the stars looks slightly fuzzy. The telescope reveals the Trapezium, a tiny cluster of four bright stars, surrounded by the faint wispy material of the nebula itself. The stars in the Trapezium are among the hottest stars known! They emit prodigious amounts of ultraviolet light that causes the surrounding interstellar gas to light up like a neon sign. Nebulae such as these are stellar nurseries; thousands of newborn stars lie deep inside.
It’s worth emphasizing that even a small telescope or binoculars will reveal thousands of deep-sky objects. And even more important than the size of your telescope is the darkness and clarity of your sky. If you are considering purchasing a telescope, you might want to resist the urge to purchase the biggest one you can afford, and simply drive out of town with a smaller, more portable instrument.

But if you already have a telescope, look for a bright yellow orb rising in the east after Gemini. That’s the planet Saturn, and its beautiful system of rings makes it one of the finest of astronomical sights!

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