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ORION
The Hunter
Ceiling Frescoe, Villa Farnese - MAP OF THE HEAVENS - Giovanni Antonio da Varese - 1575
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There is surely no finer way to begin a tour of the constellations than with the magnificent Orion. In the crisp, cold air of a winter night, direct your gaze high in the southern sky, and look for the three bright stars in a row that depict Orion's belt. They are unmistakable, and will quickly catch your eye. From there, you can't miss the large orange Betelgeuse (BEE-tel-juice) up by Orion's shoulder, and the bright blue/white Rigel (RY-jel) down by his foot. Then the giant form of Orion will suddenly appear before you, and your journey through the cosmos has begun.
In Greek legend Orion was a gift from the gods to the widower Irieus, as a reward for his generous nature. Orion had special talents that made him the greatest of all hunters. Homer called him "the tallest and most beautiful of men".
Orion also liked to chase women. He pursued and pressed his attentions on all the sisters Pleiades, as well as the sisters Hyades, who all spurned his advances. Finally, he managed to win the love of the huntress Diana (Artemis). Unfortunately, Diana had an overly protective and jealous brother, Apollo, who sent a scorpion to sting Orion, and kill him. Orion, ever the wary hunter, avoided the scorpion, only to have Apollo trick Diana into shooting her lover with an arrow and killing him.
In honor of his great skill as a hunter, Jupiter (Zeus) placed Orion's image in the sky, accompanied by the hare, Lepus, and his two hunting dogs, Canis Major, and Canis Minor. The sisters Pleiades and Hyades were placed just out of his reach, and the scorpion was placed on the opposite side of the heavens, so it could never threaten him again. By the banks of the river Eridanus, with a rabbit at his feet and his dogs by his side, Orion forever faces the horns and baleful red eye of Taurus, the bull.
The alpha star in the constellation Orion is named Betelgeuse, Arabic for armpit of the giant. But the real giant is Betelgeuse itself. Formally classified as an M2 red giant, Betelgeuse has a diameter over 800 times larger than our Sun! As shown by the Hubble Space Telescope photo below, if Betelgeuse replaced the Sun at the centre of our solar system, its surface would be way out by the orbit of Jupiter! It is so large it takes 1,265 Earth days to rotate once on its axis. Fortunately for us, Betelgeuse is very far away, out at a safe distance of 520 light years. It has a surface temperature of 3,450 ° C, which gives it an apparent magnitude of 0.58, and makes it the tenth brightest star in the sky.
The brightness of celestial objects is measured in terms of magnitude. In contrast to the magnitude scale used for earthquakes, in astronomy the smaller the magnitude number, the brighter the object, with the very brightest objects actually having negative magnitudes. Sirius, the brightest star in the night sky, has a magnitude of -1.42. There are two types of magnitudes. Absolute magnitude is the actual brightness of an object, and apparent magnitude is the brightness of an object as it appears to us on Earth. These two separate scales are necessary because of the vast distances involved. A relatively dim star that is close to Earth will appear brighter than a relatively bright star that is much farther away. Unless otherwise specified, all magnitudes referred to here will be apparent magnitudes.
The beta star in Orion is Rigel, from the Arabic Rijl Jauzah al Yusra, the left leg of the giant. It is a B8 blue giant, but nowhere near the size of Betelgeuse, only 68 times the diameter of our Sun. But being a blazing white-hot young star, it is much hotter and brighter, with a surface temperature of 11,200 ° C, twice that of our Sun. Because it is so much hotter and larger that our Sun, it is 40,600 times brighter! The only reason it doesn't outshine everything else in the sky is because it is so far away - 772 light years - half again as far away as Betelgeuse. Still, it has a magnitude of 0.12, and is the seventh brightest star in the sky. Rigel has a much fainter companion star, Rigel B, which has a magnitude of only 6.7, and although they appear close together from our perspective, the two stars are over 200 billion miles apart, and take 25,000 years to orbit each other. Modern spectroscopy has revealed that Rigel B is itself a double or binary system, consisting of two very close stars that orbit each other every 10 days.
The third brightest (gamma) star in Orion is Bellatrix, the Amazon Star, with a magnitude of 1.64. The next three brightest stars form the famous belt of Orion. Mintaka, which literally means the belt, Alnilam, belt of pearls, and Alnitak, the girdle. The last named star in Orion is Saiph, in the giant's right foot. It is a B1 supergiant, with an absolute magnitude of -6.9, which makes it technically brighter than Rigel. But Saiph's very great distance of 2,100 light years means it's apparent magnitude from Earth is only 2.06.
You never forget the first time you see The Great Orion Nebula through a telescope. In a backyard scope, it has a distinctive greenish glow about it, looking so much like a giant bat, or manta ray, or some great ghostly bird of prey with huge back-swept wings, swooping down on its prey. It is, in fact, an immense cloud of gas and dust 1,900 light years away, in which stars are being born. The Orion Nebula is unimaginably vast, being 30 light years across, or more than 20,000 times larger than our entire solar system! In the Hubble image below, the true glory of the Great Orion Nebula is exposed.
One of the most famous features in Orion is invisible to the naked eye and to most telescopes. But with long time exposures and special telescopic filters, the spectacular dark nebula with the unique shape of a horsehead suddenly appears. The Horsehead Nebula, also known as Barnard 33, is embedded within the much larger emission nebula, IC 434. The magical Hubble Space Telescope image of the Horsehead Nebula below reflects the poetry of Robert Burnham Jr., who described it as "irresistibly conjuring up surrealistic visions of a cosmic chess game."
Since the age of ancient Greece, there has been speculation about other forms of life somewhere, out there, among the stars. Over time, as science and technology revealed the true vastness of space, filled with untold numbers of suns like our own, it was assumed there must be many planets orbiting those suns. And surely on at least some of those planets, life must have evolved. It was simply a case of numbers. But there was no proof, because any planets that might be out there were just too far away for even the most advanced telescopes to see. But eventually, the technology was developed to detect the gravitational effect of far away planets on the star they orbit, as well as the spectroscopic changes in a star's light when a planet passed in front of it. The first extrasolar planet, or exoplanet, was discovered using these methods in 1992. Since then over 500 extrasolar planets have been discovered. The search for extraterrestrial life was on!
So far, no less than six planetary systems have been discovered within the boundaries of the Orion constellation. One of these systems has a star close enough and bright enough to be seen by the naked eye. The star is HD 38529, and so far we have found two planets orbiting around it. There could easily be more. So the next time you're out looking at Orion, find star HD 38529, and think about the fact there are planets orbiting that star, and on one of those planets there may be another form of intelligent life, gazing up at the night sky, looking right back at you! For more information on all the extrasolar planets in Orion, go to NASA's Planet Quest.
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