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There is surely no finer way to begin a tour of the constellations than with the magnificent Orion. It is arguably the most easily recognized and most impressive constellation in the night sky. The row of three bright stars that depicts Orion's belt is unmistakable, and will quickly catch your eye. From there, it is impossible to miss the large orange Betelgeuse (BEE-tel-juice) above, and the bright blue/white Rigel (RY-jel) below. Then the complete giant form of Orion will suddenly take shape, taking your breath away, and your journey through the constellations has begun.
Orion is not a passive image. From the moment he steps foot on the celestial stage in early November - brandishing his shield before him and swinging his club high over his head - Orion demands instant respect, taking over the night sky and commanding the stars throughout the long, cold winter, until he fades back into the western twilight in the spring.
In Greek legend Orion was the greatest of all hunters. Homer called him "the tallest and most beautiful of men". He was born from the skin of an ox, a gift from the gods to the widower Irieus, as a reward for his generous nature.
Orion also liked to chase women. He pursued and pressed his attentions on all the sisters Pleiades, as well as the sisters Hyades, but they were all able to escape his crude 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 honour 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. As shown in the Hubble Space Telescope photo below, Rigel is a binary system, with a small blue companion star in orbit around it.
You never forget the first time you see The Great Orion Nebula through a telescope. 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. Emitting a hazy but distinctly greenish glow, 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 inverted telescopic view below, the smaller nebula, M43, is below M42. (Click on photo for larger image.)
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 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."
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