Thursday, January 31, 2008

Hunting the Hunter

One of the most striking sights in the winter sky is the star-studded figure of Orion the Hunter. After the Big Dipper, Orion’s hourglass shape is perhaps the best known star pattern in the night sky. Once learned, it is recognized by most people without difficulty.

Because it straddles the celestial equator, the constellation--or some part of it--can be seen from everywhere on the planet. The celestial equator is the imaginary line that represents where the plane of the Earth’s equator, were it extended out into space, would intersect with the sky.

Diagram by Dr. Guy Worthey

Orion’s visibility around the globe has given the distinctive star grouping a prominent place in the lore of most cultures. In Greek mythology, Orion was depicted as a tall, buff, and beautiful giant, as well as an exceptionally skilled hunter. He once cleared an entire island of wild beasts in order to win the hand of the island king’s daughter. He even became the constant hunting companion of Artemis, goddess of the hunt.

There are two commonly held versions of how Orion ended up in the sky. In one, Apollo, Artemis’s brother, grew jealous of all the time she was spending with her hunting buddy. He tricked Artemis into accidentally killing Orion. The grieving Artemis paid tribute to her friend by placing him among the stars. In another tale, Orion was killed by a sting from a poisonous scorpion. Zeus placed Orion in the sky at Artemis’s request and ensured that the Hunter (winter constellation) was never in the sky at the same time as the Scorpion (summer constellation). They chase each other around the celestial sphere for eternity.

Orion the Hunter, from an 18th century star atlas
Image courtesy of G.M. Caglieris

The name Orion is believed to mean “Light of Heaven.” Although it comes down to us via Greek myths, it originated with the Akkadians, who lived along the Euphrates River from 2350 to 2200 BCE, in what is present-day Iraq. Other Middle Eastern cultures knew the constellation by names meaning the “Giant” and the “Strong One.”

We can even spot this bold constellation from urban areas, so let’s go hunting, shall we?

1) You’ll need to look south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you will be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so your sky is good and dark. The waning (shrinking) crescent moon will not rise until the wee hours of the morning and so will not interfere with your observing. New Moon, when the moon does not appear at all in the night sky, occurs on Wednesday, February 6.

3) Look south, about halfway between the southern horizon and the zenith, the point directly above your head. Now look a bit to the left, toward the eastern horizon, and there stands the Hunter. Keep in mind that if you begin observing later than an hour after sunset, Orion will be farther to the west. Since the Earth is spinning toward the east, the stars appear to move from east to west throughout the night. In the continental U.S., Orion will be due south--and at its highest above the horizon--around 9:00 p.m., after which it will continue on toward the western horizon.

4) Look for the hourglass shape: four stars marking the corners of a quadrilateral, with a diagonal line of three evenly-spaced stars cinching the middle of the hourglass. The diagonal line of three stars is quite noticeable and is known as Orion’s Belt. This is one of several asterisms (recognizable star groups) in Orion that can help us imagine the Hunter as he has traditionally been depicted. The two end stars Alnitak (pronounced ALL nitt ahk) and Mintaka (pronounced minn TAH kuh) both mean belt in Arabic. The middle star Alnilam (pronounced ALL nill ahm) is from the Arabic for string of pearls.

5) Hanging vertically below Orion’s Belt is a chain of three stars that forms the asterism of Orion’s Sword. At least, they look like three stars to the naked eye. But in the night sky, things are not always as they appear. We’ll come back to Orion’s Sword for a closer look.

6) Let’s zoom out now and look at the stars that make up the quadrilateral. Orion is unique in the sky in the number of giant and supergiant stars the constellation claims. The star at the upper left is Betelgeuse (pronounced BAY tull juice) which means armpit. This amazing red supergiant is 60,000 times brighter than our Sun. Can you see the red color of this star? The hydrogen supply that powered its internal nuclear reactor has run out, so it has begun nuclear fusion of heavier elements. This process will eventually lead to its demise in a violent supernova explosion. When it explodes, from Earth it will appear as bright as a crescent moon and will be visible in the daytime.

7) The star at the lower right, diagonal from Betelgeuse, is the brightest star in Orion, Rigel (pronounced RYE jull), which means foot. Rigel is a blue supergiant with a much hotter surface temperature than Betelgeuse. Blue stars are typically extremely hot and short-lived. And Rigel--like Betelgeuse--is dying. The scientific jury is still out on whether or not Rigel will end its life in a supernova explosion.

Sometimes it's easier to see the subtleties of star color when you can compare nearby stars of different hue. Take this opportunity to contrast the stark blue-white color of Rigel with the smoldering red-orange of Betelgeuse.

8) The star at the upper right of the quad is blue giant Bellatrix (pronounced BELL uh trix). Bellatrix is Latin for female warrior, which may be a reference to the Arabic name and a mysterious female figure lost to history. This star is sometimes called the Amazon Star. The star at the lower left of the quad is Saiph (pronounced SAFE), Arabic for sword. Saiph is a blue supergiant, hotter even than Rigel and probably destined for death by supernova.

9) Are you up for a stargazing challenge? If you are observing from a dark enough location, you can try to spot the dim asterisms of Orion’s Head and Orion’s Shield. Centered above a line connecting Betelgeuse and Bellatrix is a three-star group anchored by the star Meissa (pronounced MAY suh), Arabic for the shining one. This marks the position of Orion’s head. To the right (west) of Bellatrix is a curve of stars that marks Orion’s shield. In artistic depictions of Orion, this object is seen variously as a hide-covered shield or a lion’s pelt.

I hope you enjoyed this naked-eye tour of the magnificent Orion. Gather up your binoculars, opera glasses, telescopes, and spyglasses, because next week, we’ll take an up-close look at Orion’s Sword, wherein lies one of the most spectacular objects in the night sky. Until then, happy star trails to you.