Thursday, July 9, 2009

The Riddle of the Snake

Fans of astronomy trivia may know the answer to this riddle: what’s the only constellation that is both one and two?

Answer: Serpens the Snake. The celestial snake imagined by the ancients struggles in the unyielding grasp of Ophiuchus the Snake Handler, winding between his legs. When 20th century astronomers were laying out the boundaries of the modern constellations, they kept the ancient figure of Ophiuchus (oh-fee-YOO-kuss) intact and divided Serpens into two parts: the head, on the Snake Handler’s left, and the tail, on the Snake Handler’s right. Thus the intertwined figures of old begat the only modern constellation with two disconnected parts.

Ophiuchus and Serpens in John Flamsteed’s 1729 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Modern constellation boundaries of Ophiuchus and Serpens (in green)
Star maps created with
Your Sky

Let’s aim for the head.

1) About an hour after your local sunset time, face south. 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’ll be facing approximately south.

Looking south

2) Just under the horseshoe-shaped asterism (star pattern) of the constellation Corona Borealis (which is located on or near the meridian) and east of the bright star Arcturus, look for a small triangle of stars. This is the head of the snake. If your vision is keen and your skies dark, you may also spot the two fainter stars above the triangle. Add them to the other three and you’ll have an “X” that marks the spot where the snake’s head lies.

3) Due south of the snake’s head is a star that’s brighter than any of the “X” stars. This is Unukalhai (uh-NOO-kuh-lye), an orange giant and the brightest star in Serpens. Unukalhai is from the Arabic for serpent’s neck, the star’s location in the snaky figure of antiquity.

4) This section of Serpens containing the head and the neck is known as Serpens Caput, to distinguish it from Serpens Cauda, the section east of Ophiuchus. Serpens Caput (SIRR-penz KAH-put) is Latin for snake head, while Serpens Cauda (SIRR-penz COW-duh) is Latin for snake tail.

Serpens Cauda is a tail for another day.

Astronomy Essential: A nebula is a cloud of gas and dust in space.

The word nebula (NEBB-yoo-luh) is from the Latin word for cloud. In general, there are four types of nebulas or nebulae (NEBB-yoo-lee): diffuse nebulas, planetary nebulas, supernova remnants, and dark nebulas.

Diffuse nebulas are stellar nurseries, gas and dust clouds where stars are born. Planetary nebulas are the remains of average-sized stars (like our Sun) that have died. Supernova remnants are the remains of massive stars that died in cataclysmic explosions. Dark nebulas are gas and dust clouds where the dust is so thick it completely obscures the light from the stars behind it.