Thursday, July 2, 2009

A Pair of Ragged Claws

One of the signature sights and true delights of summer stargazing is the curvy constellation Scorpius. But preceding the scorpion in the sky are its long-lost appendages: the Claws. It was the ancient Romans who reportedly hacked them off in order to make a twelfth zodiac sign, Libra the Scales.

Scorpius and Libra in Alexander Jamieson’s star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s reunite the celestial arachnid with its pincers.

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 to Libra
Star maps created with
Your Sky

2) Just to the right (west) of the arc of stars that marks the scorpion's head is a quadrilateral of stars— two bright and two dim. The brighter ones mark the tips of the Claws. The bright one closest to the southern horizon is the star Zubenelgenubi (zoo-BENN-uhl-jenn-NOO-bee). Its name is from the Arabic for southern claw. It’s actually a double star that may be “split” with the naked eye, that is, you should be able to see the stars as two separate points, if you have relatively good eyesight and you’re in a dark location that’s relatively free of light pollution.

"The Claws" in Libra

3) The slightly brighter star above Zubenelgenubi is the blue-white dwarf star Zubeneschamali (zoo-BENN-esh-uh-MAH-lee), from the Arabic for northern claw. The luminous Zubeneschamali is about 130 times brighter than our Sun.

4) Of the two dimmer stars in the quadrilateral, only the southern one has a traditional name: Zubenhakrabi. Zubenhakrabi (zoo-BENN-hock-RAH-bee) is from the Arabic for scorpion’s claw. It’s a red giant star.

We call the northern star Gamma, for its star catalog designation. Gamma is an orange star.

It's interesting to note that Libra is the only constellation of the zodiac that represents an inanimate object. It may also be the only constellation whose brightest stars present a devilishly difficult trio of tongue twisters.

Astronomy Essential: Gravity has shaped our universe.

Gravity is one of the four fundamental forces known in the universe. Although the weakest of the four, it has shaped the large-scale structure of our universe by initiating star and galaxy formation.

Gravity pulls objects together. Every object produces a gravitational force that attracts all other objects. The more mass (quantity of matter) an object has, the stronger its gravitational force.

The gravity that causes a gas cloud in space to compress until it becomes a star is the same force that keeps us attached to the surface of planet Earth. The gravity that causes stars to congregate into massive collections called galaxies is the same force that keeps Earth in orbit around the Sun.