Cancer

StarDate: March 12, 2011

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Fame doesn't always mean brilliance -- whether with people or with constellations. Tonight's sky, for example, features a famous constellation that's so faint that you'll need a star chart to find it.

Cancer, the crab, is well up in the east as the sky darkens, and passes high overhead later on. It's famous because it's part of the zodiac, so it occasionally hosts the Sun, Moon, and planets.

Despite its brilliant visitors, Cancer's stars are dim. The brightest is Beta Cancri, but it's so faint you may not be able to see it from a suburb, let alone a city. It's an orange giant that's a lot like the bright star Pollux in the neighboring constellation of Gemini. But Beta Cancri is about nine times farther than Pollux, so it looks much fainter.

We'll talk about another of Cancer's stars tomorrow.

The constellation is better known for its star clusters. Its two most prominent are open clusters, which means their stars are spread out from one another. The younger and nearer of the two is M44, also known as the Beehive.

Farther away is the important cluster M67, which is notable for its age. Most open star clusters are much younger than the Sun, because they get torn apart by the galaxy's gravitational field. But M67 is an exception. Its age is similar to the Sun's. So when astronomers want to study stars as mature as our own, they often point their telescopes to this cluster of stars in the faint but famous constellation of Cancer.

Script by Ken Croswell, Copyright 2011

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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