Thursday, January 3, 2008

The Other Seven Sisters

The winter constellation Taurus the Bull is believed to be one of the first constellations recognized by our early ancestors. Its notable star pattern has been seen as a bull since ancient times.

The Winter Sky with Taurus and Orion

In a number of early cultures, Taurus--not Aries--was considered the first sign of the zodiac and therefore marked the beginning of the zodiacal year. The zodiac is the band formed by the twelve constellations that straddle the ecliptic, as seen from Earth. The ecliptic is the imaginary line that represents the path the Sun takes across the sky. It is marked in the image above with the gold line. Because the Earth, the Moon, and the planets all lie in roughly the same platter-shaped orbital plane, the Moon and the planets appear to stick close to the ecliptic path. This is why we see the Moon and the planets moving through the zodiacal constellations over time.

The path of the ecliptic through the zodiacal constellations
Diagram from Spacetech’s Orrery

Taurus is easy to locate because of the distinctive “V” pattern that outlines the bull’s face. This V is composed of the orange giant star Aldebaran (pronounced ahl DEBB er ahn) and the star cluster The Hyades (pronounced HIGH uh deez). This V-shaped asterism, or recognizable star grouping, is very near the most famous object in Taurus, The Pleiades (pronounced PLEE uh deez) or Seven Sisters star cluster. The image below shows a typical naked-eye view of the two clusters, the V asterism, and the two stars marking the tips of the bull‘s horns. The straight lines have been drawn in for reference.

© T. Credner & S. Kohle, AlltheSky.com

The Pleiades may have the star power, but this week we’re going to focus on the other seven sisters. In mythology, the Hyades, like the Pleiades, were the daughters of Atlas. Atlas, you may recall, was the beefy dude who was assigned to hold up the sky after his tactical error of siding in battle against the god Zeus. Also seven in number, the Hyades were half-sisters to the Pleiades, by a different mother.

Atlas holding up the celestial sphere
Image by Colin Gregory Palmer

The word Hyades comes from the Greek for “to rain,” an ancient reference to the cluster’s appearance during the rainy season. This seasonal apparition must have fueled the superstitious belief that the cluster actually caused storms to occur on land and at sea. It was said that Zeus placed the Hyades in the heavens to assuage their grief at the death of their brother, who was killed by a wild boar.

Aldebaran is the brightest star in Taurus, and it marks the position of the bull‘s eye. Its name is from the Arabic for “follower,” because it follows the Pleiades across the sky.

The beautiful, sparkling Hyades help take the sting out of a cold winter night, so let’s go take a look:

1) Plan to wait at least one hour after sunset to begin stargazing.

2) Face east. If you’re not sure of your cardinal directions, use a compass, or note where the sun sets and put your back to that spot. You will then be facing east.

3) About midway between the zenith, the spot directly above your head, and the eastern horizon, you will find the V asterism of Taurus. It will be slightly below the Pleiades (also known as M45), that prominent fuzzy-looking star cluster that looks like a little bunch of grapes or a mini-ladle. Bright planet Mars, gleaming coppery-red, is now just outside of the two stars that mark the tips of the bull’s horns, slightly toward the northeast.

© T. Credner & S. Kohle, AlltheSky.com

4) After you’ve located the V, pick out Aldebaran. It will be unmistakable as the brightest star in the V and the most reddish-colored.

5) The Hyades is a wonderful binocular object. If you have binoculars or even opera glasses, train them on the point of the V and explore the area. Although you can only see a handful naked-eye, there are more than 250 stars in this cluster. Binoculars are a great way to peer into its depths. Can you spot differences in the size, brightness, and color of the cluster’s members?

© T. Credner & S. Kohle, AlltheSky.com

6) Like all the stars that we can see in the night sky, the Hyades cluster is in the Milky Way, our home galaxy. About 150 light years away, it is notable as the nearest open cluster to us.

7) At only 60 light years away, Aldebaran is more than twice as close to us as the Hyades cluster. It is not physically associated with the cluster. It is merely a happy accident that it glows there, giving the bull’s face its most distinguishing feature.

8) As you spy upon the lesser-known but still lovely sisters and their multitude of attendants, keep in mind that this cluster--like all star clusters--is three dimensional. Its members are spread over a volume of space around 80 light years in diameter.

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