Thursday, February 11, 2010

We Are Family

I didn’t grow up with a sister (although I now have an awesome sister-in-law named Debbie). I guess that’s one of the reasons I find the Pleiades (PLEE-uh-deez) star cluster, aka the Seven Sisters, so intriguing. The idea of having six sisters is a difficult notion around which to wrap my imagination.

Right now, you can contemplate the Seven Sisters every evening, because about an hour after sunset when it’s good and dark, you’ll find them overhead, near the meridian.

They’re the sparkly little cloud of stars northwest (to the upper right) of the constellation Orion the Hunter. Lying between Orion and the Pleiades, you may notice a prominent, golden or pumpkin-colored star. This is Aldebaran (al-DEBB-uh-rahn), an orange giant that marks the eye of Taurus the Bull, the constellation within which the Seven Sisters reside.

Orion and Taurus, oriented with south at the bottom
Star maps created with Your Sky

The Pleiades are an open cluster, a group of stars that formed around the same time in the same nebula, or cloud of gas and dust. You may wish to think of an open cluster as a family group. There are several hundred members of this cluster, although you can see but a fraction with the naked eye.

Hobby astronomers use the Pleiades to test their visual acuity. Most people can pick out six separate stars when looking at the cluster naked-eye. Of course, amateurs try to push the envelope. The record for anyone of my acquaintance is 11 stars, seen at a very dark national park by a 20-something female astronomer. Yes, young eyes are a definite advantage; I’ve seen young adults easily pick out six or seven stars in light-polluted urban environments.

How many can you see? Test yourself at a dark location with no line-of-sight lights. To maximize your night vision, be sure to dark adapt first, that is, avoid all white light for a minimum of 20 minutes before attempting. To squeeze out every last star you can, also try averting your vision. In addition to looking directly at the cluster, try looking both slightly above and slightly below it. Sometimes additional stars will pop into view while using averted vision. This is because our peripheral vision is better than our straight-ahead vision.

Under a dark sky and with good observing conditions, you may notice a fuzziness or haziness associated with the Pleiades. No, you’re not imagining it. Not unlike car headlights moving through a patch of fog, the cluster is currently passing through an interstellar cloud of gas and dust called the Merope Nebula.

The Seven Sisters moniker by which most of the Western World knows the cluster refers to Greek legend: they’re the seven daughters of the giant Atlas (most famous for holding up the world) and the nymph Pleione (from whom the Pleiades get their name).

The named stars of the Pleiades
(oriented with south at the bottom)

Parents and daughters are immortalized in the names of the nine brightest stars in the cluster: Atlas and Pleione together to the east, and to the west, Alcyone (al-SIGH-oh-nee, the brightest star of the cluster), Merope (MERR-uh-pee, after which the nebula is named because it is densest near that star), Electra, Maia, Asterope, Taygeta, and Celaeno. The six that most people see naked-eye are Atlas, Alcyone, Merope, Electra, Maia, and Taygeta.

Known since antiquity, mentions of the Pleiades have been found in the written record as far back as several thousand years BCE. Nearly every ancient culture ascribed a mythological or folkloric identity to the celestial swarm. The Hindus saw a flame in the starry pattern--a symbol of their fire god. The Greek poets spoke of a flock of pigeons. In the French countryside, the cluster was known as the "Mosquito Net," a fact I’m sure you'll never see on a tourism brochure. The desert-dwelling Arabs imagined a herd of camels, and Hebrew writers memorialized a hen and her chickens. Natives of the Tonga Islands in the South Pacific named the object “Little Eyes.”

Many modern naked-eye observers of the Pleiades see the pattern of a little water dipper and mistakenly think they have found the much larger Little Dipper, the central asterism (star pattern) of the northern constellation Ursa Minor the Little Bear. Personally, I always see a little cluster of grapes. At recent public outreach events, people have told me they see items ranging from a lollipop to a microphone!

What do you see?

Unfortunately, my sister-in-law lives on the other side of the country, so I get to see the Pleiades more often than I get to see her. Sometimes, miles trump light years. But she’s often in my daily thoughts, and with the marvels of modern electronic communication, she’s always just an email away. As for those ladies of the night, my low-tech binoculars bring them— all seven— into sharp focus.

I’ve got all my sisters with me.