The Seven Sisters

Rising by 9:00 in the north-east is the most spectacular of all the clusters in the sky, the first sign of the brilliant skies to come this winter: the magnificent Pleiades. The cluster most written about in ancient times, most photographed in modern.

Even to the unaided eye, she's magnificent. Looking like a tiny dipper of six stars, she has confounded astronomers through the ages with her name, "the seven sisters." Nothing that could have happened to a star in the brief sojourn of mankind on Earth could account for the loss, but...there it is....or star isn't there.

Well, the ancients had an answer. Really, several answers. But one legend tells how the Titan Atlas had seven daughters by the nymph Pleione, and all the sisters but one married gods; the one who married a mortal hid her head in shame. (What does that say about all us mortal husbands?) They knew, even in ancient times, that one sister was missing.

Do we have a better answer to the riddle? Not an answer, but a theory. We know that one member of the cluster, a star at the limit of naked-eye visibility, is unstable. It may once have shone more brightly. Once there may have been seven sisters in the sky.

Clusters like the Pleiades are called galactic clusters. These are clusters of dozens or hundreds of stars formed from gas clouds in the plane of the galaxy. To visualize this plane, think of cutting an apple. The red line of the apple skin at the edge of the cut is like the Milky Way; the disk of the galaxy is just the plane of this cut.

This plane contains the arms of the celestial pinwheel of stars: the arms we see overhead and call the Milky Way. And like a pinwheel, these arms slowly turn -- the arm containing the Sun revolves once every two hundred million years. As they slowly turn, these arms compress clouds of interstellar gas, clouds that are found throughout the universe.

As these clouds contract, they grow hotter. When they reach the fantastic temperature of ten million degrees the atoms of hydrogen comprising the cloud fuse into helium. A thermonuclear fire begins in the cloud. Stars like the Sun, sometimes whole clusters of stars, like the Pleiades, are born.

(09/22/04)