Stars Like Diamonds

The Summer triangle sets in the west. High in the southern sky is the Autumn Square, the Great Square of Pegassus, guide to our nearest galactic neighbor, the Andromeda galaxy. Glowing in the east is ruddy Aldebaran, the eye of Taurus the bull. And just to the west of Aldebaran are two of the sky's most spectacular star clusters: the Hyades and the Pleiades. Through a small pair of binoculars, each looks like a scattering of diamonds on a jeweler's cloth.

In Greek mythology, the Hyades were the daughters of Atlas and Aethra; the Pleiades, daughters of Atlas and Pleione. The Hyades, half-sisters of the Pleiades, were the most important. They were the seven sisters entrusted with the care of the infant Dionysus, the god of wine. Their reward was to be placed in the sky for men and women to see for all time.

Modern astronomy, however, is able to tell us more than myths. We know, for example, that the Hyades is one of the closest star clusters, about a hundred light years away. They are all moving at over twenty miles a second towards the star Betelgeuse. That's how we know they all belong together. Aldebaran, though, is not one of them: it's going another way. We, with our terribly short lifetimes, will never see them move, but in a million years the Hyades will be so far away that they will have faded into invisibility.

We even know how old these clusters are. Bright stars die young, so wasteful are they of their fuel, and the very brightest stars of the Hyades have already begun to die. By contrast, none of the really bright stars of the Pleiades has yet died. So the Hyades are older than the Pleiades: about four hundred million years old, compared with a "mere" hundred million for the Pleiades.

(10/23/09)