Clusters of Stars

About halfway between the bright stars Regulus in Leo and Pollux in Gemini lies one of the most spectacular star clusters in the entire sky: the Beehive cluster. Through a pair of binoculars this cluster really does look like a swarm of bees, easy to find because its apparent size is three times that of the full moon. There are many of these clusters: the most famous is the Pleiades star cluster, to the west of Taurus the bull. They are only found along the Milky Way. To understand why, we must travel into the far reaches of outer space and see our galaxy as a whole.

The Milky Way that we see every clear night is just one of the celestial arms of the vast pinwheel of stars that is our home galaxy: home both to our own Sun and to a hundred billion other stars. Like a child's pinwheel, the galaxy spins, but ever so slowly. Our Sun, in one of the arms of this pinwheel, has traveled only a few degrees around the center of the galaxy in the entire recorded history of mankind. When these slowly turning arms encounter a cloud of interstellar gas and dust, they trigger the compression of the cloud. The cloud's own internal gravity then takes over, further compressing and heating the cloud until, when atoms of the gas are fused together, stars are formed.

That's how, over four hundred million years ago, the Beehive star cluster was formed. Hundreds of stars formed from one enormous cloud of gas were caught in one another's gravitational grip. Now as then, they circle each other as the group of stars rotates slowly around the center of our galaxy. The Pleiades cluster, much younger, was born in this way just twenty million years ago. Open star clusters, such as the Beehive and the Pleiades, are called galactic clusters to remind us not only of their place in the sky, but also how they were formed.

The stars are the meeting place of the enormous, majestically turning galaxies, and the incredibly small atoms that fuse to light it. We, the Earth, the Sun, and all the stars, are in the middle.

(2/04/09)