The Barren North

The brilliant stars of winter are even now beginning tom appear in the southeast. The autumn square is high in the southern sky. The summer triangle is setting in the southwest. The sun is lowering in the southern sky. It seems that everything is happening in the south. Only relatively few stars shine in the north, and none too brilliantly. Why?

Throughout space, in all directions, are vast clouds of gas, called nebulae. They are the stuff out of which stars are made. But having the material out of which stars are made is not the same as having stars. There has to be some "trigger" to begin compressing these clouds. Then gravity can take over, further compressing and heating the clouds until stars are formed. And the primary trigger is pressure from the slowly turning arms of the Milky Way.

The Milky Way galaxy is a vast celestial pinwheel of over a hundred billion stars, slowly turning in space. The plane of this galaxy, the stars we call the Milky Way, passes through Orion, Perseus, and Cassiopeia -- all in the southern sky. And that's where most stars are.

When we look up at the Big Dipper, we are looking directly out of the plane of the galaxy. Those stars have been formed by other means. Their initial compression was caused by nearby exploding stars, and stars like that are rare. That's why we see so few stars in that direction, and since bright stars die out really quickly, there are even fewer bright ones in the north.

What we see in the difference between the barren north and the rich south is the slow turning of the galaxy itself.

(10/25/06)