Marie Sharpe, age 13, of Nashville, Tenn., for her question:

What shape is our Galaxy?

We can look up and see the Galaxy, but the view does not tell us much about it. All we see is a wobbly band of pale light arching the heavens of the Milky Way. It does not appear at the same time nor in the same place, for our view of the heavens is constantly changing. This is because our earth rotates on its axis and revolves in orbit around the sun.

One of the best times to observe the Milky Way is after sunset in late summer. We must be far from city lights, for they can out glare the pale Milky Way. There it is, a pale, ragged hoop arching over the sky from the northeast to the southwest from horizon to horizon. It does not tell us much about the shape of our vast home in the heavens. But it is all that our eyes can see, all anyone saw until the telescope was invented.

The first to turn a telescope in the Milky Way was Galileo, This was in the winter of 1609. The telescope was not very powerful, but it showed Galileo that the pale light came from countless faraway stars. They are so far away that their dazzling light blurs before it reaches our eyes. The whole Galaxy is, in fact,, a vast system of perhaps 100 billion stars. The most amazing fact is that these dazzling suns can be compared to specks in a cloud of dust, for they are separated from each other by vast oceans of space. Also part of the Galaxy are huge clouds of gases, some dark, some ablaze with borrowed starlight. Some of these clouds get in the way of our view of the Milky Way.

Actually, the Galaxy is not shaped like a hoop, as our view of the Milky Way suggests. It is rather like a lens, a flat disk thicker in the center. Our sun is more than half way from the center and our view depends upon our position in the Galaxy. When we look up at the Milky Way, we are gazing out across the flat, lens shaped system of stars. The host of stars looks blurred and crowded because we are seeing them, as it were, on a flat table.

We are at eye level with the edge of the table and the stars appear one behind another, all blurred together.

The size of our enormous home in the heavens is measured in light years. A light year is the distance light travels in one year, which is almost six million, million miles. From side to side, the Galaxy measures about 80,000 light years and the thickness at the center is about 8,000 light years.

Nothing in the heavens stands still. The planets revolve and even the sun rotates on its axis. The Galaxy also takes part in the huge heavenly hoedown. The vast system of stars is spinning around like a cartwheel. Our Solar System is whirling around with the Big Wheel at about 600,000 miles an hour.

From the center, where the stars are thickest, the Galaxy swirls around in two spiraling arms. Our sun is in one of these star streamers, far from the center of the Big Wheel.