Ronald Baxter, age 12, of Christiana, Penna., for his question:

How big is our Universe?

A few years ago most experts were sure that our Universe, or big cartwheel Galaxy, was the largest of its kind. Some recent rechecking, however, has changed this old idea. We are not told that our Universe is average in size. There are bigger ones and smaller ones.

Universe distance is measured in light years. If you want the dimensions in miles, you simply multiply the number of light years by 6,000,000,000,000, because each light year is equal to about six million, million miles.

The width of our giant cartwheel Galaxy is roughly 100,000 light years. It thickens towards the hub, where the stars are most dense. The central thickness tapers along spiralling arms to the outer edge of sparsely scattered stars. At the control hub of the big wheel our Universe is about 10,000 light years thick, which is one tenth of the vast span from side to side.