Lacey Heilman, age 11, of North Providence, R.I., for her question:

WHAT EXACTLY IS A GALAXY?

A galaxy is a system of stars, dust and gas held together by gravity. You'll find galaxies scattered throughout the universe.

Galaxies can range in diameter from a few thousand light years to half a million light years. A light year is the distance that light travels in a year    about 5.88 trillion miles.

Small galaxies have fewer than a billion stars in them while large ones have more than a trillion stars.

Astronomers aren't sure exactly how many different galaxies can be found in the universe. But they have photographed millions of them and they believe there are billions.

Our solar system of planets, moons and the sun are located near the edge of a galaxy that we call the Milky Way. Our Milky Way galaxy is so big that it takes light about 100,000 years to travel from one end to the other.

Our solar system, astronomers say, is like a tiny speck located about 30,000 light years away from the center of the galaxy.

Only three of the billion galaxies outside the Milky Way can be seen from the Earth without a telescope. In the Northern Hemisphere, people can see the Andromeda Nebula, a galaxy more than 2 millon light years away. And people in the Southern Hemisphere can see two other galaxies: the Small and Large Magellanic Clouds, located about 200,000 light years away.

Scientists classify galaxies according to appearance and shape. There are two main types: spiral galaxies and elliptical galaxies.

A spiral galaxy is shaped like a disk with a bulge in the center. The disk looks like a pinwheel with bright spiral arms that coil out from the central bulge. The Milky Way is a spiral galaxy.

Elliptical galaxies range in shape from round to flattened globes.

All galaxies rotate, but ellipticals rotate more slowly than spirals.

Astronomers say that almost all galaxies are moving away from each other at tremendous speeds. In addition, the galaxies farthest from the Earth appear to travel the fastest. Therefore, most scientists believe the universe is expanding constantly.

There are a number of theories on the origin of galaxies.

According to one theory, masses of gas formed soon after the universe began to expand billions of years ago. Gravity slowly condensed these masses into galaxies. No new galaxies have been formed since then, and so all galaxies are about the same age.

Another theory states that new galaxies constantly appear as old ones move apart.

Galaxies give off many kinds of radiation, including radio waves, ultraviolet light, visible light and X rays.

Astronomers estimate the distance and motion of a galaxy by measuring its red shift. Red shift is the change in the wave length of light coming from an object moving away from the Earth in space.