Andy Solum, age 8, of Sarasota, Fla., for his question:
HOW LONG IS THE MILKY WAY?
During the winter months the milky Way crosses the sky near the constellations of Orion and Cassiopeia. It's a bit dimmer now than it is on dark, clear summer nights when it can be seen extending from the southern constellation Sagittarius to Cygnus, the great northern cross. The Milky Way has some black spots, caused by clouds of dust called nebula.
Many billions of stars make up the glowing band of starlight in the sky that we call the MIlky Way. They are all part of our galaxy which is shaped like a pancake.
At night we see a milky‑looking strip of stars because we are inside it. The stars fan out from the center in curving, wide arms that give our galaxy a spiral or coil shape. Astronomers call the Milky Way a spiral galaxy.
The galaxy is about 10 times longer than its thickness. It is so long that light, traveling at the rate of 186,282 miles per second, takes about 120,000 years to get from one end to the other. Our solar system, made up of our sun and planets, is just a tiny speck located about 30,000 light‑years from the center of the galaxy, about midway between the upper and lower edges of the galaxy.
Our earth is just a tiny part of the universe. Earth and our known planets circle the sun as part of the solar system. Our sun is actually a star and is just one of billions of stars in the huge galaxy that we call the Milky Way.
Astronomers say that the Milky Way is only one of countless galaxies scattered throughout space. And you'd have to travel several billion light‑years to find the next one.
Much of the Milky Way is made up of clouds that contain dust and gas.
Gravity holds the Milky Way together as all of its stars rotate around its center. Depending on their position in relation to the mass, some stars rotate faster than others.
Our sun, for example, is moving on a circular path at a speed of about 170 miles per second. Yet it takes our sun 200 million years to make its way completely around the center of the Milky Way. Stars closer to the center move faster since they are attracted with greater force, yet those very close to the center move slowly. For the ones near the center. most of the mass lies toward the edges of the Milky Way. A study of this motion has shown that the Milky Way's mass is equal to 200 billion suns.
Astronomers have not been able to study the center of the galaxy since light from this part of the Milky Way cannot reach us through the dust. However, scientists know from studying infrared rays and radio waves that there is a tremendous amount of activity in the center. They also know that gas shoots out from the center at 1,000 miles per second, but as yet they do not know why this happens.