Tammy Curtiss, age 12, of Cumberland, Md., for her question:
HOW MANY STARS ARE IN THE MILKY WAY?
Look heavenward on a clear night and you'll be able to see the Milky Way. It is a flattened, lens shaped aggregation of about 200 billion stars. It also has enough interstellar gas and dust to equal the mass of another 20 billion stars.
The Milky Way is classified as a spiral galaxy because it has a dense central region with several spiral arms coiling around it in the same plane. The diameter of the galaxy is about 10 times greater than its thickness.
The galaxy is so big that light, which travels 186,282 miles per second, takes about 100,000 years to travel from one end to the other. Our solar system is a tiny speck located about 30,000 light years from the center of the galaxy.
From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon.
The hazy or milky appearance of the Milky Way results from the combined light of stars too far away to be seen individually by the unaided eye, although many can be observed through a telescope. Visible stars appear distinct and separate because of their relative nearness '' to earth.
The Milky Way contains both the so called type I stars which are brilliant, blue stars, and also the type II stars which are giant red stars. The central Milky Way is composed of the type II population. Most of this region is obscured behind dust clouds which prevent visual observation.
Radiation from the central region has been recorded by use of such special devices as photoelectric cells, infrared filters and radio telescopes.
Surrounding the central region is a fairly flat disk of both type II and type I stars.
The Milky Way rotates around the axis joining the galactic poles, or the points farthest from the central line of the system. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise and the spiral arms trail in the same direction.
The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system, the period of rotation is more than 200 million years.
The speed of the solar system due to the galactic rotation is about 170 miles per second. The galactic equator, or galactic circle, passes midway between the galactic poles.
Gravity holds the Milky Way together and all of its stars rotate around the center. However, not ail stars rotate with the same speed. Their speed depends on their position in relation to the mass in the galaxy. Stars such as our sun, which are far from the center, rotate around the Milky Way's center much as the planets rotate around the sun.