Charles Y. Robertson, age 11, of Atlanta, Georgia, for his question:
Is it true that galaxies rotate?
Our entire Universe spins around like a dizzy heavenly hoedown. The earth rotates on its axis and so does the moon. So do the other moons, planets, and even the gaseous sun. Rotation is the most fashionable celestial motion and very complicated. Our sun and its orderly planets travel around an immense circle through the Milky Way. With 100 billion other starry suns, we orbit around with the rotating Galaxy. Apparently all galaxies must rotate to offset the pull of gravity, otherwise they would collapse.
The Milky Way contains enough material to make perhaps 200 billion average stars like our sun. About half of its mass is in the form of vast, gaseous cosmic clouds. But all of its entire mass is governed by the force of gravitation, exerting its enormous pull from the crowded center. If gravity had its way, it would pull all the material in our stars, planets and gaseous clouds into a dense ball. But the spinning motion of rotation works against the force of gravity.
Our Galaxy rotates somewhat like a celestial sized pinwheel, streaming its billions of stars and dusty clouds around as it goes. It is typical of countless galaxies strewn through the vast reaches of outer space. They must rotate or collapse. However, some of these outer galactic star systems appear to be at different stages of development. The ages of their stars and other factors suggest that older galaxies tend to become flattened ellipses. Our Galaxy is one of the mature types, with a thick crowded center and long spiralling arms thinning out toward the rim of the disk.
Astronomers tabulated the motions of our neighborhood stars to trace the moving picture of our rotating Milky Way. Some of our neighbors are coming closer to us and others are receding. Together they reveal a system of celestial traffic, with stars near the center orbiting faster than those in the outer lanes. Those in the inside lanes catch up and move ahead of us. Those in outer lanes, nearer the rim, lag behind as we catch up and pass them. Hence, the various stars in our local traffic are either approaching closer to us or receding.
The diameter of our spinning Galaxy is estimated to be about 100,000 light years. Our spinning Solar System is located about 30,000 light years from the hub, more than half way to the outer rim. In our lane, the starry traffic moves at about 130 miles per second, which is almost 470,000 miles per hour. Yet even at this fantastic speed, our Solar System takes about 200 million years to complete one orbit around with the rotating Galaxy.
Other galaxies rotate more or less like our own enormous pinwheel. It rotates as a unit, though individual stars orbit around at different speeds. Their orbital speeds are radial velocities and each orbital swing is a cosmic year. Our sun has completed more than 20 cosmic years since its birth, spade flue billion years ago.