Do the stars have orbits ?

The stars in their fixed patterns arch over the sky every right  or so it seems. Through the year, an orderly parade of constellations passes overhead with the changing seasons   or so it seems. These apparent motions, however, are caused by the earth turning on its axis and orbiting the sun. But the stars really do move in the heavens, though you would have to live a thousand lifetimes to notice any changes in their positions.

The Big Dipper is a constellation of so called fixed stars. Some of its distant stars are nearer to us than others and if we saw them from, say Sirius the Dog Star they would form a different pattern. The group of stars in a constellation are not necessarily neighbors of each other and, what’s more, they may be moving in different directions..

The two pointer stars of the Big Dipper, for example, are movingg in opposite directions. Three stars in the handle are moving in the opposite direction from the star at the end of the handle. In tide year 100, 000 A. D, our Big Dipper will be turned upside down. The handle will be the two stars which now point to Polaris and there will be three stars instead of two on the bottom of the bowl. By that remote date, all our present constellations will have changed beyond recognition.

This is because the stars in our sky are part of vast pinwheel system called the Galaxy. The pinwheel rotates and the individual stars rotate with it. The stars near the center of the Galaxy rotate "aster than those near the rim. Every star, we think, is orbiting :round the center of the Galaxy along its own path and at its own speed. Stars near the

Stars near the center of the Galaxy crawl around their small orbits. At 3,200 light years from the center, the orbital speed of the stars is about 10 miles per second and with every additional 3,200 light years from the center, about 10 miles per second is added to the orbital speed.

The orbital speed of our sun through the Galaxy is 170 miles per second, which means that in two thousand million years it makes one complete trip around the Milky Way. This period of time is called the Cosmic Year. Stars nearer the rim of the Galaxy orbit faster than the sun but, it is thought, none of them orbit faster than 200 miles per second. This is the escape velocity, the speed needed for a star to escape the gravitational pull of the Galaxy which keeps the whirling pinwheel in motion.

The stars in our sky are widely separated and situated at different points in the rotating Galaxy. Though they travel at fantastic speeds, they change their positions very slowly, even when two which seem to be neighbors are traveling in different directions.