John Perkins, age 10, of Shreveport, Louisana, for his question:
What is the actual share of the Milky Way?
Our caveman ancestors gazed up in wonder at the Milky Way in the starry skies. Nobody knew what it was and for thousands of years people took it at face value. They thought that it was some kind of pearly white mist looped in a great arch high above the earth.
The American Indians thought that the Milky Way was a stairway to take their heroes to heaven. In the Old World, early astronomers had charted its ragged streamers among the fixed stars. Some of them knew that the pearly haze circled through the skies, clear around the whole world. But no one guessed what it really was until the great Galileo spied it through his little home made telescope. This was in the winter of 1609. Galileo was able to tell the world that the Milky Way actually is made of teaming, swarming stars. They are so crowded and so far away that their starry light blends together in a hazy blur.
Our information rested there until late in the 18th century. Then William Herschel undertook the stupendous job of charting the details of the celestial scenery, bit by patient bit. His system of stellar astronomy led to many discoveries. One of the most important of these was a true picture of the Milky Way. By measuring and fitting together jigsaw pieces of sky, he was able to show that the enormous star system is shaped somewhat like a cosmic cartwheel, with a thick hub and spiraling spokes that become thinner towards the outer rim.
Astronomers have added a great deal to the discoveries of Galileo and Herschel, but their basic findings have been proved correct. We now know that the width of the big pinwheel spans a distance of roughly 100,000 light years. The thickness of the central hub is about one tenth this distance. We know that our visible stars are all part of the enormous wheel and our own Solar System is but a small item in this cosmic system.
The fact that we view the Milky Way Galaxy from inside makes it hard to estimate its shape and its scope. We have an edgewise view of the starry wheel, facing out across and through its flattened surface. What's more, parts of its crowded center are hidden from us by cosmic clouds of dusty gases. The hub lies somewhere behind Sagittarius where cloudy blotches hide the brilliance of the crowded stars. Our sun lies on one of the spiraling arms, roughly two thirds of the way from the center. Astronomers used the motions of local stars to figure the wheeling motion of the entire Galaxy. This enabled them to estimate the center and the total mass of the Milky Way. From this they figured that we share our stupendous home in the heavens with about 100 billion other starry suns.
The hub of the big wheel turns more rapidly than its spiraling arms. The stars in our neighborhood revolve at speeds around 100 miles per second. Our sun, together with its family of planets, takes roughly 200 million years to complete each turn around the Milky Way. This time period is called the "cosmic year." Our neighborhood stars ¬revolve faster or slower on inner or outer traffic lanes. As we all wheel around together, other stars gradually change their positions in relation to the earth.