Amy O'Brien, age 13, of St. Paul, Minn., for her question:
WHAT EXACTLY IS THE MILKY WAY?
It seems the sky above does its best to fool our eyes. For example, the planets look like stars and the moon appears to swell up and shrink. For countless ages, people saw the Milky Way as a pale ghostly arch over the night sky. Then Galileo spied it through a telescope and saw that it was crowded with faraway stars.
It's easier to spot the Milky Way when you are in the country, where the sky is not blurred by haze and city lights. Even the light of the full moon may be bright enough to spoil the view. In winter the pale foggy arch loops through the Big Dipper, Orion the Hunter and Taurus the Bull. The bright stars in these winter constellations stand out against the pale background, which is wide enough for 10 full moons to glide side by side.
In the year 1609, the great Galileo built his small telescope and discovered that the milky glow is actually billion of stars, so far away that their light is blurred by distance. Later astronomers figured out that all these crowded stars belong in an enormous system, which we called the galaxy. And far from the center is our starry sun.
Astronomers estimate that the total number of assorted stars in our Milky Way galaxy is about 100 billion. It is shaped like an enormous wheel and spins around and around. Most of its stars are crowded together in the central hub, and the starry population gets thinner toward the outer rim.
The stupendous size of the Big Wheel is measured in a light‑year, which is the time it takes a beam of light to travel in one earth year. Light travels at 186,000 miles per second, and the distance of one light‑year is 6 trillion miles.
The diameter of our enormous galaxy is estimated to be about 100,000 light‑years. This means that a spaceship traveling nonstop at 186,000 miles per second could cross from side to side in 100,000 earth years. The starry system is fairly flat, and the thickness of the central region is estimated to be about 16,000 light‑years. It turns like a wheel, though in our region it takes about 200 million years to make one complete swing around the Milky Way.
Our galaxy, one might suppose, must include everything in the entire universe. Not at all. It is surrounded by vast reaches of outer space, strewn with thousand:, of other starry galaxies‑‑some older, some younger than ours. Our best telescopes can probe billions of light‑years into space‑‑and there are galaxies all the way to the limits.