5/11/08 – 5/17/08
by C. Zaitz
We tend to think of the night sky as having an infinite number of stars, but in all reality we can only see one or two hundred in our local sky. This fact seems to contradict poems and prose that refer to the sky full of "countless stars." So where are all the stars?
From earth, we see the individual stars that make up the familiar constellations like Orion and Leo. Stars like Betelgeuse and Sirius are either fairly close, or really big, or both. They appear much brighter than the rest of the stars in the sky. They are so noticeable that they were granted proper names, rather than just catalogue numbers like the rest of the visible stars. But there are only a few hundred bright stars with proper names. The rest are either just too far away, too faint and unnoticeable. We see them as the blurry path in the sky called the Milky Way.
When we look up at night, nearly everything we can see is part of our galaxy. The Milky Way is home to several hundred billion stars in different stages of life and death. We only see several hundred of them when we look up at night due to light pollution from sources like civilization and the moon. On a dark, clear night away from a metropolitan area, maybe thousands of stars are visible. But where are the rest of them? Where are the other billions?
Our galaxy is a flat disk of stars with a bulge in the middle, like a flying saucer made out of sand. Each “sand grain” is a star like our sun, but there are other things in the galaxy, like clouds of gas and dust called nebulae. These clouds might be stars waiting to form, or stars that have exploded. Often when these giant clouds form stars, they form in big groups called clusters. When you put honey into a bowl of granola, the granola clumps around the honey. Imagine that gravity is the honey. Gravity causes the gas to clump and out of the clumps are born stars. Most stars that you see in the sky formed in groups, but over time they scatter. Our own sun seems to stand alone, but most likely formed in a group of stars that long ago scattered. Most of the stars in the Milky Way are too far away for us to see individually now. They are scattered throughout the galaxy, which is over 100,000 light years end to end.
Other clouds are from stars that have exploded, flinging their gas and dust back whence they came; cold, empty space. Dark clouds or nebulae block light from stars beyond them. As we look along the galaxy, along the flat disk of milky faraway stars, we can detect these dark clouds. They look like dark smoke hiding the bright stars behind them. The best time to see the dark clouds and stars of our galaxy is in the summer time. The Milky Way looks like a swath of milky light stretching from north to south overhead, but only from places where there are very few lights, or on an evening with no moon. Summer time is a good time because we often get to leave our cities and find places with fewer lights, lower populations, and a much better view of our home galaxy.
You may not see the billions of stars, but you'll get enough starlight in your eyes to appreciate our tiny place in the vast galaxy, the Milky Way.
Until next week, my friends, enjoy the view.