Sky watch: Star clusters' fluid movements

Years of living and working on this planet have taught me many lessons, but one seems to be truer than most: Whatever is temporary is permanent, and whatever is permanent is temporary.

It applies everywhere: buildings, governments, relationships . . . things we're convinced are everlasting are almost always destined for change. And those that we're told will be only temporary - taxes, detours, visits by in-laws . . . well, they often turn out to be permanent!

Even the heavens, which we perceive as eternal and unchanging, are only temporary. While we often refer to the stars and constellations as being "fixed" - especially when we watch the moon and planets drift in front of them - they are actually moving in seemingly random directions.

But, since the stars reside trillions of miles away, we're not able to notice their movements without sophisticated instruments, or without waiting inconceivably long periods of time. So it's only natural that we perceive them as fixed.

If we are patient, however, we can see reality.

Take, for example, the Big Dipper. To most of us in the Northern Hemisphere, this great celestial icon is one of first star groupings we learn to recognize.

Every scout has used it to find the North Star and to tell time during the night.Its shape has been represented by various cultures as a bear followed by three warriors, a wagon, a plow and - to the Chinese - as the government itself. Its "drinking gourd" shape even served in song to inspire pre-Civil War slaves to flee northward to freedom.

We've come to recognize the seven stars of its bowl and handle to be permanent. But what we see as the "Big Dipper" is not a solid structure - it's an optical illusion that occurs because of where we live in our galactic neighborhood. If we could travel to the other side of those stars, we would discover that our perception of the Dipper would change.

The "dipper" shape is fluid over time as well. Its five central stars are stellar siblings; they were born from the same interstellar cloud some 500 million years ago and still travel together.

, and how the shape we know as the Big Dipper would change.

A hundred thousand years ago, the first Homo sapiens in the Middle East might have recognized this stellar collection as a celestial fishhook rather than a dipper. Our descendants a hundred millennia in the future will also see the shape of a dipper, but one that is distorted and reversed.

So the next time you head outdoors, remember: Whatever is temporary is permanent, and whatever is permanent is temporary!