How far away are the stars?

It takes a scientific way of thinking to appreciate the immensity of outer space.

The sun - the closest star to Earth - is beyond any distance we'd even think of traveling. It is 93 million miles away. Light, which goes 186,000 miles a second, takes eight minutes to leave the sun and arrive on Earth.

If you were riding in a car going 65 miles per hour, you would have to travel nonstop 24 hours a day for almost 200 years to reach the sun. (And that's only one way.)

The next-closest star to our sun is called Proxima Centauri. It is about 25.2 trillion (a 25 with 12 zeroes after it) miles away. (And you thought walking to school was a trip.)

Everyday measurements don't work very well when we want to talk about how far away the stars are. So astronomers have come up with other ways to express interstellar distances.

Astronomers combine units of distance and time into a term called the light-year. A light-year is how far light travels in one Earth-year through the vacuum of space. One light-year equals about 6 trillion miles.

Dividing the distance to Proxima Centauri in miles by 6 trillion, you get 4.2. So we say that Proxima Centauri is 4.2 light-years from Earth.

Put another way, light traveling at 186,000 miles per second takes more than four years to go from Proxima Centauri to Earth. And remember, that's just the second-closest star to our planet. Billions and billions of stars are much farther away.

Another useful term astronomers use is the 'astronomical unit,' or AU. One AU is the distance between the sun and the earth (93 million miles, remember?).

Think of it: There are more stars in the sky than grains of sand on a beach. The stars are thousands or millions of astronomical units from Earth. It's enough to make you want to just flop down on your back on a clear night and look in awe at all the twinkling lights above.

How the stars became constellations

Over the centuries, people in various cultures created a map of the night sky. They did so by giving names to groups of stars that formed imaginary shapes or people, animals, and objects. We call these groups of stars "constellations" today, after a Latin word meaning "set with stars."

All the stars inside the boundaries of a particular constellation are said to be "in" that constellation, no matter that some of the stars are much farther away than others. [See illustration at the top of page 18.]

Humans have drawn boundaries around groups of stars in the sky the same way they have drawn imaginary boundaries around the nations of Earth. One key difference: Countries don't move. Stars do. They seem to revolve in immense orbits, in the same way Earth orbits the sun.

In fact, the reason constellations seem to orbit is that the earth orbits the sun. Where you live, the time of day, and where the earth is in its solar orbit determine what you see in the sky.

In the Northern and Southern Hemispheres, there are 88 named constellations, combined. Each has its own space. No constellation overlaps another.

With some constellations, their shape is obvious - Orion the hunter, for example. Others are not so clear. Cetus, for example. Cetus was seen as a sea monster in ancient times, threatening the maiden (constellation) Andromeda. Today, Cetus is more commonly pictured as a whale.

The companion book to the "Starry Night" software program I have (see "For more info" on page 18) gives a more precise definition of "constellation": It's "a wedge of the universe with Earth at its center that extends to the farthest reaches of space."

Constellations, like countries, were named by humans. Some have been named since the earliest of times by Egyptians, Babylonians, and Greek stargazers. Others are more recent, named after the invention of the telescope in the 1400s.

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