The stars are a looong way off. The closest is more than four light-years away -- about 25 million million miles -- a distance that's tough to even think about.

Even so, scientists have learned quite a bit about these points of light in the night sky -- from how they move to what makes them shine.

An example is the star Rasalhague. It represents the head of Ophiuchus, the serpent bearer. It's in the east at nightfall, and climbs high across the south later on.

Rasalhague is 47 light-years away. Astronomers measured this distance by plotting the star's position against the other stars at different times in Earth's year-long orbit around the Sun. The effect is like holding out your finger and looking at it with one eye, then the other -- the finger appears to move against the background. The size of the shift reveals the finger's distance -- and the same thing works for stars.

We also know that Rasalhague is a double star. By plotting the orbit of the two stars, astronomers determined their masses. One is about three times the mass of the Sun, while the other is only half as massive as the Sun.

The larger star shines pure white, which tells us that its surface is thousands of degrees hotter than the Sun.

And the combination of temperature, mass, brightness, and other factors reveals that the star is later in life than the Sun is.

So we know a great deal about the head of the serpent bearer -- even though it's a looong way off.

Script by Damond Benningfield, Copyright 2008

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