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Rasalgethi

StarDate: 
May 7, 2010

Every time you look at a star, your eye is the final destination for a long celestial journey -- a journey that's lasted for years, and covered trillions of miles.

Consider, for example, a star known as Rasalgethi, one of the brightest stars of Hercules. It rises in early evening, and soars high across the sky during the night.

Rasalgethi is about 380 light-years away. In other words, the light you see from the star tonight has been traveling across the galaxy since about the time the first Pilgrims landed at Plymouth Rock.

As the light traveled across the galaxy, it passed through big clouds of gas and dust. These clouds absorbed and scattered some of the starlight, making the star look a bit redder than it actually is -- just as Earth's atmosphere makes the rising or setting Sun look redder than it really is.

Rasalgethi is pretty red to begin with. It's a red supergiant -- a star that's far larger than the Sun. Its surface is far cooler than the Sun's, too, which makes it look orange or red.

The star is so faint as seen from Earth that the color doesn't register with the unaided eye. But astronomical instruments measure its color. They also split the light into its individual wavelengths, allowing astronomers to learn about the star's composition, its motion through space, and much more.

So as you gaze at the star, you're marking the end of the journey for a tiny bit of its light -- a journey that's lasted for almost four centuries.

Script by Damond Benningfield, Copyright 2010

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