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Our home planet resides in that just-right region known as the habitable zone -- the distance from the Sun where the temperature is just right for liquid water -- a critical ingredient for life.
Every star has its own habitable zone, but its distance from the star varies by quite a bit. For one of the bright stars of Orion, for example, it's billions of miles.
Rigel marks the hunter's foot. The bright star is to the lower right of Orion's Belt early this evening.
Rigel is a blue supergiant. It's much bigger and heavier than the Sun, and thousands of degrees hotter, so it shines blue-white. That high temperature means that Rigel also pumps out a lot of ultraviolet energy, which produces sunburn and other problems.
When you add up the ultraviolet, visible light, and other wavelengths, Rigel shines tens of thousands of times brighter than the Sun. In fact, depending on Rigel's exact distance, it could be up to a hundred thousand times brighter than the Sun.
With so much energy streaming its way, a planet would need to be billions of miles out to be a safe abode for life. And it would need a thick ozone layer to screen out the ultraviolet.
And even then, such a planet wouldn't be a good long-term home. In a few million years, Rigel is likely to blast itself to bits as a supernova. The energy and shock wave would make quick work of life on any world around it -- making the habitable zone uninhabitable.
More about Orion tomorrow.
Script by Damond Benningfield, Copyright 2009