Bears have stubby little tails -- except for the great bear in the sky, the constellation Ursa Major. His tail stretches a long way across the sky, and forms part of the most famous star pattern in the sky -- the handle of the Big Dipper.
The tail ends at a star named Alkaid. It's almost exactly 100 light-years away, so the light we see from Alkaid tonight actually left the star a century ago.
Although many of the stars that are visible to the unaided eye are bigger, heavier, or farther away than Alkaid, few are hotter. Its surface is many thousands of degrees hotter than the Sun's, so it shines white with a hint of pale blue.
Alkaid is so hot because of its heft -- it's about six times as massive as the Sun. As a result, the star burns through the hydrogen fuel in its core much faster than the Sun does. This fast pace produces much more energy than the Sun does, so it heats Alkaid's outer layers to much higher temperatures. But it also means that Alkaid will live only a fraction as long as the Sun.
Because of the extra heat, Alkaid shines brightly not only in wavelengths that are visible to our eyes, but in ultraviolet wavelengths as well. When you add it all up, the star puts out about 700 times as much energy as the Sun, making it easily visible across a hundred light-years of space.
Alkaid is low in the northeast at nightfall, but circles high across the north during the night -- marking the "tail end" of the great celestial bear.
Script by Damond Benningfield, Copyright 2009
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