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The Sun shines because nuclear reactions in its core convert hydrogen into helium. Several billion years from now, though, the core will run out of hydrogen, so the Sun will begin to burn the hydrogen in a layer around the core. This will cause the Sun to expand, cool, and brighten, until about a billion years later it becomes a red giant.
Tonight, if you know where to look, you can see a star that’s already entered the red-giant stage of life. It’s part of the Great Square of Pegasus, which passes high overhead this evening.
In mythology, Pegasus was the winged horse. In the sky, it’s one of the largest constellations. Its most prominent feature is a giant square made of four fairly bright stars.
The star that marks the northwestern corner of the square is a red giant called Beta Pegasi. It’s close to a hundred times the Sun’s diameter. If it took the Sun’s place in our own solar system, it would extend all the way to the orbit of Venus, the second planet out. And from the surface of the burned-out Earth, it would stretch half way across the sky.
Beta Pegasi shines about 350 times more brightly than the present-day Sun. But because it’s 200 light-years away, it looks like just another star.
Still, Beta Pegasi is one of the brightest red giants in the night sky. If you have a pair of binoculars, you might even be able to glimpse its red color. In that tiny red point of light, you’ll see the future of our own Sun.
Script by Ken Croswell, Copyright 2011