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Rare Gems

October 3, 2015

Stars come in many varieties. Some of those varieties are common, while others are quite rare. And at this time of year, you can see two stars of an especially rare type in the same constellation.

Alpha and Beta Aquarii are the brightest stars in Aquarius, the water bearer. Both stars are about 500 light-years from Earth, and both are yellow supergiants. They’re similar in color and temperature to the Sun, but they’re more than 2,000 times brighter. So in one minute, each of these stars casts more light into space than the Sun emits in an entire day.

Yellow supergiants are rare for several reasons. First, they were born much more massive than the Sun, and such heavy stars are rare. Second, massive stars don’t live long — they explode after a relatively short lifetime. And finally, a star doesn’t stay in the yellow supergiant stage for long. Instead, this phase is a brief transition as a massive star changes from a blue supergiant to a red supergiant or vice versa. Alpha and Beta Aquarii were probably born together, and are now going through this same stage of life together.

And with the help of a good star chart, you can see both stars tonight. Aquarius is in the southeast as darkness falls, and stands highest in the sky, due south, around 11 p.m. Because of their great distance, Alpha and Beta Aquarii look fairly faint. Even so, they’re rare stellar gems: yellow supergiants that are nearing the ends of their lives.


Script by Ken Croswell, Copyright 2015

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