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Summer Triangle

July 8, 2012

As the color of twilight drains from the eastern sky this evening, a pyramid of bright stars fades into view: the Summer Triangle. It’s visible from just about anywhere — even light-polluted cities.

The apex of the pyramid is marked by Vega, the leading light of Lyra, the harp. It’s about half-way up the eastern sky. It’s one of the brightest stars in all the night sky, so it’s hard to miss.

Vega is classified as a “main-sequence” star — a star that’s in the prime of life. It’s steadily “burning” the hydrogen in its core to make helium. It’s a process that will continue for hundreds of millions of years. More about Vega tomorrow.

The star that marks the lower right corner of the triangle is also on the main sequence: Altair, in Aquila, the eagle. It’s not quite as massive as Vega, though, so it doesn’t shine as brightly.

The star at the opposite corner of the triangle has completed its time on the main sequence and has moved into the next phase of life — a supergiant. Deneb is no longer fusing hydrogen to make helium, but may be about ready to fuse the helium to make even heavier elements.

The changes in its core have caused its outer layers to puff up, making the star about a hundred times wider than the Sun. And it’s also tens of thousands of times brighter than the Sun, so it’s easily visible even though it’s close to 1500 light-years away — about 60 times farther than Vega, and close to a hundred times farther than Altair.


Script by Damond Benningfield, Copyright 2012


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