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Summer Triangle

June 25, 2017

The sky offers plenty of calendar markers to help note the turning of the seasons. At the start of summer, for example, the Sun is farthest north in the sky for the entire year. And by the time the last rays of the Sun fade away, another marker is in good view: the Summer Triangle.

The brightest point of the triangle is Vega, in Lyra, the harp. It’s high in the east-northeast as darkness falls. It’s one of the brightest stars in all the night sky, so you just can’t miss it. And it’s a close neighbor, at a distance of just 25 light-years — one of the reasons it looks so bright.

Well to the lower left of Vega, look for the faintest point of the triangle, Deneb. It marks the tail of Cygnus, the swan. Its fainter appearance is deceiving, though. The star is a supergiant, so it’s tens of thousands of times brighter than the Sun. It looks fainter than Vega only because it’s about 1400 light-years farther — one of the most-distant stars that’s easily visible to the eye alone.

Finally, look far to the lower right of Vega and Deneb for the triangle’s southernmost point, Altair, in Aquila, the eagle. It’s the closest member of the triangle, just 17 light-years from Earth. It’s actually the least impressive member of the triangle. Even so, it’s bigger and heavier than the Sun, and a good bit brighter, helping it stand out as a member of the season’s signature star pattern: the Summer Triangle.

Tomorrow: noticing the tug of an unseen planet.


Script by Damond Benningfield

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