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March 23, 2015

Hydra, the water snake, is the longest of the 88 modern-day constellations. It’s so long that it takes many hours for the entire thing to climb above the horizon. Yet Hydra is also one of the fainter constellations. In fact, from most cities, only one of its stars is easy to spot.

Alphard is about a third of the way up the southeastern sky as night falls. There are no other bright stars anywhere near it, so you shouldn’t have much trouble picking it out.

Alphard is about 175 light-years away. At that great distance, the fact that we can see it so clearly tells us that the star is pretty remarkable. In fact, it’s a giant — it’s puffed up to about 40 or 50 times the diameter of the Sun.

The star is about 400 million years old — roughly one-tenth the age of the Sun. Yet it’s also about three times as massive as the Sun. Heavier stars “burn” through their nuclear fuel more quickly than less-massive stars, so they age more quickly.

Alphard is already entering the final stages of life. As it uses up the fuel in its core, its outer layers puff outward. They get cooler as they do so, which gives Alphard a reddish-orange color.

In a fairly short time — astronomically speaking — those outer layers will stream away into space. That will leave only the star’s hot but dead core — a tiny remnant known as a white dwarf. It’ll be far too faint to see with the eye alone — depriving the water snake of its one bright light.


Script by Damond Benningfield, Copyright 2015

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