The Keystone

You can never judge a star by its appearance alone. The four stars at the center of Hercules, for example, all look pretty much alike. But each is different from all the others.

The stars form a sort of lopsided square called the Keystone. It’s in the east-northeast at nightfall.

Its brightest star, at the upper right, is called Zeta Herculis. Moving clockwise, the other three are Epsilon, Pi, and Eta Herculis.

Zeta is the closest of the four, just 35 light-years away. It actually consists of two stars. One of them is bigger, heavier, and brighter than the Sun, while the other is smaller, lighter, and fainter. The larger one is nearing the end of its life, so it’s undergoing some changes. They’ve caused it to puff up and shine even brighter.

Epsilon probably also is a binary. And it may even have three stars. Some studies indicate the presence of a third star, but others show only two. Like the Sun, the system’s main star is in the stellar equivalent of middle age.

Pi is the most impressive member of the Keystone. It’s about 375 light-years away — more than 10 times farther than Zeta. It’s so large that it would just about fill the orbit of Mercury, the closest planet to the Sun. And even though it’s just a small fraction of the Sun’s age, it won’t live much longer.

And finally, Eta is also more impressive than the Sun. And it, too, is nearing the end of its life — a stage the Sun won’t enter for billions of years.

Script by Damond Benningfield