Straight ‘A’s

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Straight ‘A’s

Five of the stars that make up the Big Dipper are like a great report card: straight “A”s. All five stars are members of class A — stars that are bigger, hotter, and brighter than the Sun.

The dipper is in the northwest at nightfall, with its bowl below the handle. The stars at the tip of the handle and the lip of the bowl are outliers — they line up in the same direction as the rest of the dipper, but otherwise they’re unrelated.

But the five stars in the middle have a lot in common. They were born together, from the same giant cloud of gas and dust. Their nursery has long since broken up, but they still hang out together — they move through the galaxy at about the same speed, and in the same direction. They’re all roughly 80 light-years from Earth. And all five stars are class A. That puts them in the top one percent or so of all the stars in the galaxy.

The stars range from about one and a half to three times the mass of the Sun, and from about 15 to a hundred times the Sun’s brightness. And as a reflection of their common birth, they’re all just a fraction of the Sun’s age — a few hundred million years old.

From the second star in the handle to the lower corner of the bowl, the class-A stars are Mizar, Alioth, Megrez, Phecda, and Merak. Megrez, which connects the handle to the bowl, is by far the smallest and faintest. But it’s still impressive — one of the “straight-A” stars of the Big Dipper.

Script by Damond Benningfield

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