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There’s more than one way to gauge the brightness of a star. And the difference reveals important details about the star itself.

An example is Alpheratz — a binary star system that’s almost a hundred light-years away. Officially, it’s one of the brighter stars of the constellation Andromeda. But it’s also the brightest corner of the Great Square of Pegasus. Right now, it’s at the top of the square, which is low in the west and northwest at nightfall. It’s to the right of the brilliant planets Venus and Jupiter, so it’s easy to find.

One way to measure a star’s brightness is by its absolute magnitude. That tells us how bright a star would look to the eye from a distance of 10 parsecs — about 33 light-years. And on that scale, the brighter star of Alpheratz — star A — would look almost exactly 100 times brighter than the Sun.

But that’s only part of the story. On a scale known as luminosity, Alpheratz is 240 times the Sun’s brightness.

The difference is the result of surface temperature. Alpheratz A is many thousands of degrees hotter than the Sun. At that temperature, it shines a lot bluer than the Sun, which is yellow. More important, hot stars produce a lot more ultraviolet light than the Sun does. So when you add up all wavelengths, Alpheratz A shines even brighter — a brilliant corner of the Great Square of Pegasus.

Tomorrow: Even hotter stars whip up something heavy.
 

Script by Damond Benningfield

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