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Bigger and Hotter

January 11, 2016

Some stars are much more powerful than others, emitting far more light. How much light a star emits depends on just two things: its diameter and its surface temperature.

Every star shines because it’s hot, and a simple rule says that the hotter it is, the more energy every square inch of its surface gives off. The rule is named for Josef Stefan and Ludwig Boltzmann, two physicists in Austria who discovered this relationship in the late 1800s. For a star that’s twice as hot as the Sun, every square inch of its surface emits 16 times more light than the same-sized patch of the Sun.

And the larger the star, the more square inches of surface, so the more total light it emits into space. A star that’s the same temperature as the Sun but twice its diameter, for example, emits four times more light.

To see the Stefan-Boltzmann law in action, consider Sirius, the brightest star in the night sky. It climbs into view in the southeast in early evening, and arcs across the south later on. Sirius is both hotter and bigger than the Sun. So every square inch of its surface emits more light than the Sun does, and there’s a lot more surface to radiate light into space. When you add it all up, Sirius emits about two dozen times more light than the Sun does.

And Sirius is a close neighbor — just eight-and-a-half light-years away — enhancing our view of this truly bright star. We’ll have more about Sirius and its constellation — the big dog — tomorrow.


Script by Ken Croswell, Copyright 2015

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