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November 18, 2012

As befits its name, Cetus, the whale or sea monster, is one of the largest constellations in the sky. It glides across the southern sky on November evenings.

Despite its size, though, Cetus isn’t prominent, because like most autumn constellations, it lacks brilliant stars. Still, Cetus does boast one of the brightest red-giant stars in the heavens.

The star is known as Menkar or Alpha Ceti, and it shines in the whale’s head. A good star map can help you find the star and the rest of the constellation.

Menkar is about 250 light-years from Earth, so the light you see from the star tonight actually left its surface shortly before the signing of the Declaration of Independence.Menkar was once a main-sequence star, so it generated energy as the Sun does — by converting hydrogen into helium in its core. But the star burned up the hydrogen in its core, so it had to burn hydrogen in a thin layer around the core. This caused the star to expand. Since an expanding gas cools, Menkar’s surface temperature dropped. So the star turned yellow, then orange, and then finally red. Today, the star is a red giant — a star that’s so large that if it were to take the Sun’s place in our own solar system, it would engulf the planet Mercury.

And in fact, Menkar gives us a preview of the Sun’s distant future. In several billion years, the Sun’s core will run out of hydrogen, and our star will slowly expand until it becomes a red giant — just like Menkar.


Script by Ken Croswell, Copyright 2012

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