Delta Capricorni

StarDate: December 7, 2009

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The planet Jupiter dominates the western evening sky this month. It's at least twice as bright as any other planet or star in the evening sky, so it's hard to miss.

Jupiter is passing by a true star that's not hard to miss. Delta Capricorni is the brightest star of Capricornus, the sea goat. It's only one percent as bright as Jupiter, but it's a star that has a lot going on.

Delta Capricorni consists of at least four stars. Two of them form a tight pairing -- they orbit each other in a little over a day. They cross in front of each other as seen from Earth, causing the system's brightness to drop a bit.

The brighter of the two stars is several times brighter than the Sun. It's also several thousand degrees hotter, so it looks pure white. Such hot stars also produce a lot of ultraviolet energy -- the kind of energy that produces sunburn.

The star is probably nearing the end of its "normal" lifetime, when it fuses together hydrogen atoms in its core to make helium. The transition to the next phase of life appears to be making the star unstable, so it may pulse in and out like a beating heart.

The system's fainter star is smaller and cooler than its companion, but other than that, it's pretty much a mystery.

Delta Capricorni is in the south at nightfall, just to the left of Jupiter. Jupiter will move closer to the star over the next few nights, then quickly leave it behind as it treks eastward -- toward other intriguing stars.

Script by Damond Benningfield, Copyright 2009

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