More Moon and Jupiter

StarDate: May 16, 2009

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The last-quarter Moon and the planet Jupiter stage a beautiful encounter in the wee hours of tomorrow morning -- they're just a few degrees apart. Jupiter looks like a brilliant star to the right or lower right of the Moon. They rise around 2 or 3 o'clock, and are well up in the southeast at first light.

Jupiter is the giant of our solar system. It's more massive than all the other planets and moons put together.

Most of the planets discovered in other star systems are a good bit heavier than Jupiter. But most of them may not be that much bigger. That's because a planet with greater mass also has stronger gravity. The gravity pulls its material inward, squeezing it more tightly and limiting its size.

This squeezing process heats up the planet's core. The heat radiates through the planet's outer layers and into space in the form of infrared energy. In fact, Jupiter radiates more energy into space than it gets from the Sun. More-massive planets would produce even more energy.

If an object gets heavy enough, its core is heated to millions of degrees. That's hot enough to ignite nuclear fusion -- a process that combines lightweight atoms to make heavier ones. Such an object isn't a planet -- it's a star. The cutoff is about 80 times the mass of Jupiter. Interestingly enough, though, a star of that mass is about the same size as Jupiter -- a planet that's just about as puffed up as a planet can get.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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