More Moon and Jupiter

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The two most prominent objects in the night sky right now snuggle close together tonight -- the Moon and Jupiter. They're low in the south at nightfall. Jupiter looks like a dazzling star a little above the Moon. It's not a star, though. Instead, like our own Earth, it's a planet -- an object that orbits a star.

As planets go, Jupiter's a doozy -- the biggest in the solar system. In fact, it's actually about the same size as many of the stars -- those classified as red dwarfs.

What sets Jupiter apart from those stars, though, is mass. Even the lightest red dwarf is close to a hundred times more massive than Jupiter. The star's gravity squeezes it tightly, though, so it's about the same size as Jupiter, but a good bit denser.

A red dwarf is also a good bit hotter, and that's the key difference. Temperatures in the cores of red-dwarf stars reach millions of degrees -- hot enough to ignite nuclear fusion. Hydrogen atoms ram together to form heavier atoms, producing a lot of energy in the process. This energy makes its way to the surface -- and the star beams its light into the universe.

The core of Jupiter is hot, too. But the planet just isn't heavy enough to generate the extreme temperatures needed to shine as a star. Instead, Jupiter shines by reflecting the light of the Sun. So it's plenty bright in our night sky -- but a mere flicker compared to a true star -- even a red dwarf.

We'll talk about another planet tomorrow.

Script by Damond Benningfield, Copyright 2008

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