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Autumn Star

September 24, 2016

The best sign in the night sky that fall has arrived is the appearance of the “autumn star.” It’s the only bright star that puts in its best showing during the nights of autumn: Fomalhaut, the leading light of Piscis Austrinus, the southern fish.

Fomalhaut is only about 450 million years old — just one-tenth the age of the Sun. It’s also a good bit bigger and heavier than the Sun, which makes it hotter and brighter — one reason that Fomalhaut shines so brightly in our sky. The other reason is that it’s just 25 light-years away, which makes it a close neighbor.

The most interesting thing about Fomalhaut isn’t the star itself, though. Instead, it’s a wide disk of dust that encircles it. The disk seems to be bounded by a giant planet that’s about 15 billion miles from Fomalhaut — more than a hundred times farther than Earth is from the Sun. Hubble Space Telescope has snapped pictures of the possible planet, although it’s so small and far away that it’s nothing more than a tiny dot.

Its orbit is stretched out, so the planet’s distance from Fomalhaut varies by almost 300 billion miles. That odd orbit may suggest that another planet orbits much closer to Fomalhaut, deep within the dust that surrounds the “autumn star.”

Look for Fomalhaut low in the southeast not long after night falls, and due south around midnight. There are no other bright stars or planets anywhere near it, so you can’t miss the signature star of autumn nights.


Script by Damond Benningfield

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