Late September skies are still rich with summer stars. Just after sunset, the northern Milky Way stretches across the top of the sky, with Deneb, Vega, and Altair huddled around the zenith, and Sagittarius crossing the meridian. In another month, however, these stars will slide off toward the west, and the autumnal constellations will begin taking their place, although without much fanfare from bright stars.
There is, however, one notable exception. Low in the southeast around 9 o'clock is a lone white beacon, a member of the otherwise dim constellation Piscis Austrinus the southern fish. The star's name is Fomalhaut, from the Arabic Fum al Hut, the fish's mouth. The appearance of the star this week signals the beginning of the end of summer.
Fomalhaut is one of the most southerly of all the prominent stars visible from latitude 40°N. In southern locales such as Chile or southern Australia, however, the star is nearly overhead this week by 11 o'clock. Because of the absence of nearby bright stars in this region of sky, Fomalhaut was an important star for ocean-going navigators.
To find Fomalhaut, you'll need a horizon with a clear, unobstructed view to the south and southeast. If your skies are hazy, use binoculars to help you penetrate the muck. Look just above treetop height for a solitary star, twinkling madly in the atmosphere. Its distance is only 22 light-years, meaning that the light you see from that star tonight left it 22 years ago.
Fomalhaut rises above the southeastern horizon. 9 p.m. September 15.
Ostracized as it appears, Fomalhaut is nonetheless an important star. Radio images of the star show a doughnut-like structure of dust girdling this neighboring sun. The disk, which is roughly the size of our solar system, is cleared out in the middle, nearest the star, leading some astronomers to conclude that rocky planets may have already formed there. A similar dusty disk has also been observed orbiting Vega in Lyra (June 7 - 13) as well as stars in the southern constellations of Centaurus and Pictor. If these disks are indeed nascent solar systems, planets in our galaxy may be more common than previously believed. Unfortunately, like Vega, Fomalhaut is a hot star with an inherently short lifespan and it is likely it will die long before planets have formed.
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