The lonely star of the south skies

SkyCaramba weekly astronomy blog for the week ending November 12, 2011

For northern hemisphere sky watchers, there’s a bright star that seems to be all by itself in the southern part of the evening sky this time of year. Its Arabic name means mouth of the fish. Fomalhaut is the brightest star in the Southern Fish, or Piscis Austrinus. 

Fomalhaut looks lonely because the stars around it are much dimmer. Sometimes, it’s even called Lonely One. 

At 30 degrees south of the celestial equator, Fomalhaut passes high overhead from southern locations like Chile, South Africa, and New Zealand. It spends more time above the horizon there. For people watching from the Unites States, Europe, and Asia, however, it may be up just a few hours. 

Fomalhaut is bright, estimated at 16 times brighter than the sun viewed at the same distance. At 25 light years away, it’s one of the brightest stars in the sky at first magnitude. It’s also fairly young as stars go, maybe just 100 million years old. 

If we could see the star in infrared, it may be even brighter. Astronomers know that when stars emit lots of infrared radiation, they usually have disks of material around them. Advanced imaging techniques have revealed a disk orbiting around Fomalhaut’s equatorial plane, somewhat like the rings orbiting around Saturn’s equator. The debris disk is a possible spawning ground for planets. 

There is at least one planet orbiting Fomalhaut and it’s been imaged in visible light. It’s been named Fomalhaut b. We don’t know yet, but there may be more planets going around the star. 

Many stars are double stars. Fomalhaut isn’t counted as a double star because no other stars are known to be orbiting it. However, there is another star near it moving in the same direction and at the same speed. 

A consequence of watching a star that stays low on the horizon, many in the northern hemisphere think Fomalhaut sparkles and changes colors. It could even appear to move. Depending on how close you are to an airline flight path, you may need to watch a few minutes to make sure you’re not actually seeing an airplane. 

One way to find Fomalhaut is the spot the Great Square of Pegasus and look south of there. Another is to know when it will cross directly over the meridian, the north-south line in the sky. It does that around 9pm local time in early November. 

The constellation Fomalhaut is part of doesn’t really look like a fish. The fish reference may be because, whatever it is, it’s in a stream of water being poured by nearby Aquarius. 

In our sky, Fomalhaut, Arcturus, and Capella are almost as far apart as they can get for stars that can all be up at the same time. But these three are actually within 65 lights years of each other. Fomalhaut is almost exactly the same distance from each of the other two. 

Links to more sites about Fomalhaut: