November 18, 2007


To finish up the Summer Triangle, today I will talk about the constellation Aquila the Eagle. The most common Greek mythology associated with this constellation is that eagle is a servant of Zeus (or in some cases, represents Zeus himself), sent down to the Earth to fetch the mortal Ganymede, a handsome young man who served as the water bearer for Mount Olympus. Ganymede is now represented by the constellation Aquila is swooping towards, Aquarius.

Altair is the brightest star in Aquila, and it is flanked closely by a bright star on either side. At this time of year, if you are looking sort of Northwest so that the Northern Cross of Cygnus is upright, then Altair is beneath it. Below and to the left (south) of Altair is Alshain, the neck of the Eagle. Above and to the right (north) is Tarazed, from the Persian title for this constellation, Shahin tara zed, which translates to the star-striking falcon (from Allan's book).

These three stars together span about five degrees in a rough line and are called the Family of Aquila. In the Chinese story, Alshain and Tarazed are the two children of the Herd-Boy (Altair) and the Weaving-Girl (Vega) who are separated by the Milky Way river. Altair is about 16 light years away and so one of the closer stars to us. It is about 10 times more luminous than our Sun and is noted for its extremely rapid rotation (how we know this is a story for another time). It rotates so quickly (once every 6.5 hours compared to our own Sun, which takes a month!) that we speculate is is very flattened, with its equatorial diameter double that of its polar diameter.

With the Family of Aquila forming a short line that points SW to NE in the evening sky during the Fall, you can follow a line Southeast along the body of the eagle, which brings you do Delta Aquilae, also called Deneb Okab, which translates to tail of the eagle. Following that line further Southeast leads you to Lambda Aquilae, the outstretched claws of the eagle. Going a little further than that leads to the spectacular Scutum Star Cloud, which contains Messier objects M11 and M26. More about that another time when I talk about Scutum.

Almost exactly one full moon diameter due South of Lambda is the spectcular planetary nebula NGC 6751, which resembles a glowing eye. If you follow the line formed by the Family of Aquila toward the Southwest, you run into Theta Aquilae, a bright spectroscopic binary about 300 light years distant. If instead you go more due South, you run into Eta Aquilae, the third brightest Cepheid variable in the sky, behind Polaris and the prototype: Delta Cephei itself. This star has a very precise light curve period just over seven days, during which time its magnitude varies from 4.5 to a maximum of 3.7 (remember, the magnitude scale goes backwards).

Above and to the right of Deneb Okab by about five degrees is NGC 6781, a very nice planetary nebula similar to the Ring Nebula (M57) in Lyra. Also nearby, about 1.5 degrees to the right (a little North of West) of Tarazed is the very neat dark nebula known as the E nebula, or B142 and B143, one of the few dark nebula visible through a small telescope silhouetted against the background of thousands of faint stars. The B designation comes from a catalog of nebulae compiled by Edward Barnard around 1900.

Just scanning a small telescope over the body of this constellation, which lies across the band of the Milky Way, will lead you over many bright nebula and star clouds. This region between Sagittarius and Cygnus is one of the best in the sky for such telescopic free ranging.

Aquila is also the home of the most brilliant nova explosion in recorded history, Nova Aquilae 1918. The object was first noted by Barnard and another (later famous) comet-hunter and variable star observer by the name of Leslie Peltier (as usual, lots of really interesting information found from Burnham). This nova brightened suddenly and for a few days was the brightest star in Aquila. This was likely a white dwarf companion to a massive star, and when too much matter had been transferred from the massive star to the white dwarf, it ignited explosively on the surface.

In the 20th century, five novae reached a brightness of 1st magnitude in our sky like this, the most recent in 1942. Who knows when we will see another?

Posted by Observer at November 18, 2007 08:38 AM

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