The Summer Triangle is still visible in the western half of the sky after Sun has set, even though autumn has begun. This triangular shape is a nice example of an asterism -- a recognizable pattern of stars that is not a specific constellation. It is composed of stars in three different constellations; Vega is in Lyra, Deneb is in Cygnus, and Altair is in Aquila. Vega, Altair, and Deneb are the fifth, twelfth, and nineteenth brightest stars, respectively, visible in the night sky. The two faintest are the subject of this discussion.

    Both Aquila and Cygnus are located in the faint band of light we call the Milky Way. Of course all the stars we see with our naked eye here in the northern hemisphere are members of the Milky Way Galaxy, but the stars in these two constellations are actually superimposed on the glow of the myriad of distant stars producing this hazy band of light.

    The only two stars in Aquila besides Altair that are easy to locate are its two faint neighbors (on the sky), Alschain to the left and Tarazed to the right. These three stars form the Family of Aquila. The names of the two fainter stars are derived from the Persian name for the constellation, Shahin tara zed, the Star-striking Falcon of Al Nasr al Din. The name Altair is part of the Arabic name for the constellation, Al Nasr Al Tair, the Flying Eagle, which the Arabs applied only to these three stars. The other stars in the constellation shown on this month's star chart were included by the Romans in their Aquila, the Eagle, although Allen reports in Star Names: Their Lore and Meaning, that it is "supposed to be represented by the bird figured on a Euphratean uranographic stone of about 1200 B.C."

    Altair is a white star similar to Sirius, the brightest star in the night sky. Altair is about twice as far away as Sirius, and consequently fainter. Its apparent magnitude is +0.77. Altair is most notable for its extremely rapid rotation. The speed at its equator, determined by measuring the width of its spectral lines, is about 160 miles per second, resulting in a complete rotation of the star in about 6 1/2 hours. Our star, Sun, requires a little more than 25 days for a complete rotation.

    Deneb, in addition to being represented as the tail of Cygnus the Swan, is also the top of an asterism contained within the boundaries of Cygnus -- the Northern Cross. This cross shaped pattern of six stars with Deneb at the top will be just above the northwestern horizon, standing upright when it first becomes dark around the time of Christmas in December. The remaining stars which form the shape of a Swan are difficult to find and require considerable imagination. They are swallowed up by the Milky Way. Deneb is from the Arabic Dhanab which means tail. This word appears in the name of the star which forms the tail of Leo the Lion, Denebola, and in the tail of Cetus the Whale, Deneb Kaitos.

    Deneb is in a class with Rigel, the blue-white star that forms the left knee of Orion. It is a blue super-giant with a surface temperature nearly twice that of Sun. Its distance is about 1600 light years, making it one of the most distant of the first magnitude stars. It is emitting radiant energy at a rate 60,000 times that of Sun, and is believed to contain about 25 times as much mass as Sun in a sphere that is probably 60 times Sun's diameter. Our Sun viewed from the same distance would appear to be a thirteenth magnitude star, requiring a telescope of at least 8-inch aperture to see easily. Deneb seems to have a very slight variability in brightness and has a variable velocity along the line of sight. These observations suggest that Deneb is vibrating and are consistent with predictions of stellar structure theorists that such a massive star should be slightly unstable.