DENEB (Alpha Cygni). One of the truly great stars of our Galaxy, Deneb serves a three-fold role among the constellations. Its very name tells the first. "Deneb" is from an Arabic word meaning "tail," as this first magnitude (1.25) star, the 19th brightest as it appears in our sky, represents the tail of Cygnus the Swan, a classical figure seen flying perpetually to the south along the route of the Milky Way. As the constellation's luminary, the star is also Alpha Cygni. The
First magnitude Deneb (Alpha Cygni) shines at far right, while fourth magnitude Xi Cygni lies at the far left edge just below center. The color contrast is obvious, Deneb a white class A supergiant, Xi Cyg a much cooler mid-class K orange supergiant. In between (and closer to Xi) is the faint reddish glow of the North America Nebula, NGC 7000; the fainter Pelican Nebula is down and to the right.
reversal of Cygnus makes the asterism of the Northern Cross, with Deneb now at the top, the cross seen rising on its side in early northern summer, standing upright in the west in early northern winter evenings. Deneb also makes the western apex of the famed Summer Triangle, which also incorporates Vega and Altair. All three of these white class A stars (Deneb an A2 supergiant) have similar surface temperatures, Vega, at 9500 Kelvin, the warmest, Deneb radiating at 8400 Kelvin. Though Vega and Altair are really quite luminous, they are first magnitude primarily because they are close to us, averaging only 25 light years away. Deneb, on the other hand, may be as far as 2600 light years. Based on that distance, its awesome luminosity of 160,000 Suns makes it about the intrinsically brightest star of its kind (that is, in its temperature or spectral class) in the entire Galaxy. If placed at the distance of Vega, Deneb would shine as bright as a well- developed crescent Moon. Deneb is a true supergiant, its diameter, calculated from its temperature and luminosity, is 200 times that of the Sun. Direct measurement of its tiny angular diameter (a mere 0.002 seconds of arc) gives a very similar value of 180 solar. If it were placed at the center of our Solar System, Deneb would extend to the orbit of the Earth. While far from the largest star in the Galaxy, Deneb is one of the biggest of its kind. It is evolving and has stopped fusing hydrogen in its core. Just what it is doing, however, we do not know. Having begun its life as a star a bit over 20 solar masses, its fate is almost certainly to explode as a supernova sometime within the next million years or so. The pace of evolution for such stars is rapid. If at 20 solar masses, Deneb ended its dwarf (hydrogen-fusing) lifetime a mere 40,000 years ago as a hot class B star. The star is constant in its light, but its spectrum, its light as seen when stretched into a rainbow, is slightly variable. Blowing from its surface is a wind that causes the star to lose mass at a rate of 0.8 millionths of a solar mass per year, a hundred thousand times the flow rate from the Sun. Deneb, among the most magnificent stars you can see with the unaided eye, was a halfway decent pole star (standing 7 degrees off the North Celestial Pole) 18,000 years ago, and will be again around the year 9800.
Written by Jim Kaler, revised 5/25/07. Return to STARS.