DENEB (Alpha Cygni). One of the truly great stars of our Galaxy, a class A (A2) supergiant, 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.
Deneb The region of Deneb. 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. See a labelled version.
The 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 evenings, standing upright in the west in early northern winter after sunset. Deneb also makes the western apex of the famed Summer Triangle, which also incorporates Vega and Altair. These three white class A stars have roughly similar surface temperatures, Vega at 9500 Kelvin, Altair at 7550, Deneb in the middle, radiating at 8500. 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, comes in at an amazing (and somewhat uncertain) 1425 light years (second Hipparcos reduction). Based on that distance, its awesome luminosity of 54,400 Suns makes it among the intrinsically brightest stars of its kind (that is, in its temperature or spectral class) in the Galaxy. If placed at the distance of Vega, Deneb would shine at magnitude - 7.8, 15 times more brightly than Venus at her best, be as bright as a well-developed crescent Moon, cast shadows on the ground, and easily be visible in broad daylight. Deneb is a true supergiant, its diameter, calculated from its temperature and luminosity, is 108 times that of the Sun, half the size of Earth's orbit. Direct measurement of its tiny angular diameter (a mere 0.0025 seconds of arc) gives a very similar value of 114 solar, and excellent match, showing that the various stellar parameters are close to being correct. While far from the largest star in the Galaxy, Deneb is certainly one of the biggest of its kind. With a rotation velocity of at least 30 kilometers per second, Deneb might take as long as half a year to make a full rotation. The star is evolving and has stopped fusing hydrogen in its core. However, it's hard to know just what is going on. It might be expanding and cooling with a dead helium core and on its way to becoming a red supergiant, or it might have advanced to the state of core helium fusion. Having begun its life as a hot class B (or even class O) star of 15 to 16 solar masses just over 10 million years ago, its fate is almost certainly to explode sometime astronomically soon as a grand supernova. 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 a millionth of a solar mass per year, 40 million 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 6/19/98; last revised 6/26/09. Return to STARS.