Star Facts: Deneb

Deneb Star
Image Credit: Fred Espenak

Deneb (Alpha Cygni) is the most luminous star in the constellation Cygnus, and with an apparent magnitude of +1.25, it is the 19th most luminous star in the entire night sky. The star forms one of the vertices of the Summer Triangle asterism, and marks out the “head” of the Northern Cross. It also shares its name, or variations of its name, with at least seven other stars, and in all cases, the derived name refers to the tail of the animal the star is said to represent.

Quick Facts

• Constellation: Cygnus
• Coordinates: RA: 20h 41m 26s|Dec: +45° 16″ 49″
• Distance: 3,230 light years
• Star Type: A2 Ia
• Mass: 19 sol
• Radius: 203 sol
• Apparent Magnitude: 1.21 to 1.29 Variable
• Luminosity: 196,000 sol
• Surface Temperature: 8,525K
• Rotational Velocity: 20 km/s
• Age: Uncertain
• Other Designations: a Cygni, Alpha Cyg, 50 Cyg, Arided, Aridif, Gallina, Arrioph, HR 7924, BD +44°3541, HD 197345, SAO 49941, FK5 777, HIP 102098.


Deneb is located in the northern constellation of Cygnus, and can be seen from +90° and -40° of latitude, with its best viewing month in August when it reaches its midnight culmination point on the 15th. For observers in the southern hemisphere below latitude 45 degrees south, however, Deneb never rises above the horizon, which means that for observers in Tasmania and southern New Zealand, the star is only just visible above the horizon during the southern winter. Due to precession, though, future generations will be able to see Deneb as the Pole Star from around the year 9800 AD.


In the northern hemisphere, Deneb is easy to spot as one of the vertices of the huge Summer Triangle asterism, the other two members of which are the bright stars Vega in Lyra and Altair in the constellations of Aquila. The Summer Triangle is best seen during the summer months in the northern hemisphere. Deneb is also easily visible as the “head of the Northern Cross asterism, which is made up of the bright stars Beta- (Albireo), Gamma, Delta, and Epsilon Cygni. For observers above latitude 45 degrees north, Deneb never sets below the horizon, while at its lowest elevation above the horizon, the star just grazes the horizon from locations such as Minneapolis, Montreal, and Turin.

Summer Triangle

Physical Properties

Size, Mass and Brightness

As a blue-white supergiant (A2Ia), Deneb is one of the biggest white stars known, and is 203 times bigger than the Sun, and around 19 times more massive. In fact, direct measurements of its angular diameter, a miniscule 0.002 arc seconds, translate into a diameter that would extend all the way to Earth’s orbit if the star were to be placed in the centre of the solar system.

Deneb is also among the brightest of the stars that have magnitudes brighter than +1.5, even though at 3,230 light years away it is more distant, by a factor of nearly two, than the 30 most luminous stars known. Furthermore, it happens to be the prototype of the Alpha Cygni variables, a class of variable stars whose surfaces typically undergo non-radial fluctuations in brightness, as well as small changes in their spectral types. As with other variable stars of this class, Deneb’s variations in brightness occur without a clear, unambiguous period. As a further point of interest, Deneb’s solar wind is blowing away material at a phenomenal rate resulting in it losing mass at a rate that is 100,000 times higher than the rate at which the Sun is losing its mass.

Evolutionary Stage

In terms of its evolution, there is much about Deneb that remains unknown, or uncertain. However, it is generally accepted that Deneb spent its youth as a 23 solar mass O-type main sequence star that has now exhausted its supply of hydrogen fuel. As a result, it has now evolved into a giant star, but because it is a blue-white giant, instead of a red giant, it is uncertain whether Deneb has entered a second evolutionary stage.

In this second stage, massive stars do not explode as supernovae; instead, massive giant stars merely blow off their outer layers which causes the core to heat up again. In this state, but depending on their initial masses, these stars may explode either as yellow hyper giants, or as luminous blue variables. Such stars may however also evolve into very hot Wolf-Rayet stars that could explode in type Ib or Ic supernova events. At this point it is not known if Deneb is evolving into a red giant phase, or if it is evolving further into the blue giant phase, but determining this would serve to clarify the constraints on the classes of stars that explode as either red super giants, and those that evolve to explode more violently as hotter blue giants.

It is well-known that stars that evolve toward the red giant phase are enveloped in a shell of material that is not hot enough to initiate the process of fusing carbon into oxygen, but although some fusion products are dredged up from the depths of the star by convection currents, these products are generally not seen at the surface of normal red giant stars. However, it is expected that these fusion products would be visible at the surface in post-red giant stars due to the action of vastly stronger convection currents, but in the case of Deneb which is thought to be evolving toward the red giant phase, there are observed elements in its spectrum that cannot yet be explained by the standard models of stellar evolution.


Cygnus the Swan
Image Credit: Larry Landolfi

Deneb, the brightest star in the constellation of Cygnus, marks the tail of a magnificent celestial swan, with its name coming from the Arabic word ‘dhanab’ meaning ‘tail’. However, its name derives from the longer Arabic phrase ‘Al Dhanab al Dajajah’ which translates into ‘Tail of the Hen’, obviously harking back to an earlier age when Cygnus represented not a swan, but a slightly less majestic chicken. Even less flattering, German poet and author Philipp von Zesen (1619 –1689) dubbed the star Uropygium, referring to the rump of a bird called the “Parson’s Nose.”

Chinese astronomers, on the other hand, saw Deneb as part of an asterism named “the Celestial Ford”, which includes Deneb plus eight other stars, with Deneb occupying the fourth place in this asterism, hence its Chinese name ‘Tian Jin sì’, which means the “Fourth Star of the Celestial Ford”. Note that in this context, “Ford” means “river crossing”, with Deneb forming part of the bridge that allowed the Weaver Girl from Chinese mythology to cross the celestial river (Milky Way) once a year to visit her Cowherd husband, represented by the nearby bright star Altair.

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