Deneb, all about the Swan Alpha star

The most beautiful “hen tail” in the Universe

summer triangle star deneb
The Summer Triangle. Deneb is the star in the middle, on the left.
Credit: NASA, ESA
A. Fujii

It’s called “hen’s tail” (from the Arabic Dhanab to the Dajajah). Its appearance, however, resembles little that of the ruffled tail of the biped of our flowerbeds… It is Deneb, the alpha star of the Swan constellation. We reveal some curiosities about one of the stars of summer skies…!

It will have happened to you during the summer nights to raise your eyes to the sky and see a stellar conformation that resembles a cross. The “Northern Cross“, or Swan, “flutters” on our heads in these evenings, with its main stars Deneb, Albireo, Sadir, Gienah acting as borders to the constellation.

The Arabs used to see the Swan as a hen, so much so as to appeal to its main star, Deneb (“the tail of the hen”). It was the Greeks who assimilated asterism to the figure of the majestic Swan. Today we therefore know Deneb as the “Swan’s tail“.

Main features

Deneb is a white supergiant. Of spectral class A2Iae, it places in the range of the most brilliantly intrinsic stars.

In the sky the star appears to us as the most brilliant nineteenth, immediately after Pollux and Formalhaut. Its apparent magnitude is +1.25 but unlike other apparently brighter stars, Deneb is at a considerable distance from us. Therefore what we see is the fruit, above all, of its absolute magnitude. It was not calculated accurately, but it is estimated to be between -6.88 and -8.46 (approximately). In any case, with high probability Deneb is one of the first-magnitude stars farthest from our Sun, and one of the intrinsically most brilliant stars.

The color of the stars

The color of the stars generally indicates their surface temperature. Deneb is a white supergiant of spectral class A2Iae. The letter A indicates that the star is white; the brightness class I indicates that it is a supergiant. The subclass Ia indicates the brightest supergiants while the final letter e indicates that emission lines are present in the spectrum. The white color denotes that its surface temperature is much higher than that of the Sun. However, astronomers have not calculated accurately its temperature yet.

The mysterious and fascinating Deneb is in fact a variable star: its magnitude varies from +1.210 to + 1.290 (General Catalog of Variable Stars (Samus + 2007-2012)).

summer triangle sky
The Summer Triangle.
Credit: Wikipedia

Observe Deneb

To find it, it is enough to find the constellation of the Swan, which is easy to identify thanks to its cross shape. Or follow the tracks of the Summer Triangle, the typical asterism of summer skies that also presents Vega and Altair as vertices of the triangle. Favorable for the observation of Deneb in the northern hemisphere is the summer period. During the winter until January it will be low on the horizon. In the southern hemisphere the best period for observation is between August and September.

Curiosity

On Mars the star Deneb currently holds the role of “Polar Star” (Nadine G. Barlow, Mars: an introduction to its interior, surface and atmosphere, Cambridge, Cambridge University Press, 2008) as it indicates the north.

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