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Star of the week: Deneb
By Larry Sessions in BRIGHTEST STARS | August 18, 2016
Deneb is one of the most distant stars you will see with your eye alone. That’s because
it’s one of the most luminous stars in the Milky Way galaxy.
Image via Fred Espenak
Deneb is one of the most distant stars you will see with your eye alone. That’s because
it’s one of the most luminous stars in the Milky Way galaxy. The exact distance to Deneb
is unclear, with estimates ranging from about 1,425 light-years to perhaps as much as
7,000 light-years. Whatever its exact distance, when you gaze at Deneb, know that you
are gazing across thousands of light-years of space.
The best estimates for Deneb’s distance likely are those obtained by the Hipparcos Space
Astrometry Mission in the 1990s. A simple calculation from initial Hipparcos data gives
the figure of 3,230 light-years, whereas the refined data yield just over 1,400 light-years.
At any of these estimates distances, Deneb is one of the farthest stars the unaided human
eye can see. It is so far, that the light that reaches the Earth today started on its journey
well more than 1,000 years ago.
Deneb (bottom half of frame) is 200 times bigger than the sun. Image via AstroBob
In order for us to see it at its enormous distance, Deneb must also be tremendously bright
and energetic. Among the 20 brightest stars, only Rigel in Orion surpasses Deneb in
intrinsic brightness. Deneb is an A2Ia star, which says that it is white hot (A2) and a
supergiant star (Ia). Prof. James Kaler, using the figure of 2,600 light years as the
distance, estimated a diameter 200 times greater than our sun, and about a quarter of a
million times brighter in visible light. Considering its spectral classification (A2), Deneb
must have a surface temperature between about 8500 to 9000 kelvins (roughly 14,800 to
15,700 degrees F).
This is the famous Hertzsprung-Russel diagram, which shows the luminosities of stars.
See Deneb at the top of the diagram? It is one of the most luminous stars known.
Deneb is not the biggest star in the Milky Way. Betelgeuse and Antares, among others,
are larger. But Deneb may well be the most energetic (based on the amount of
electromagnetic radiation it produces) of the A-type stars. The famed North American
nebula, a large cloud of glowing gas in the shape of the continent, likely is glowing in a
process similar to fluorescence, energized by the radiation of Deneb.
How to see it
White Deneb is the 20th brightest star in the sky. It is one of the most remote stars visible
to the eye alone. You can gaze at this faraway star starting in late spring, when Deneb
rises over the northeastern horizon by mid-evening. A month later, Deneb is already in
the sky by the time it gets dark. In standard time, the midnight culmination date is about
August 1. Using Daylight Time, the midnight culmination is about August 15. The word
‘culmination’ indicates the star’s highest point in the sky.
Deneb is part of several famous star patterns, which overlap each other. It’s the brightest
of the stars composing the constellation Cygnus the Swan, where it marks the Swan’s
Tail. When you hear ‘deneb’ in a star name, it always means ‘tail.’ Simultaneously,
Deneb marks the head of an asterism (a readily recognizable grouping of stars that is not
an official constellation), known as the Northern Cross. Plus it is one of the three stars
known as the Summer Triangle. The other two stars are Vega and Altair. Deneb is the
northernmost and dimmest of the three Triangle stars, but its association with the other
bright stars makes it easy to identify.
Deneb is circumpolar as seen from locations of about 45 degrees north latitude, roughly
the northern tier of US states. In other words, from the northern U.S. and similar
latitudes, Deneb never sets but instead circles round and round the pole star. It cannot
ever be seen south of about 45 degrees south latitude. That includes Antarctica, far
southern Argentina and Chile, and perhaps the far southern tip of New Zealand’s South
Island. Aside from that, just about anyone should have a chance to see Deneb at one time
or another.
History and Myth
The name Deneb derives from the Arabic Al Dhanab al Dajajah meaning Tail of the Hen.
It obviously dates from an earlier incarnation of Cygnus not as a swan but as a chicken.
Like many bright stars, Deneb has been called by a number of other names, but the
oddest, according the Richard Hinckley Allen, who cites the Arabic name above, was
Uropygium, meaning the posterior part of a bird’s body from which feathers grow, and
oddly sometimes called the “Pope’s nose.”
The constellation Cygnus the Swan. The bright star Deneb represents the Tail of Cygnus.
Image via Constellation of Words
In Chinese mythology Deneb is associated with the story of the Celestial Princess or the
Weaver Girl. In this story a Girl (the star Vega) is separated from her beloved (a cowherd
represented by the star Altair) by the Milky Way. Once a year, the girl and the cowherd
are allowed to meet briefly when a large flock of magpies form a bridge across the starry
river. Deneb represents the bridge.
Deneb’s position is RA: 20h 41m 26s, dec: +45° 16′ 49″.