Summer Triangle star: Deneb is distant and luminous
Deneb or Alpha Cygni is the northernmost star in the famous Summer Triangle, a prominent asterism visible in the east on July and August evenings. The Summer Triangle is made of three bright stars in three separate constellations. Deneb’s constellation is Cygnus the Swan. In a dark sky, you can imagine the Swan, flying along the starlit trail of the summer Milky Way. The constellation Cygnus also makes an obvious cross shape, and that’s another asterism. That is, it’s another prominent star pattern. It’s called the Northern Cross.
Okay, we’ve given you a lot of names here: Summer Triangle, Cygnus, Northern Cross.
Just remember, the constellation Cygnus the Swan contains the asterism of the Northern Cross. The Cross is – more or less – just another way to see the Swan. Deneb is at the top of the Cross, but at the tail of the Swan (the star name “deneb” always means “tail”). The little star Albireo is at the head of the Swan, but at the base of the Cross.
Deneb is very far away, and very luminous
The star Deneb in the constellation Cygnus the Swan is one of the most distant stars you’ll ever see with your eye alone. That’s because it’s one of our Milky Way galaxy’s most luminous stars.
Deneb is somewhere around 1,500 light-years away. That’s in contrast to most visible stars in our sky, located tens to hundreds of light-years away.
But astronomers still aren’t certain of the exact distance for this very luminous star. There are varying estimates for its distance. Why?
For some decades, the most important distance measurement for Deneb has been that from ESA’s Earth-orbiting Hipparcos satellite, which operated from 1989 to 1993. Hipparcos was the predecessor to the Gaia space observatory, which is currently in space and collecting data, with the goal of creating a 3D map of our Milky Way galaxy.
Both Hipparcos and Gaia gather what’s called astrometric data on the stars. That is, they measure stars’ positions, motions and brightnesses not just once, but again and again. Those measurements let earthly astronomers calculate a distance, see how the star is moving, and much more.
Early analyses of Hipparcos data indicated a distance of somewhere around 2,600 light-years for Deneb. Then, in 2009, a newer study – which used more powerful analysis techniques on Hipparcos data – gave a distance for Deneb that’s about half the widely accepted value, closer to 1,500 light-years.
Today, that value – around 1,500 light-years – is the most widely accepted value for Deneb’s distance.
When will Deneb’s distance estimate be updated?
Gaia has now released three sets of data. Why haven’t Gaia’s newer measurements let astronomers measure Deneb’s distance more precisely? It’s mainly because Gaia isn’t geared toward observing such a bright star as Deneb. Astronomer Anthony G.A. Brown of Leiden Observatory in the Netherlands – a member of the Gaia team – told EarthSky in July 2021 that Gaia data still haven’t been used to determine a new distance for Deneb. He said:
The Hipparcos distance estimate still stands.
Deneb is so bright that we can only observe it with Gaia through specially programmed observation sequences (the star is not automatically picked up by the observing instruments on the spacecraft). We have observations of
Deneb in hand but these will require a dedicated processing which we have not yet started.
So, for now, the updated Hipparcos number is still the best one for Deneb’s distance. The best distance estimate for Deneb is approximately 1,500 light-years, for now.
And that’s impressive. In order for us to see a star shine so brightly in our sky from this great distance away, the star must be very powerful. Deneb is thought to be one of the most luminous stars – one of the brightest stars, intrinsically – that we can see with the eye.
Deneb in history and mythology
By the way, 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 to 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.”
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 forms a bridge across the starry river. Deneb represents the bridge.
How to see Deneb
You can gaze at this faraway star in the evening starting around May, or late spring in the Northern Hemisphere. From this hemisphere, in July and August, Deneb shines in the east at nightfall and appears high up in the sky around the middle of the night.
Like all stars, Deneb is found about one degree farther west at the same time each day, and climbs to its highest point about four minutes earlier per day, 1/2 hour earlier per week, or two hours earlier per month.
Deneb is circumpolar as seen from locations of about 45 degrees north latitude, roughly the northern tier of U.S. states. In other words, from the northern U.S. and similar latitudes, Deneb never sets but instead circles round and round the pole star.
This star 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. When you do see it, think of the power of this mighty star shining over such a great distance in space!
Deneb’s position is RA: 20h 41m 26s, dec: +45° 16′ 49″.
Bottom line: Information on the star Deneb, plus how to see it in your sky.
Our Summer Triangle series includes:
Deneb is distant and very luminous