Well up in the eastern sky during the late-evening hours is the Summer Triangle, not a constellation, but a very noteworthy star pattern of the summer sky consisting of three bright stars from three constellations.
Brightest of the three is Vega, in the constellation of Lyra, the Lyre. Next in brightness is Altair in Aquila, the Eagle. Finally, there is Deneb in Cygnus, the Swan.
To our eyes, Altair appears to shine only half as bright as Vega and Deneb seems only one-third as bright as Vega. So at first glance, Vega appears to rule this section of the sky. In reality, however, a bit of celestial deception is at work here, for Vega is appears very bright chiefly because of its proximity to us. Vega is 25 light-years away, so when you are looking at it tonight, you are really seeing it as it was back in 1982.
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Altair is 17 light-years away, closer than Vega, yet appearing somewhat dimmer. That tells us that in terms of overall luminosity between the two, Vega is indeed the brighter star: it is 48 times brighter than our sun, while Altair is only 12 times brighter.
But what about the third star in the triangle, Deneb? Here, there is no comparison between either Altair or even Vega, for Deneb is one of the greatest supergiant stars known. It shines about 80, 000 times brighter than our sun, but because of its vast distance from us — 1, 467 light-years away — it appears as just a fairly conspicuous but by no means noteworthy star.
If it were somehow possible to move Deneb to Vega's distance from us, it would then appear to shine 16 times brighter than Venus is now, and capable of casting distinct shadows and be visible even in the daytime. Conversely, if we were to move Vega out to where Deneb is, it would shine only as bright as a ninth-magnitude star; you would need a star atlas to precisely locate it, and it would only be visible in good binoculars or a telescope.
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With the bright moon now pretty much out of the evening sky this week, there is no better time than now to observe the beautiful summer Milky Way. With a good pair of binoculars or a telescope you can now observe millions of sparkling little stars that make up this glowing, irregular belt of luminosity. It appears to arch from the north-northeast to the south-southeast, with its brightest and most spectacular region running across the Summer Triangle and beyond toward the south-southeast horizon.
There appears to be a great black rift (sometimes called the bifurcation) dividing it into two streams, beginning with Cygnus and extending down toward the south. Also in Cygnus is the black void known as the Northern Coal Sack. This Coal Sack and the Rift are not holes in the Milky Way, but rather are vast clouds of dust drifting in interstellar space which, from our Earthly perspective, presents a solid and impenetrable curtain between us and the more distant stars.
This particular weekend, the Milky Way involves a poignant Japanese legend. The star Vega represented Orihime, who produced brilliantly colored fabrics. Across the Heavenly River (the Milky Way), Altair represented the cowherd Kengyu. After meeting each other they received divine permission to marry, whereupon both abandoned their occupations. This angered the gods who separated them and sent them back to their original jobs on opposite sides of the heavenly river. The couple however, received permission from the gods to get together for one night each year.
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As a result, the evening of July 7 has evolved into a young-people's holiday in Japan called Tanabata. Prayers are then offered for clear skies so that Orihime and Kengyu, the star-crossed lovers can be reunited. One popular Tanabata custom is to write one's wishes on a piece of paper, and hang that piece of paper on a specially erected bamboo tree, in the hope that the wishes come true.The bright star Deneb is part of the famous Summer Triangle asterism. Its constellation Cygnus the Swan flies across the July evening sky.
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. Three bright stars make up the Summer Triangle. They are the brightest 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.
Just remember, the constellation Cygnus the Swan contains the asterism of the Northern Cross. The Cross is 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.
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The constellation Cygnus represents a graceful swan. But many also see it as a cross, and so these stars have become known as the Northern Cross. Deneb marks the Tail of Cygnus the Swan … and the head of a cross-like pattern known as the Northern Cross.
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
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.
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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, ESA’s Earth-orbiting Hipparcos satellite, which operated from 1989 to 1993, provided the most important distance measurement for Deneb. 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.
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Early analyses of Hipparcos data indicated a distance 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.
Astronomers use the parallax method to find distances to nearby stars. But Deneb is too far away for accurate parallax measurements from Earth’s surface. Image via NASA/ ESA/ A. Feild (STScI).
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 in July 2021 that Gaia data still haven’t been used to determine a new distance for Deneb. He said:
Deneb (α Cygni)
The Hipparcos distance estimate still stands. Deneb is so bright that we can only observe it with Gaia through specially programmed observation sequences (the observing instruments on the spacecraft do not automatically pick up the star). 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. So, 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 one of the most luminous stars – one of the brightest stars, intrinsically – that we can see with the eye.
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Deneb is a blue-white supergiant star with a spectral class of A2Ia. It is the 19th brightest star in the sky shining at +1.25 magnitude. It’s about 196, 000 times more luminous than our sun. Deneb contains about 20 solar masses, and as mentioned above its distance is uncertain. Deneb has a diameter about 203 times that of the sun. And that makes Deneb one of the largest type A spectral class stars known.
Deneb is the prototype for the Alpha Cygni variable stars. Its brightness varies due to non-radial fluctuations on the surface of the star. The fluctuations originate from areas on the surface of the star either expanding and contracting at the same time. They can last for days to weeks and their origin is unknown. The change in brightness is minimal, but detectable. For example, Deneb varies in brightness from 1.21 to 1.29 magnitude. Another Alpha Cgyni variable star is Rigel, in the constellation of Orion the Hunter.
Deneb was once a spectral class O-type main sequence star with a
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