July 05 – Cygnus, a Summer Constellation

If I looked hard enough, I could probably find a Soviet Kosmos launch to mention, but generally nothing much seems to happen up there on July 5th, and as yesterday was the birthday of Henrietta Swan Leavitt, I’m going to use the occasion as a tenuous link to my having a quick look at one of the more impressive summer constellations, Cygnus, the swan.

Cygnus: the Swan (background map credit: Stellarium; very rough locations of deep sky objects: me).


As you will immediately see, Cygnus has a well-defined and very recognisable shape, even under most urban skies, and unlike some other asterisms (patterns formed by stars) you can relatively easily imaging the long neck, sweeping wings and tail of a member of the cygnini tribe of large waterfowl (probably a mute swan, given that the mythology around the name is Mediterranean). This particular asterism is known as the “Northern Cross”.

The first object that stands out when you look up at Cygnus (or down at the above map) is the bright star, Deneb, or Alpha Cygni, named after the Arabic word for “tail”, which gives you the orientation of the bird, and lets you know that you are indeed looking at a swooping swan, and not a braking peacock.

As well as the Northern Cross, Deneb is also part of another asterism, the Summer Triangle, comprising Deneb, Altair in the constellation Aquilla) and Vega (in Lyra).

At over 2,600 light years distant, Deneb is farther away than any of the other brightest stars in the night sky. To achieve this level of brilliance, Deneb obviously needs to be either very bright or very big. It’s both. Deneb is a supergiant, with an estimated luminosity of 200,000 times that of the Sun, and has a radius 200 bigger than our star.

While Deneb is a solitary star, the same cannot be said for many of the other significant members of the constellation. Epsilon Cygni has three constituent parts, while Beta Cygni (aka Alberio) is a double star and Delta Cygni is a binary.

There are several interesting deep sky objects in Cygnus, three of which I will mention briefly.  The map above only shows the approximate locations of these. Messier 39 is a open cluster about 1,000 light years away, with a good collection of binary systems. Discovered by Guillame de Gentil, and incorporated into Messier’s catalogue some years later, M39 lies East-North East of Deneb, and is best viewed with binoculars.

M39 (image: HST/NASA)

Also in Cygnus is NGC6826 (or Caldwell 15), known as the Blinking Eye Nebula due to it’s tendency to seemingly disappear and reappear as stargazers attempt to use the technique of averted vision to avoid just seeing the very bright central star.

NGC6826 (image: NASA/ESA/HST)

Caldwell 15 is a planetary nebula, a shell of gas blown off by an elderly red giant, and represents a very brief stage of a star’s multi-billion year life cycle, perhaps only lasting ten or twenty thousand years.

Finally, while nebulae and clusters are relatively straightforward to spot, the same cannot be said of Cygnus A. It’s a radio galaxy, notable for being one of the first discovered, and for being one of the strongest radio sources known. Cygnus A is also now known to harbour a quasar, a class of object which were previously only known in the most distant galaxies. At 232 megaparsecs, Cygnus A is a relatively local place to find one.

Cygnus A (image: Mhardcastle, English Wikipedia)