November 13, 2007

Cygnus

In my next three constellation stories, I will do the three constellations of the Summer Triangle. They are still visible pretty high up in the Western sky after sunset, and they are easy to find even when the sky brightness isn't ideal. First up is Cygnus the Swan, better known to many as the asterism "The Northern Cross". In most cultures, this is known as a bird of some kind, flying either North or South.

The brightest star in Cygnus is Deneb, which comes from the Arabic for "The Hen's Tail", Al Dhanab al Dajajah. In fact, Deneb appears in the names of many stars that represent tails (for example in Leo and Cetus, which I'll be talking about soon). Deneb is, indeed, the tail of the swan which is flying south across the sky river of the Milky Way, eternally chasing Aquila the Eagle. This star is very far away, with a distance between 1600 and 3200 light years (the nearest star is just over 4), and to be as bright as it is at that distance requires a considerable intrinsic luminosity, over 60,000 times that of our Sun. The reason for the distance uncertainty has to do with the difficulty of measuring such a small parallax angle.

This supergiant is incredibly large in size compared to our Sun as seen by this comparison image from Wikipedia. It's luminosity is such that it is thought to be responsible for lighting up the nearby reflection nebula known as the North America Nebula (NGC 7000), but that depends on whether our distance estimate is accurate since we know the nebula is about 1600 light years away. The region of the sky immediately surrounding Deneb is rich in glowing nebulae and star clusters, as I'll describe later.

At the bottom of the Northern Cross is the star also known as the beak of the Swan: Albireo. The original Arabic name is Al Minhar al Dajajah, "the beak of the hen", and that name was translated to ab ireo in Ptolemy's Almagest, then later mistaken for Albireo by translators. This is perhaps the finest double star in the sky visible through a small telescope. The color contrast between the yellow topaz and blue sapphire star is unmistakeable. Burnham claims that the star clouds just to the NE of Albireo are probably unequaled in splendor in the entire heavens. The current thinking is that this is a physical pair, as they seem to move together through space, though no orbital motion has been detected as yet (it could just be a very long period binary system).

Gamma Cygni is called Sadr, and it is at the center of the cross. It is between Sadr and Albireo that the richest star clouds in Cygnus will be found through a telescope. There are a great many faint but interesting stellar objects in this region, and I'll hit a few of the highlights. First is the binary system known as 61 Cygni. This is the first object for which a parallax angle was successfully obtained by Bessel in 1838 (it is sometimes called Bessel's Star in his honor).

This 700-year period binary system has been observed continuously since the 17th century in some form or other, and the orbiting companion has visibly moved about halfway around its elliptical path in that time. This object was noted to have a very high proper motion (about half that of the record holding Barnard's Star) in 1792. It was later found to be at a distance of about 11 light years, which places it as number four in the list of stars closest to Earth, behind the Alpha Centauri system, Sirius and Epsilon Eridani.

SS Cygni is a bright nova (also called a Cataclysmic Variable) that recurs about every 50 days or so, brightening by a factor of about 40 from 12th to 8th magnitude. This is one of the most heavily observed systems in the sky, but we still aren't sure what is causing the flares. Is it a standard nova or some kind of interaction between the two stars that is causing a recurring flare on the main sequence star? Normally, a nova is caused when a star dumps matter onto a white dwarf companion. Every so often that matter on the surface of the white dwarf ignites fusion and burns up, creating a big show but leaving the two stars intact.

One of the more famous novae is V1500 Cygni, which last erupted in 1975 and hasn't gone off again since. Its eruption increased the system's brightness by a factor of nearly 100 million, a record but still within the theoretically calculated range of possibilities for such objects. Novae have different outburst frequencies which depend upon the separation of the objects and the nature of the companion that is losing its mass to the white dwarf. P Cygni is thought of as a "permanent nova" because it is a star with a very strong stellar wind. The star is surrounded by an expanding shell of gas that leaves a distinctive spectral line signature thanks to the Doppler shift pattern.

Cygnus X-1 is one of the 1st detected sources of X-rays in the sky, found about halfway between Sadr and Albireo along the bottom stem of the cross. The source is likely very hot gas in an accretion disk surrounding a black hole. Matter from a bright, hot star is falling in toward an object that is at least 10 or 15 solar masses according to its orbital properties, but we can't see the companion. So we figure it must be a black hole, one of the first indirectly detected black holes.

Among the deep sky objects are M29 and M39, two relatively small star clusters. M39 is about nine degrees ENE from Deneb and a good binocular object. It is a cluster of about 30 bright stars spanning a field the size of the full moon. If you are facing the cross at Deneb is at the top, then East is to the left. Also to the left of Deneb by only about 3 degrees is the spectacular North America Nebula, one of the most photographed objects in the sky, along with its companion, the Pelican Nebula.

A more difficult object is the Veil Nebula, sometimes called the Great Cygnus Loop (parts of it are also known as the Witch's Broom). This is a filamentary, almost circular band of gas that is likely an expanding remnant of a supernova explosion that occured thousands of years ago. As this expanding shell sweeps up the surrounding gas, it is like a star-forming front, leaving an empty region behind. If you count the stars inside and outside the loop, you find a lot more inside since the dust is being swept out of your line of sight.

The extragalactic star of the show here is Cygnus A, the second strongest source of radio waves in the sky (not a good object for small telescopes, though ... too faint). This is a galaxy about 600 million light years away, and the energy is coming from two enormous lobes of excited gas on either side of the galaxy. The lobes are created by jets of energetic particles being ejected from the core of this strange galaxy in opposite direction. The precise source of this phenomenon is still poorly understood.

Posted by Observer at November 13, 2007 06:51 PM
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