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Draco the Dragon

High in the northern sky the coils of a great serpent-like dragon seem perilously close to Ursa Minor, the smaller bear. Draco, the dragon, constantly circles Polaris, the pole star. Click to see how Draco and Ursa Minor were depicted in early nineteenth century Uraniaís Mirror.

Although the constellation Draco is of ancient origin, and listed by Greek astronomer Ptolemy in the second century, it isn't easily recognizable. The stars are quite spread out, and the three brightest stars are second magnitude. The shape is difficult to pick out in the absence of bright stars - in fact, impossible in a light-polluted area.

Dragons played a part in the creation myths of the ancient Middle Eastern civilizations. As envisaged thousands of years ago, they were winged, and originally so was Draco. However from about the sixth century BCE onward, the Greek representation of the dragon became wingless and more serpentine.

Not surprisingly, many stories are associated with Draco. The most popular one relates the dragon to the Graeco-Roman tale of the eleventh labor of Hercules. The dragon Ladon was the guardian of the golden apples of the Hesperides which Hercules was sent to steal. He killed the dragon with a poisoned arrow and, with the help of Atlas, acquired the apples, as ordered.

When Johann Bayer (1572 - 1625) was assigning labels to the stars in a constellation, he usually designated the brightest star as the alpha star. This meant that the order of the Greek letters roughly showed the order of the visual magnitudes of the stars. Draco is a major exception. Gamma Draconis (Eltanin) is an orange giant and the brightest star of Draco. Itís also nearly four times brighter than Alpha Draconis, traditionally called Thuban.

Although Thuban is far from being the brightest star, it is of great historical significance. Thuban was the north polar star for over a thousand years, from about 3900 BCE to 1800 BCE. (The location of the pole star changes slowly due to precession brought about by a wobble in the Earth's axis.)

For the ancient Egyptians there was something special about the stars we call circumpolar, the ones so close to the pole that they never set. The Egyptians called them ďundyingĒ and the pole star itself was the most important of the undying stars. They built the pyramids so that one side faced north, and in the deep burial chamber of the pharaoh Khufu a shaft used to point to Thuban.

In another 19,000 years or so Thuban will be the pole star again.

Thuban isnít a single star. Itís a binary in which the primary is a blue-white giant, about 250 times brighter than the Sun. It has a close unseen companion which is a low-mass dwarf star.

As of April 2013 there were three stars in Draco that had known planets. Two are planets several times more massive than Jupiter, but one of them, Kepler 10, has at least two planets that are much smaller. One of these is close to Earth-sized, but it isnít in the starís habitable zone.

Deep sky objects
There are some interesting deep sky objects in Draco. One is NGC 5866, the Spindle Galaxy. This may well be a spiral galaxy, but we see it so precisely edge-on that we canít see any structure. Itís classified as a lenticular galaxy but has a considerable dust disk. This is unusual for a lenticular galaxy, but not for a spiral galaxy.

The Tadpole Galaxy (Arp 188) is a barred spiral galaxy that has been disrupted by the gravitational effects of another galaxy. When two galaxies interact, there is often a tidal tail of gas that can only be imaged in the infrared. However here the disruption has turned the long gas tail (280,000 light years long) into a series of sites for star formation with numerous young blue stars and star clusters.

My favorite of the deep sky objects in Draco isnít a galaxy, but a planetary nebula. Itís NGC 6543, also known as the Catís Eye Nebula. William Herschel discovered it in 1786 and in his telescope it looked like a blurry, blue-green disk. The disk made it look planet-like which is why Herschel called them planetary nebulae. However they are created when a Sun-like star runs out of hydrogen fuel, expands and sloughs off its outer layers. This one is the most complex ever seen and its mystery has yet to be unraveled.

The nebulaís modern nickname comes from its appearance when imaged by a large telescope. Click for a photo of the Catís Eye Nebula to see if you agree with its nickname.

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