Draco – the Dragon

Draco – the Dragon
High in the northern sky the coils of a great serpent-like dragon seem about to capture the small bear Ursa Minor. It's Draco, a circumpolar constellation that circles the north celestial pole, never setting below a northern hemisphere horizon.

Draco was already an ancient constellation when the Greek astronomer Ptolemy catalogued it in the 2nd century. Dragons were part of the creation myths of the Babylonians a few thousand years ago. Their primordial goddess Tiamat gave birth to the younger gods, including dragons and serpents, and she herself may also have taken dragon form at times. The Babylonians envisaged dragons with wings, and originally the constellation Draco was also winged. But from about the sixth century BCE onward, the Greek dragon became wingless and more serpentine.

Not surprisingly for such an old constellation, many classical stories are associated with Draco. In the most popular surviving one, the dragon was Ladon, the guardian of the golden apples of the Hesperides. For his eleventh labor, Hercules was sent to steal the apples. He killed the dragon with a poisoned arrow and, with the help of Atlas, acquired the apples.

Stars and planets
Draco's sinuous shape makes it believably dragon-like. However the constellation is very spread out and most of its stars aren't bright, so it can be difficult to see, except for the “eyes of the dragon”. These are the stars Gamma Draconis and Beta Draconis, which you can find to the north-northwest of Lyra's bright star Vega.

The star labels with Greek letters are Bayer designations. People usually assume that Johann Bayer (1572-1625) named them in order of brightness, alpha being the brightest. In fact, Bayer had various criteria for the naming order, and in Draco, Alpha Draconis (Thuban) is outshone by seven other stars. The brightest star is Eltanin (Gamma Draconis). Eltanin is a cool orange giant 150 light years away, which has expanded to nearly 50 times the Sun's width. Its surface is cooler than that of the Sun, but since it's much bigger, it's about 500 times brighter. Interestingly, it's also moving towards us, and in 1.5 million years it will be the brightest star in our sky.

Thuban, although far from being the brightest star, is of great historical significance. It was the north polar star long ago when the pharaohs were building the pyramids. The heavens haven't changed, nor has the tilt of Earth's axis. The change, called precession, is due to a little wobble in Earth's axis. In another 19,000 years or so Thuban will be the pole star again. [Animation credit: AstroBob]

The Kepler Space Telescope was launched in 2009 to search for exoplanets in an area of the sky that included Draco. Kepler was retired in 2018 with 2700 confirmed discoveries and a similar number of exoplanet candidates. As of July 2019, over 140 of the confirmed planets were in Draco.

Astronomers are particularly interested in smaller planets and in planetary systems, so Kepler-90 has had a lot of attention. It's a Sun-like star 2500 light years away, much younger and somewhat hotter than the Sun, and with a system of 8 planets. As in our Solar System, the smaller planets orbit closer to the star, and the gas giants orbit farther out. But the Kepler-90 system is very compact. Its outermost planet is at the same distance from the star as the Earth is from the Sun. [Image: NASA / Ames Research Center / Wendy Stenzel]

Deep sky objects
NGC 5907 was just a bit of nebulosity to its 18th century discoverer, William Herschel. Today we know it's a spiral galaxy 50 million light years away. We can't see the spiral structure because it's edge-on to us. The header image shows why one of its nicknames is the Splinter Galaxy. [Image credit: R. Jay GaBany]

A more exotic spiral galaxy is the Tadpole Galaxy (Arp 188). It's been disrupted by the gravitational effects of a close encounter with another galaxy. When two galaxies interact, a tidal tail of gas can often only be seen in the infrared. However this tail, which is 280,000 light years long, is visible because all along the tail the gas collapsed into numerous young blue stars and star clusters.

Draco has not only galaxies, but also clusters of galaxies. One massive cluster, Abell 2218, is about 3 billion light-years away. It acts as a gravitational lens for background galaxies even farther away, allowing astronomers also to study those galaxies.

But there's something bigger than a cluster of galaxies: a supercluster, i.e., a cluster of clusters. Part of the biggest known structure in the observable universe can be seen in Draco's southern region. It's the Hercules-Corona Borealis Great Wall, a colossal supercluster 6-10 billion light years in size, and containing billions of galaxies.

Nonetheless my favorite object isn’t a galaxy, it's NGC 6543, nicknamed the Cat’s Eye Nebula. It's another Herschel discovery. His telescope showed it as a fuzzy blue-green disk, the reason he gave the name planetary nebulae to such objects. In fact, they're created by Sun-like stars running out of hydrogen fuel, and sloughing off their outer layers. The Cat's Eye is one of the most complex and beautiful of the planetary nebulae.

You Should Also Read:
ABC of Astronomy – B is for Bok globule
ABC of Astronomy – G Is for Gravitational Lens
Search for Earth's Twin – book review

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