Draco

Draco is a constellation in the far northern sky. Its name is Latin for dragon. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy and remains one of the 88 modern constellations today. The north pole of the ecliptic is in Draco. Draco is circumpolar (that is, never setting), and can be seen all year from northern latitudes. The constellation’s right ascension is 17, its declination +65, and it covers an area 1083 sq. deg., ranking 8th in size.

Draco coils around the north celestial pole, as depicted in Urania's Mirror, a set of constellation cards published in London c.1825

Dragons in Greek mythology that may have inspired the constellation’s name include Ladon, the dragon who guarded the golden apples of the Hesperides. Hercules killed Ladon during his 12 labors; he was tasked with stealing the golden apples. The constellation of Hercules is depicted near Draco. Sometimes, Draco is represented as the demon son of Gaia, Typhon.

In Greco- Roman legend, Draco was a dragon killed by the goddess Minerva and tossed into the sky upon his defeat. The dragon was one of the Gigantes, who battled the Olympic gods for ten years. As Minerva threw the dragon, it became twisted on itself and froze at the cold North Celestial Pole before it could right itself.

The most commonly accepted version of Draco’s arrival in the heavens is that Draco was the dragon killed by Cadmus. Cadmus was the brother of Europa, who was carried off to Crete by Jupiter in the form of a bull (Taurus). Cadmus was ordered by his father to go in search of his sister, and told he could not return unless he brought Europa back with him. As Ovid narrates in his Metamorphosis, “Cadmus wandered over the whole world: for who can lay hands on what Jove has stolen away? Driven to avoid his native country and his father's wrath, he made a pilgrimage to Apollo’s oracle, and begged him to say what land he should dwell in.”

Cadmus followed Apollo’s advice and found a suitable site for his new city. He sent his attendants to find fresh water to offer as a libation to Jupiter, and they wandered into a cave with springs. As they were getting water, however, they were all killed by “the serpent of Mars, a creature with a wonderful golden crest; fire flashed from its eyes, its body was all puffed up from poison, and from its mouth, set with a triple row of teeth, flickered a three-forked tongue.” After his companions did not return, Cadmus himself went into the cave and discovered the dragon. He killed it with his spear, and then (upon the order of Minerva) sowed the dragon’s teeth in the ground. From the teeth sprung warriors, who battled each other until only five were left. These five, along with Cadmus himself, were the first people of the city of Thebes.

Ovid himself does not equate the dragon of Mars with Draco. In fact he describes the dragon killed by Cadmus in terms of the constellation: “It was as huge as the Serpent that twines between the two Bears in the sky, if its full length were seen uncoiled.”

[http://www.comfychair.org/~cmbell/myth/draco.html]

Traditional Arabic astronomy does not depict a dragon in modern-day Draco, which is called the Mother Camels. Instead, two hyenas, represented by Eta Draconis and Zeta Draconis are seen attacking a baby camel (a dim star near Beta Draconis), which is protected by four female camels, represented by Beta Draconis, Gamma Draconis, Nu Draconis, and Xi Draconis. The nomads who own the camels are camped nearby, represented by a cooking tripod composed of Upsilon, Tau, and Sigma Draconis.

A Babylonian creation story tells of Tiamat, who turned herself into a dragon but was later defeated and split into two parts. One part became the heavens and the other, the Earth.

A Chinese tale sees the stars as the dragon who eats the Sun or Moon (possible represented by the north star Polaris) in an eclipse. During a real eclipse, ancient Chinese would make as much noise as possible, banging on pots and pans to try and scare away the dragon which was eating the Sun or Moon.

A Norse creation myth tells of a dragon who gnaws at the roots of Ygdrasil, the tree that covers the world.

Because Thuban was the pole star 5000 years ago the ancient Egyptians keenly observed it. Some of Draco’s stars were part of their constellation of Hippopotamus and some were of the Crocodile. They appear on the planisphere of Denderah and the walls of the Ramesseum at Thebes. Draco’s stars were also said to represent the falcon headed god Horus.

The Pharaoh Khufu ruled ancient Egypt around 2550 BCE and was buried in the largest of the Giza pyramids when he died. Khufu’s burial chamber was fashioned deep inside the Great Pyramid. Two skinny shafts bore outward from the chamber. For decades, scholars thought they were airshafts. But in the 1960s, astronomers found that they have an astronomical purpose. It was found that one of the shafts pointed directly towards Thuban. The other was aimed at the belt of Orion, which symbolized Osiris. The stars close to the pole never set. The Egyptians described these stars as ‘imperishable’ or ‘undying.’ Khufu expected that when he died, he would join not only with the Sun, but with Thuban as well, maintaining order in the celestial realm, just as he had on Earth.

Around 800 BCE, the prehistoric Adena people who lived in the Ohio area of the United States created Serpent Mound which is believed to mirror the constellation Draco. This huge mound is nearly a quarter mile long.

The Persians have regarded Draco as a man-eating serpent called Azhdeha.

In early Hindu worship, Draco is given the form of an alligator known as Shi-shu-mara.

[http://starryskies.com/The_sky/constellations/draco.html]

[https://infograph.venngage.com/p/191012/draco-constellation-ajcr11951]

[https://www.pa.msu.edu/people/horvatin/Astronomy_Facts/constellation_pages/draco.htm]

Thuban (α Draconis) was the northern pole star from 3942 BCE, when it moved farther north than Theta Boötis, until 1793 BCE. Due to the effects of precession, it will again be the pole star around the year AD 21000. It is a blue-white giant star of magnitude 3.7, 309 light-years from Earth. The traditional name of Alpha Draconis, Thuban, means ‘head of the serpent.’

There are two other stars above magnitude 3 in Draco. The brighter of the two- and the brightest star in Draco- is Gamma Draconis, traditionally called Etamin or Eltanin. It is an orange giant star of magnitude 2.2, 148 light-years from Earth. The aberration of starlight was discovered in 1728 when James Bradley observed Gamma Draconis.

Beta Draconis, traditionally called Rastaban, is a yellow giant star of magnitude 2.8, 362 light-years from Earth. Its name shares a meaning with Thuban, ‘head of the serpent.’

The four stars forming the dragons head (beta Dra, gamma Dra, xi Dra and nu Dra), build a conspicuous asterism called the Lozenge.

Epsilon Dra is a good double to be observed with scopes at a moderate magnification. Its companions are about 3 arc sec apart. The binary mu Dra is a good test for a scope with an aperture of 60mm. Good optics might split that pair consisting of two F7 main sequence stars (5.83 mag and 5.80 mag).

An outstanding binary is nu Dra. The two white stars (an A6V and an A4m) have magnitudes of 4.88 and 4.87. They are a good object for binoculars. Small scopes reveal the 8th mag blue companion of the G9IIIbCN-0.5 star omicron Dra (4.66 mag). A good binocular may be sufficient to split the pair of psi Dra. Small scopes show a 5th mag star and a 6th mag star. Another easy pair for small telescopes is 40-41 Dra. The two yellow stars are of 6th mag. Its coordinates are roughly RA=18h and DECL=+80 degrees.

A really impressive triple system is 39 Dra. Field glasses show a wide double; in larger scopes a third star close to the brighter one occurs.

Another attractive triple is 16-17. In binoculars two blue-white stars of 5th mag are revealed. Viewing with a telescope shows another star of 7th mag close to one of the first two.

[http://www.crystalinks.com/draco.html]

The constellation contains the star recently named Kepler-10 which has been confirmed to be orbited by Kepler-10b, the smallest rocky Earth-sized planet yet detected outside of the Solar System. Yet it orbits its star in less than a day, at less than a twentieth of the distance from Mercury to the Sun. Its surface temperature on the star lit side is approximately 1833 K which is as hot as a blast furnace and hot enough to melt iron.

[https://en.wikipedia.org/wiki/Kepler-10b]

One of the deep-sky objects in Draco is the Cat’s Eye Nebula (NGC 6543), a planetary nebula approximately 3,000 light-years away that was discovered by English astronomer William Herschel in 1786. It is 9th magnitude and was named for its appearance in the Hubble Space Telescope, though it appears as a fuzzy blue-green disk in an amateur telescope. NGC 6543 has a very complex shape due to gravitational interactions between the components of the multiple star at its center, the progenitor of the nebula approximately 1,000 years ago. It is located 9.6 arcminutes away from the north ecliptic pole to the west-northwest:

The Cat’s Eye Nebula

The full beauty of the Cat’s Eye Nebula (NGC 6543) is revealed in this new, detailed view from NASA’s Hubble Space Telescope. The image from Hubble’s Advanced Camera for Surveys (ACS) shows a bull’s eye pattern of eleven or even more concentric rings, or shells, around the Cat’s Eye. Each ‘ring’ is actually the edge of a spherical bubble seen projected onto the sky – that’s why it appears bright along its outer edge.

Observations suggest the star ejected its mass in a series of pulses at 1,500-year intervals. These convulsions created dust shells, each of which contain as much mass as all of the planets in our solar system combined (still only one percent of the Sun’s mass). These concentric shells make a layered, onion-skin structure around the dying star. The view from Hubble is like seeing an onion cut in half, where each skin layer is discernible.

The bull’s-eye patterns seen around planetary nebulae come as a surprise to astronomers because they had no expectation that episodes of mass loss at the end of stellar lives would repeat every 1,500 years. Several explanations have been proposed, including cycles of magnetic activity somewhat similar to our own Sun’s sunspot cycle, the action of companion stars orbiting around the dying star, and stellar pulsations. Another school of thought is that the material is ejected smoothly from the star, and the rings are created later on due to formation of waves in the outflowing material.

[https://www.nasa.gov/multimedia/imagegallery/image_feature_211.html]

Three Galaxies in Draco

This intriguing trio of galaxies is sometimes called the Draco Group, located in the northern constellation of (you guessed it) Draco. From left to right are edge-on spiral NGC 5981, elliptical galaxy NGC 5982, and face-on spiral NGC 5985- all within this single telescopic field of view spanning a little more than half the width of the full moon. While the group is far too small to be a galaxy cluster and has not been catalogued as a compact group, these galaxies all do lie roughly 100 million light-years from planet Earth. On close examination with spectrographs, the bright core of the striking face-on spiral NGC 5985 shows prominent emission in specific wavelengths of light, prompting astronomers to classify it as a Seyfert, a type of active galaxy. Not as well-known as other tight groupings of galaxies, the contrast in visual appearance makes this triplet an attractive subject for astrophotographers. This impressively deep exposure hints at faint, sharp-edged shells surrounding elliptical NGC 5982, evidence of past galactic mergers. It also reveals many even more distant background galaxies.

[https://apod.nasa.gov/apod/ap131016.html]

There are several faint galaxies in Draco, one of which is the lenticular galaxy NGC 5866 (sometimes considered to be Messier Object 102) that bears its name to a small group that also includes the spiral galaxies NGC 5879 and NGC 5907:

NGC 5866 Edge-On

This is a unique NASA Hubble Space Telescope view of the disk galaxy NGC 5866 tilted exactly edge-on to our line-of-sight. Hubble’s sharp vision reveals a crisp dust lane dividing the galaxy into two halves. The image highlights the galaxy’s structure: a subtle, reddish bulge surrounding a bright nucleus, a blue disk of stars running parallel to the dust lane, and a transparent outer halo.

Some faint, wispy trails of dust can be seen meandering away from the disk of the galaxy out into the bulge and inner halo of the galaxy. The outer halo is dotted with numerous gravitationally bound clusters of nearly a million stars each, known as globular clusters. Background galaxies that are millions to billions of light-years farther away than NGC 5866 are also seen through the halo.

NGC 5866 is a disk galaxy of type ‘S0’ (pronounced s-zero). Viewed face on, it would look like a smooth, flat disk with little spiral structure. It remains in the spiral category because of the flatness of the main disk of stars as opposed to the more spherically rotund (or ellipsoidal) class of galaxies called ‘ellipticals.’ Such S0 galaxies, with disks like spirals and large bulges like ellipticals, are called ‘lenticular’ galaxies.

The dust lane is slightly warped compared to the disk of starlight. This warp indicates that NGC 5866 may have undergone a gravitational tidal disturbance in the distant past, by a close encounter with another galaxy. This is plausible because it is the largest member of a small cluster known as the NGC 5866 group of galaxies. The starlight disk in NGC 5866 extends well beyond the dust disk. This means that dust and gas still in the galaxy and potentially available to form stars does not stretch nearly as far out in the disk as it did when most of these stars in the disk were formed.

The Hubble image shows that NGC 5866 shares another property with the more gas-rich spiral galaxies. Numerous filaments that reach out perpendicular to the disk punctuate the edges of the dust lane. These are short-lived on an astronomical scale, since clouds of dust and gas will lose energy to collisions among themselves and collapse to a thin, flat disk.

For spiral galaxies, the incidence of these fingers of dust correlates well with indicators of how many stars have been formed recently, as the input of energy from young massive stars moves gas and dust around to create these structures. The thinness of dust lanes in S0s has been discussed in ground-based galaxy atlases, but it took the resolution of Hubble to show that they can have their own smaller fingers and chimneys of dust.

NGC 5866 lies in the Northern constellation Draco, at a distance of 44 million light-years (13.5 Megaparsecs). It has a diameter of roughly 60,000 light-years (18,400 parsecs) only two-thirds the diameter of the Milky Way, although its mass is similar to our galaxy. This Hubble image of NGC 5866 is a combination of blue, green and red observations taken with the Advanced Camera for Surveys in November 2005.

[http://www.wolaver.org/Space/NGC5866.htm]

NGC 5907, located approximately 50 million light years from Earth, has long been considered a prototypical example of a warped spiral in relative isolation. In 2006 an international team of astronomers announced the presence of an extended tidal stream surrounding the galaxy that challenges this picture and suggests the gravitational perturbations induced by the stream progenitor may be the cause for the warp:

[https://en.wikipedia.org/wiki/NGC_5907]

The Star Streams of NGC 5907

Grand tidal streams of stars seem to surround galaxy NGC 5907. The arcing structures form tenuous loops extending more than 150,000 light-years from the narrow, edge-on spiral, also known as the Splinter or Knife Edge Galaxy. Recorded only in very deep exposures, the streams likely represent the ghostly trail of a dwarf galaxy- debris left along the orbit of a smaller satellite galaxy that was gradually torn apart and merged with NGC 5907 over four billion years ago. Ultimately this remarkable discovery image, from a small robotic observatory in New Mexico, supports the cosmological scenario in which large spiral galaxies, including our own Milky Way, were formed by the accretion of smaller ones. NGC 5907 lies about 40 million light-years distant in the northern constellation Draco.

[https://apod.nasa.gov/apod/ap080619.html]

A dwarf galaxy in Draco is the Draco Dwarf Galaxy, one of the least luminous galaxies with an absolute magnitude of −8.6 and a diameter of only about 3,500 light years, discovered by Albert G. Wilson of Lowell Observatory in 1954. Another one found in this constellation is PGC 39058:

Bright star - faint galaxy

Astronomers are used to encountering challenges in their work, but studying the prosaically-named galaxy PGC 39058 proves more difficult than usual. Due to a stroke of bad luck, a bright star happens to lie between the galaxy and the Earth, meaning our view is partly obscured by the glare of the star. The astounding image from the NASA/ESA Hubble Space Telescope shows the nearby star easily outshining the more distant galaxy PGC 39058. The galaxy is about 14 million light-years away and contains millions of stars- many of them not unlike the bright star in the foreground.

The bright foreground star seems to shine with incredible intensity due to the power of Hubble. Most Earth-bound observers would however consider the star to be quite faint. At magnitude 6.7, binoculars or a small telescope are needed to see it at all. That the image manages to capture both objects serves to further highlight Hubble’s excellent optics and sharp vision.

PGC 39058 is a dwarf galaxy, which explains its faintness despite its modest distance by galaxy standards. The sharp Hubble image easily resolves it completely into its component stars and also reveals many much more distant galaxies in the background.

This star and galaxy pair is located within the constellation of Draco (the Dragon). It is visible in the northern hemisphere, appearing to slither over a large portion of the sky around the north celestial pole. The ancient Greeks claimed that Draco represented Ladon, the dragon with 100 heads. One of Hercules’ twelve near-impossible tasks was to steal golden apples guarded by Ladon. The difficulty of this challenge is perhaps on a par with observing such a faint galaxy obscured by a bright star.

This picture was created from images taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through yellow (F606W, shown as blue) and near infrared (F814W, shown as red) were combined. The exposure times were 20 minutes and 15 minutes respectively and the field of view is 2 × 1.6 arcminutes.

[https://www.spacetelescope.org/images/potw1021a/]

Draco also features several interacting galaxies and galaxy clusters. One such massive cluster is Abell 2218. It acts as a gravitational lens for even more distant background galaxies, allowing astronomers to study those galaxies as well as Abell 2218 itself; more specifically, the lensing effect allows astronomers to confirm the cluster’s mass as determined by x-ray emissions:

Abell 2218: A Galaxy Cluster Lens

Gravity can bend light, allowing huge clusters of galaxies to act as telescopes. Almost all of the bright objects in this released Hubble Space Telescope image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. As a result, multiple images of these background galaxies are distorted into long faint arcs- a simple lensing effect analogous to viewing distant street lamps through a glass of wine. The cluster of galaxies Abell 2218 is itself about three billion light-years away in the northern constellation Draco. The power of this massive cluster telescope has recently allowed astronomers to detect a galaxy at redshift 5.58, the most distant galaxy yet measured. This young, still-maturing galaxy is faintly visible to the lower right of the cluster core.

[http://apod.nasa.gov/apod/ap011007.html]

One of the most well-known interacting galaxies is Arp 188, also called the ‘Tadpole Galaxy.’ The tail of stars drawn off the Tadpole Galaxy appears blue because the gravitational interaction disturbed clouds of gas and sparked star formation:

Arp 188 and the Tadpole’s Tail

In this stunning vista, based on image data from the Hubble Legacy Archive, distant galaxies form a dramatic backdrop for disrupted spiral galaxy Arp 188, the Tadpole Galaxy. The cosmic tadpole is a mere 420 million light-years distant toward the northern constellation Draco. Its eye-catching tail is about 280 thousand light-years long and features massive, bright blue star clusters. One story goes that a more compact intruder galaxy crossed in front of Arp 188- from right to left in this view- and was slung around behind the Tadpole by their gravitational attraction. During the close encounter, tidal forces drew out the spiral galaxy’s stars, gas, and dust forming the spectacular tail. The intruder galaxy itself, estimated to lie about 300 thousand light-years behind the Tadpole, can be seen through foreground spiral arms at the upper right. Following its terrestrial namesake, the Tadpole Galaxy will likely lose its tail as it grows older, the tail’s star clusters forming smaller satellites of the large spiral galaxy.

[http://apod.nasa.gov/apod/ap121108.html]

Q1634+706 is a quasar that holds the distinction of being the most distant object usually visible in an amateur telescope. At magnitude 14.4, it appears star-like, though it is at a distance of 12.9 billion light-years. The light of Q1634+706 has taken 8.6 billion years to reach Earth, a discrepancy attributable to the expansion of the universe.

Draco is home to the February Eta Draconids, a meteor shower that was discovered on February 4, 2011. Observers noted six meteors with a common radiant in a short period. Its parent is a previously unknown long-period comet.

Also, the Hercules- Corona Borealis Great Wall, possibly the largest known structure in the universe, covers a part of the southern region of Draco.

[https://en.wikipedia.org/wiki/Draco_%28constellation%29]

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