Thuban (α Draconis) was the northernpole star from 3942 BC, when it moved farther north thanTheta Boötis, until 1793 BC. TheEgyptian Pyramids were designed to have one side facing north, with an entrance passage geometrically aligned so that Thuban would be visible at night.[2] Due to the effects ofprecession, 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".[1]
There are three stars above magnitude 3 in Draco. The brighter of the three, and the brightest star in Draco, isGamma Draconis, traditionally called Etamin or Eltanin. It is an orange giant star of magnitude 2.2, 148 light-years from Earth. Theaberration of starlight was discovered in 1728 whenJames Bradley observed Gamma Draconis. NearbyBeta 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".[1] Draco also features several interacting galaxies and galaxy clusters. One such massive cluster is Abell 2218, located at a distance of 3 billion light-years (redshift 0.171).
Draco is home to several double stars and binary stars.Eta Draconis (traditionally called Athebyne[3]) is adouble star with a yellow-hued primary of magnitude 2.8 and a white-hued secondary of magnitude 8.2 located south of the primary. The two are separated by 4.8arcseconds.[4]Mu Draconis (traditionally called Alrakis) is abinary star with two white components. Magnitude 5.6 and 5.7, the two components orbit each other every 670 years. The Alrakis system is 88 light-years from Earth.Nu Draconis is a similar binary star with two white components, 100 light-years from Earth. Both components are of magnitude 4.9 and can be distinguished in a small amateur telescope or a pair of binoculars.Omicron Draconis is a double star divisible in small telescopes. The primary is an orange giant of magnitude 4.6, 322 light-years from Earth. The secondary is of magnitude 7.8.Psi Draconis (traditionally called Dziban[3]) is a binary star divisible in binoculars and small amateur telescopes, 72 light-years from Earth. The primary is a yellow-white star of magnitude 4.6 and the secondary is a yellow star of magnitude 5.8.16 Draconis and17 Draconis are part of a triple star 400 light-years from Earth, visible in medium-sized amateur telescopes. The primary, a blue-white star of magnitude 5.1, is itself a binary with components of magnitude 5.4 and 6.5. The secondary is of magnitude 5.5 and the system is 400 light-years away.[1]20 Draconis is a binary star with a white-hued primary of magnitude 7.1 and a yellow-hued secondary of magnitude 7.3 located east-northeast of the primary. The two are separated by 1.2 arcseconds at their maximum and have an orbital period of 420 years. As of 2012, the two components are approaching their maximum separation.[4]39 Draconis is a triple star 188 light-years from Earth, divisible in small amateur telescopes. The primary is a blue star of magnitude 5.0, the secondary is a yellow star of magnitude 7.4, and the tertiary is a star of magnitude 8.0; the tertiary appears to be a close companion to the primary.40 Draconis and41 Draconis are a binary star divisible in small telescopes. The two orange dwarf stars are 170 light-years from Earth and are of magnitude 5.7 and 6.1.[1]
R Draconis is a redMira-type variable star with a period of about 8 months. Its average minimum magnitude is approximately 12.4, and its average maximum magnitude is approximately 7.6. It was discovered to be a variable star byHans Geelmuyden in 1876.[5]
The constellation contains the star recently namedKepler-10, which has been confirmed to be orbited byKepler-10b.
One of thedeep-sky objects in Draco is theCat's Eye Nebula (NGC 6543), aplanetary nebula approximately 3,000 light-years away that was discovered by English astronomerWilliam Herschel in 1786.[4] It is 9th magnitude and was named for its appearance in theHubble Space Telescope, though it appears as a fuzzy blue-green disk in an amateur telescope.[1] NGC 6543 has a very complex shape due to gravitational interactions between the components of themultiple star at its center, the progenitor of the nebula approximately 1,000 years ago.[6] It is located 9.6arcminutes away from thenorth ecliptic pole to the west-northwest. It is also related toIC 4677, a nebula that appears as a bar 1.8 arcminutes to the west of the Cat's Eye Nebula. In long-term exposures, IC 4677 appears as a portion of a ring surrounding the planetary nebula.[4]
PGC 39058, a dwarf galaxy found within the Draco constellation – picture taken by ESA/Hubble & NASA.
Draco also features severalinteracting galaxies andgalaxy clusters. One such massive cluster isAbell 2218, located at a distance of 3 billion light-years (redshift 0.171). It acts as agravitational 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 byx-ray emissions. One of the most well-knowninteracting galaxies isArp 188, also called the "Tadpole Galaxy". Named for its appearance, which features a "tail" of stars 280,000 light-years long, the Tadpole Galaxy is at a distance of 420 million light-years (redshift 0.0314). The tail of stars drawn off the Tadpole Galaxy appears blue because the gravitational interaction disturbedclouds of gas and sparkedstar formation.[6]
Q1634+706 is aquasar 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 theexpansion of the universe.[4]
TheHercules–Corona Borealis Great Wall, possibly the largest known structure in the universe, covers a part of the southern region of Draco.
TheCatasterismi attributed toEratosthenes identify Draco asLadon, the dragon who guarded the golden apples of theHesperides.[12] WhenHeracles was tasked with stealing the golden apples during histwelve labors, he killed Ladon andHera transformed Ladon into a constellation.[15] In the sky,Hercules is depicted with one foot on the head of Draco.[16] Sometimes, Draco is represented as the monstrous son ofGaia,Typhon.[12]
Traditional Arabic astronomy does not depict a dragon in modern-day Draco, which is called theMother Camels. Instead, twohyenas, represented byEta Draconis andZeta Draconis are seen attacking a baby camel (a dim star nearBeta Draconis), which is protected by four female camels, represented byBeta Draconis,Gamma Draconis,Nu Draconis, andXi Draconis. The nomads who own the camels are camped nearby, represented by a cooking tripod composed ofUpsilon,Tau, andSigma Draconis.[17] However Arabic astronomers also knew of the Greek interpretation of the constellation, referring to it inArabic asAt-Tinnin (التنين, 'the dragon'), which is the source of the formal name of Gamma Draconis, Eltanin, fromraʾs al-tinnīn ('the head of the dragon').[18]
The OctoberDraconids, also called Giacobinids, is ameteor shower associated with the periodic comet21P/Giacobini-Zinner. The shower peaks on 8 October and it has experienced storms in 1933 and 1946, when thezenithal hourly rate (ZHR) was up to 10,000 meteors per hour.[19] Further outbursts were observed in 1985, 1998,[19] and 2011. During the 2011 outburst, ZHR reached 400 meteors/hour, however it was largely unnoticed visually due to interference by the bright Moon.[20]
TheFebruary Eta Draconids is a meteor shower that was discovered on February 4, 2011. Observers noted six meteors with a commonradiant in a short period. Its parent is a previously unknownlong-period comet.[21]
The main character in the 1996 filmDragonheart gets his name from this constellation. The film also reveals that Draco is actually a dragon heaven, where dragons go when their time in this world is complete, if they have upheld the oath of an ancient dragon to guard mankind, with dragons otherwise fading into nothing upon their deaths. At the conclusion of the film, Draco, the last dragon, ascends into the constellation after he sacrifices himself to destroy an evil king.[citation needed]
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