Centaurus

Centaurus (the Centaur), is a large southern constellation that lies north of Crux and at the northern end of the Milky Way. It surrounds the Southern Cross forming one of the richest celestial associations.

Notable features

Constellation of Centaurus

In this constellation is Proxima Centauri, the closest star to the Solar System, located only 4,243 light-years away. It is a red dwarf of spectral type M5.5Ve whose luminosity is equivalent to 0.17% of the solar luminosity. Although its luminosity is very low, it is a flare star that undergoes spectacular increases in brightness due to magnetic activity. Of apparent magnitude +11.05, it is not visible to the naked eye. The existence of two planets around Próxima Centauri is known; the innermost is at least 1.18 Earth masses and completes an orbit every 11.2 days. The outermost, with a mass seven times Earth mass, is a "super-Earth" type; or "minineptune" and has an orbital period of 1928 days.

α Centauri, the third brightest star in the night sky, is a binary to which Proxima Centauri appears to be gravitationally bound. The two stars that make up α Centauri are a G2V-type solar analog star —named Rigil Kentaurus— and an orange dwarf —Toliman—, whose orbital period around the common center of mass is 79.9 years. Proxima Centauri is 13,000 astronomical units (AU) from the binary.

β Centauri, officially named Hadar, is another star in the 1st magnitude constellation. It is a triple star system formed by three bluish-white stars. The main component, in turn a spectroscopic binary, is classified as a giant of type B1III; around it, the third component of the system completes one orbit every 357 years.

Followed in brightness by Menkent (θ Centauri), an orange giant with a radius 11 times that of the Sun, and γ Centauri, a binary with two identical stars of spectral type A1 classified as subgiants or giants. The separation between the two γ Centauri stars varies between 8 and 67 AU over the course of their 84.5-year orbital period.

ε Centauri is a hot blue star of spectral type B1 and 23,900 K of surface temperature; it is a Beta Cephei variable with a brightness variation of just 0.02 magnitudes. In fact, many of the bright stars in Centaurus are of spectral type B, as they belong to the Scorpius-Centaurus stellar association, the closest OB stellar association to the Sun. Among them, ζ Centauri and κ Centauri are binary systems where the main component is a subgiant of type B2IV or B2.5IV. Some of them, such as η Centauri, μ Centauri, and ν Centauri, are surrounded by a circumstellar disk related to the fast rotation rate of the star (in the case of η Centauri, equal to or greater than 310 km/s). In contrast, stars such as ξ² Centauri and ρ Centauri are part of the so-called Gould belt, a partial ring of stars that includes a multitude of recently formed hot and massive stars.

Centaurus contains several interesting variable stars. R Centauri is a peculiar Mira variable: its brightness varies between magnitude +5.30 and +11.80 in a period of 546.2 days but its light curve presents a double maximum, and, in addition, its period and amplitude have been steadily declining over the past 50 years. UY Centauri is a semiregular variable and prototype of SC stars, very cool stars with intermediate characteristics between S stars and carbon stars. On the other hand, RR Centauri is a contact binary whose two components share its outer shell of gas; the two stars are only 3.94 solar radii or 0.018 AU apart. a Centauri (V761 Centauri) is a chemically peculiar Bp star that presents a great variation in the abundance of helium, with 2 clearly differentiated hemispheres, one rich in helium and the other poor in this element. In this sense, Przybylski's Star (V816 Centauri) is an extreme peculiar star, with strong spectral lines of lanthanide elements; it is a pulsating star prototype of rapidly oscillating Ap (roAp) stars. Another Ap star, V827 Centauri, has one of the largest magnetic fields ever detected, 17,300 times that of the Sun. Another notable object is Krzeminski's Star (V779 Centauri), an eclipsing binary system made up of a neutron star (Centaurus X-3) and a hot blue giant. The neutron star is an X-ray pulsar with a period of 4.84 seconds.

Circular disk around HR 4796 A (ESO VLT Image)

In addition to Proxima Centauri, there are other stars with planetary systems in this constellation. HD 113538, an orange dwarf of spectral type K9V considerably cooler and dimmer than the Sun, has two similar planets to the Saturn-Jupiter pair, in this case at 0.7 and 2.4 astronomical units respectively. Much more similar to the Sun is HD 114729, around which a planet with a mass equal to or greater than 95% of the mass of Jupiter moves in an eccentric orbit. Similarly, Dofida —name designating HD117618— is another yellow dwarf with an extrasolar planet 0.17 AU from the star. Another Sun-like star, HR 4523, hosts a planet whose mass is at least 16 times that of Earth and which completes an orbit every 122 days; this star forms a wide binary system with a red dwarf whose current separation is 221 au. On the other hand, around the young star HR 4796, only 8 million years old, a protoplanetary disk has been found that appears to contain granules of complex tholin-type organic matter.

WD 1310-472 is a lone and cold white dwarf in this constellation, its temperature being only 4158 K. Since white dwarfs do not generate energy by nuclear fusion, their surface temperatures decrease over time, the estimated age of WD 1310-472—as a stellar remnant—of 8.87 billion years.

Omega Centauri globular cluster. Credits: ESO.

Centaurus is home to one of the peculiar objects in the universe, the Boomerang Nebula, a planetary protonebula whose temperature is only 1 K (−272 °C) —one degree above absolute zero—, the coldest temperature that has been recorded outside of a laboratory. It is the only known object with a temperature below the microwave background radiation.

Omega Centauri (NGC 5139) stands out among the star clusters in the constellation. This globular cluster was already recorded by Ptolemy, being rediscovered by Edmond Halley in 1677, who identified it as a non-stellar object. It is estimated to contain about ten million stars and a total mass equivalent to four million solar masses. It is also the largest and brightest globular cluster in the Milky Way, as well as the most luminous globular cluster in our galaxy, its luminosity being more than a million times greater than that of the Sun. Centaurus also has two open clusters: NGC 3766 or the Pearl cluster is a relatively young cluster —it has an estimated age of 14.4 million years— and is located about 5,700 light years from Earth; for its part, NGC 5460 is a sparse cluster 2,300 light-years away.

Numerous galaxies can be observed in this constellation, including Centaurus A (NGC 5128), the fifth brightest galaxy in the sky and one of the closest radio galaxies to Earth. There is no consensus as to how far away it is: various sources place it at a distance between 11 and 15 million light-years. At its core is a supermassive black hole that ejects massive jets of matter that emit radio waves due to synchrotron radiation. Centaurus A is thought to have merged with at least one spiral galaxy in the past.

Infrared image of RCW 86, rest of the supernova SN 185

NGC 4945 is a galaxy with characteristics similar to the Milky Way that can be observed near ζ Centauri. It is part of the Centaurus cluster, one of the closest clusters of galaxies to ours. Another spiral galaxy in this cluster, which appears head-on to our point of view, is NGC 4622. It is a rare example of a spiral galaxy with arms pointing in opposite directions, probably the result of its interaction with another galaxy in the past.

On the border between this constellation and Circinus, near α Centauri, a supernova today called SN 185 could be observed at the end of the year 185. Recorded by Chinese astronomers on December 7 of that year, it is thought to be the first supernova of which there is a historical record.

Main Stars

Image of Next Centauri, the star closest to the solar system, obtained by the Hubble space telescope.

Relative sizes of the stars of the α Centauri system compared to the Sun (to the left)

• α Centauri (Rigil Kentaurus), star of magnitude -0,01, the brightest fourth in the sky that is only 4,37 light years from Earth. It is a triple star system whose secondary star can be solved with medium-open telescopes. The third component, Next Centauri, is a red dwarf with 1/7 of the solar diameter at a distance of 4.2 light years from the Sun, making it the closest star to Earth (after the Sun).
• β Centauri (Hadar or Agena), second brightest star of the constellation with magnitude 0.61 and the brightest tenth of the sky. Blue Giant located about 390 light years away, has a separate companion 1.3sec arc. In turn, the main star is a spectroscopic binary.
• γ Centauri, double visual star whose components are two white stars of equal shine. A telescope of at least 150 mm of opening is required for its resolution.
• δ Centauri, of magnitude 2,60 is a variable star Gamma Cassiopeiae.
• ε Centauri, hot blue star of magnitude 2,29, is a Beta Cephei variable with a variation of 0.02 magnitudes.
• γ Centauri, of magnitude 2,55 is a spectroscopic binary star.
• MIL Centauri, white-azulada Be star with a ring disk around. It's also a Gamma Cassiopeiae variable.
• θ Centauri (Menkent), with magnitude 2,06 is the third brightest star of the constellation. Unlike other stars of the constellation that are white-blue, Menkent is an orange giant located 61 light years from Earth.
• . Centauri, white star of magnitude 2.75 located at 59 light years of our planet. Like Vega, he has a circum-stellar disk around him.
• κ Centauri, white-azulada star with a tenuous companion of magnitude 11 to 3.9 seconds of arc.
• λ Centauri, white-azulada giant of magnitude 3,12.
• μ Centauri, white-azulada star of magnitude 3,46 surrounded by a circum-stellar disk.
• ν Centauri, star of magnitude 3.39 and variable Beta Cephei.
• φ2 Centauri, star system of magnitude 4,26 whose main star is a white-azulada star.
• ρ Centauri, white-azulada star of magnitude 3.96.
• σ Centauri, almost as bright as the previous one, is also a white-azulada star.
• τ Centauri, white star of the main sequence of magnitude 3,85.
• /25070/ Centauri, binaria eclipsante of average magnitude 4.05.
• C2 Centauri, star Am of magnitude 5.26.
• j Centauri (HD 102776), star Be of magnitude 4,31.
• 1 Centauri, white-yellow star of magnitude 4,23 and spectroscopic binary.
• 2 Centauri, red giant and semi-regular variable of average magnitude 4,19.
• 3 Centauri, binary whose components are separated 8 seconds of arc; the most brilliant is a chemically peculiar star.
  

  Light curve of the V Centauri cefeide
• R Centauri, variable Mira whose brightness ranges between magnitude 5.3 and 11.8 in a period of 546.2 days.
• T Centauri, semi-regular variable north of the constellation whose magnitude varies between 5.90 and 9.0.
• U Centauri, variable Look of variable shine between magnitude 7 and 14 with a period of 220,28 days.
• V Centauri, a strained variable ranging from 6.40 to 7.20 in a 5-day and 11-hour cycle.
• RR Centauri, contact binary and eclipsing binary of magnitude 7.45.
• SS Centauri, eclipsing binary of magnitude 9,4.
• SV Centauri, variable W Ursae Majoris composed of two warm stars of type B.
• SZ Centauri, 8,89 magnitude eclipsing binary composed of two white stars.
• TT Centauri, variable Mira with a period of 462 days.
• UY Centauri, semi-regular variable and prototype of the SC stars.
• XX Centauri, also a strain whose magnitude varies between 7.30 and 8.31.
• V636 Centauri, binaria eclipsante of maximum magnitude 8,70.
• V761 Centauri (a Centauri), a white-azulada star that exhibits extreme variability in terms of its helium abundance.
• V824 Centauri (HD 114365), Ap star and variable Alpha2 Canum Venaticorum.

Artistic impression of the protoplanetary disc that surrounds HD 113766 A.

• Krzeminski Star (V779 Centauri), binary system composed of a neutron star (Centaurus X-3) and a blue giant hot.
• HR 4523 (HD 102365), binary star at 30.1 light years, composed of a yellow dwarf and a red dwarf.
• HR 4796 (HD 109573), young binary star where a protoplanetary disk has been detected around the main star.
• HD 101930 and HD 114386, both orange dwarfs; each of them houses an extrasolate planet.
• HD 113538, dark orange dwarf with two extra-solar planets.
• HD 113766, also a binary system with a hot dust disk around one of the stars.
• HD 114729, old yellow dwarf that houses a planetary system.
• HD 116713, barium star of magnitude 5.11.
• HD 131399, three-star system with an exoplanet, HD 131399 Ab.
• Gliese 438, red dwarf at 27.4 light years of the Earth.
• Gliese 542 (HD 125072) and Gliese 435 (HD 101581), orange dwarfs located at 38.6 and 40.8 light years respectively.
• Przybylski Star (HD 101065), the most extreme peculiar star whose chemical anomalies still remain unexplained.
• GJ 3770 (WD 1310-472), dark white dwarf very cold and old.

Deep Sky Objects

Boomerang nebula, distant planetary protonebulous about 5000 light years.

Cumulus and nebulae

• NGC 3766, also known as the "Cluster of Pearl", is an open cluster discovered by Nicolas-Louis de Lacaille in 1752.
• NGC 3918, planetary nebula of magnitude 8.1. It has a roughly spherical outer wrap and is about 3000 light years away.
• NGC 5307, distant planetary nebula about 10,000 light years, of magnitude 11.2. It has a very symmetrical spiral or molinete structure.
• Omega Centauri (NGC 5139). Ascension Recta: 13h 26m 48.0s Declination: -47°29'00" (Epoch 2000). Glybular cluster visible to the simple view as a diffuse patch, spectacular with small and binocular telescopes. Orbit the Milky Way and is one of the largest and brightest globular clusters associated with our galaxy. It contains several million population II stars with an age of 12 million years.
• IC 2944 is an open cluster with an associated emission nebula.
• Boomerang Nebula, planetary protonebulous whose temperature of 1 K makes it the coldest known object in the universe.
• Very close to Alfa Centauri is RCW 86, rest of the supernova SN 185.
• Remains of supernova SNR G292.0+01.8, Kesteven 17 and SNR G306.3-00.9; the first, distant 6000 pársecs, contains the PSR complex J1124-5916.

Galaxies

Image of NGC 4622 obtained with Hubble space telescope.

• NGC 4603, spiral galaxy to 107 million light years that is part of Centauro cluster.
• NGC 4622, non-barred spiral galaxy front view. It has a very prominent ring structure and is 111 million light years away.
• NGC 4650A, annular polar galaxy of magnitude 13,9.
• NGC 4696, elliptical galaxy and the brightest member of the Centaur cluster. It is 116 million light years from Earth.
• NGC 4945. Discovered by James Dunlop in 1826, it is one of the most brilliant galaxies in the Centaurus A/M83 group. It is 11.7 million light years away.
• Centaurus A (NGC 5128). AR: 13h 25m 30.0s Dec: -43°01'00" (Epoca 2000). Lenticular galaxy distant 14 million light years approximately. It is one of the closest radiogalaxias to Earth, so its active galactic core has been widely studied by professional astronomers. It is, with magnitude 6.9, the fifth most brilliant galaxy in the sky.
• NGC 5253. AR: 13h 39m 54.0s Dec: -31°39'00" (Epoch 2000). Irregular Galaxy discovered by John Frederick William Herschel on March 15, 1787. It is part of the M83 subgroup within the Centaurus A/M83 group.
• NGC 5408. Irregular Galaxy located about 15.7 million light years.

Mythology

According to Greek mythology, Zeus decided to kill Ixion, king of Thessaly, so that he could live among the gods and thus overcome the mental torture he suffered for killing his father-in-law. However, far from being grateful, Ixion tried to seduce Hera. Zeus realized this and formed a Hera made of clouds: Nephele. Ixion slept with her and Kentauros was born, a being with the legs and body of a horse and the head, arms and torso of a human. Kentauros bred the Centaurs with some mares. This race did not respect the laws of hospitality, nor was it governed by the rules of peaceful coexistence. The only exceptions were Pholo and Chiron.

Chiron mentored several heroes, including Ajax, Hercules, Achilles, Jason, and Theseus. Chiron died accidentally wounded by Heracles, who had been using arrows poisoned with the blood of the Hydra of Lerna, so Zeus, in honor of Chiron, gave him a place among the constellations.

Text references

General references

• _ Catasterism (Καταστερισμο).
  • 40: Centauro (EURείρων, Κενταυρός; Chiron, Centaurus): Quirón.
    • Spanish text on the site of the University of Cantabria.
      • Latin text in Google Books; electronic facsimile.
        • Greek text on the Internet Archive; electronic facsimile.
• HIGINO: Poetry astronomy (Astronomica).
  • 38: Centauro.
    • English text on Mary Grant's Theoi site; 1960 trad.