Centaurus

• 1. Introduction

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  Centaurus is a bright constellation in the southern sky, and it harbors a star that sits closer to our Sun than any other in the universe. That star is Proxima Centauri, a faint red dwarf just 4.4 light-years away, and it is only one feature of a constellation so rich and vast that it outstrips every other patch of sky in the number of stars visible to the naked eye. With 281 stars above magnitude 6.5, Centaurus holds the record. But sheer star count is only the beginning. Inside its borders lie the nearest star system to Earth, the largest and brightest globular cluster in the Milky Way, a galaxy with jets of matter blasting from a supermassive black hole, and a white dwarf that scientists have nicknamed "Lucy" after a Beatles song. What is Centaurus, and why does this single constellation contain so many of the sky's most extreme objects? How did it travel from the equatorial sky of ancient Babylon to the deep southern heavens we see today? And who is the centaur the Greeks immortalized here, and why does his story end with a poisoned arrow?

• 2. A Sky Full of Records

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  Alpha Centauri is not one star but three, bound together in a system with an overall magnitude of -0.28. Two yellow-hued stars form a close binary pair, orbiting each other on an 80-year cycle. They will appear closest together as seen from Earth in 2037 and 2038. The third member is Proxima Centauri, a red dwarf of magnitude 11.0 orbiting at roughly 2 degrees of apparent separation from its companions, on a period of approximately one million years. Proxima is a flare star, capable of brightening by over a magnitude in outbursts that last only minutes. The Arabic name Rigil Kentaurus, meaning "foot of the centaur," was given to the Alpha Centauri system long before astronomers understood its true nature.

  Beta Centauri, also called Hadar and Agena, sits 525 light-years from Earth. A blue-hued giant of magnitude 0.6, it serves alongside Alpha as one of the two "southern pointer stars" that guide observers to the constellation Crux. In roughly 4,000 years, the proper motion of Alpha will have carried it to within half a degree of Beta, an exceptionally close pairing by celestial standards.

  Theta Centauri, officially named Menkent, carries a distinction that neither Alpha nor Beta can claim: it is the only bright star of Centaurus easily visible from mid-northern latitudes. An orange giant of magnitude 2.06, it marks the northerly reaches of this otherwise southern-sky constellation.

• 3. The Diamond Star and the Baby Planet

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  BPM 37093 is a white dwarf whose carbon atoms are thought to have crystallized, and scientists nicknamed it "Lucy" after the Beatles song "Lucy in the Sky with Diamonds." Diamond is also carbon arranged in a crystalline lattice, though in a different configuration, making the analogy imprecise but irresistible to the researchers who coined the name.

  PDS 70, also catalogued as V1032 Centauri, is a young, low-mass T Tauri star. In July 2018, astronomers captured the first conclusive image of a protoplanetary disk containing a newly forming exoplanet, designated PDS 70b. The image was the first of its kind, documenting a planet in the act of formation rather than inferring its existence indirectly.

  Centaurus also hosts R Centauri, a Mira variable star that swings between a minimum magnitude of 11.8 and a maximum of 5.3 over a period of 18 months. At about 1,250 light-years from Earth, its regular pulsations made it a well-studied example of a class of stars named after the star Mira in Cetus. V810 Centauri is another variable in the constellation, classified as semiregular rather than following a strict cycle.

• 4. Omega Centauri

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  Omega Centauri, catalogued as NGC 5139, appears to the naked eye as a fuzzy patch roughly half a degree across, the same apparent size as the full Moon. That apparent fuzziness conceals an object of extraordinary scale: a globular cluster 17,000 light-years away, 150 light-years in diameter, containing several million stars. It is the largest and brightest globular cluster in the Milky Way, ten times the size of the next-largest cluster, and shines at magnitude 3.7.

  Most of its stars are yellow dwarfs, but the cluster also contains red giants and blue-white stars. Their average age is 12 billion years, a fact that has led astronomers to suspect that Omega Centauri was once the core of a dwarf galaxy absorbed by the Milky Way long ago. The cluster has a Shapley class VIII designation, indicating a loosely concentrated center, and it is one of only two globular clusters to receive a Bayer letter stellar designation rather than simply a catalogue number.

  Edmond Halley, the English astronomer, determined in 1677 that Omega Centauri was a nonstellar object, though the ancients had seen it simply as a star. Its true nature as a globular cluster was established by James Dunlop in 1827. It is also the most luminous globular cluster in the Milky Way, exceeding one million solar luminosities.

• 5. Galaxies at the Edge

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  Centaurus A, catalogued as NGC 5128, sits just 11 million light-years from Earth, making it one of the closest active galaxies known. A supermassive black hole at its core fires massive jets of matter outward; those jets emit radio waves through a process called synchrotron radiation. The galaxy's most striking visual feature is a dark dust lane cutting across its elliptical body, a structure unusual in elliptical galaxies and interpreted as evidence of a past merger, probably with a spiral galaxy. Under perfect conditions, NGC 5128 can be seen with the naked eye, placing it among the most distant objects a human can spot without optical aid.

  NGC 4622, a spiral galaxy 200 million light-years away, presents a unique puzzle: its spiral arms wind in both directions simultaneously, making it nearly impossible to determine which way the galaxy rotates. Astronomers suspect a collision with a smaller companion near the core may have reversed the direction of one set of arms.

  NGC 4650A, a polar-ring galaxy 136 million light-years away, shows a core of older stars surrounded by an outer ring of young stars orbiting on a distorted plane. The stars in that outer ring move too fast for the visible mass to explain, which has made NGC 4650A a reference case in dark matter research. The leading explanation is a surrounding dark matter halo providing the gravitational pull that keeps those stars in orbit.

• 6. From Babylon to Greece

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  Centaurus has not always occupied its current position in the far southern sky. At the dawn of civilization, precession had not yet carried it southward, and it rode the celestial equator where many early cultures could observe it. In roughly 7,000 years, precession will swing it back toward high northern visibility.

  The figure traces its roots to a Babylonian constellation known as the Bison-man, catalogued with the designation MUL.GUD.ALIM. This being appeared in two forms: a four-legged bison with a human head, and a creature with a man's torso joined to the hindquarters of a bull or bison. From early times the Bison-man was closely linked to the sun god Utu-Shamash. By the late 3rd millennium BCE, however, the Bison-man had been displaced by a new constellation, the Wild Boar.

  The Greeks gave it the centaur form and the name it carries today. Eudoxus mentioned it in the 4th century BC, and Aratus described it in the 3rd century BC. In the 2nd century AD, Claudius Ptolemy catalogued 37 stars in Centaurus. The constellation was even larger in ancient times. Lupus was treated as an asterism within it. The Southern Cross, now a separate constellation, was regarded as merely the centaur's legs. And the small modern constellation Circinus occupied undefined stars beneath the centaur's front hooves.

• 7. Chiron's Legacy

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  The Roman poet Ovid, in Fasti v.379, identified the constellation with Chiron, the centaur who served as tutor to Heracles, Theseus, and Jason, the leader of the Argonauts. Chiron was exceptional among centaurs for his wisdom and gentleness, qualities that set him apart from the more warlike figure represented by Sagittarius, the other centaur of the zodiac.

  The myth that placed Chiron in the sky carries a painful irony. According to the legend, Hercules accidentally struck Chiron with a poisoned arrow. Chiron, immortal but unable to heal from the wound, was subsequently placed among the stars. The constellation he became is the one that holds Proxima Centauri, the nearest star to the Sun, and Omega Centauri, the cluster now thought to be the surviving core of a consumed galaxy. Chiron the teacher, who shaped the heroes of an age, now marks the part of the sky where astronomers captured the first image of a planet being born around PDS 70b.