Crux

• 1. Introduction

  ›

  Crux is a constellation of the southern sky built around four bright stars in a cross-shaped pattern known as the Southern Cross, and it is the smallest of all 88 modern constellations. Every one of those four main stars is bright enough to see easily, each shining at magnitude 2.8 or better, yet by 400 AD the same stars had vanished entirely from the skies over most of Europe. How does a landmark so small, so far south that it disappeared from an entire continent's view, end up on the flags of five nations and in the hands of navigators, gauchos, and star-mapping explorers half a world apart? And what is actually hiding inside that tiny patch of sky, beyond the four stars everyone recognizes?

• 2. The Vanishing Stars

  ›

  Ptolemy, the Greek astronomer, folded the bright stars of Crux into the larger constellation Centaurus rather than treating them separately, and in the fourth millennium BC those same stars were visible as far north as Britain. The precession of the equinoxes slowly dragged them below the European horizon over the following centuries, and scholars still debate whether Dante's description of four bright southern stars in the Divine Comedy, written in the 14th century, was a genuine sighting or an allegorical coincidence.

  Venetian navigator Alvise Cadamosto likely spotted the cross on exiting the Gambia River in 1455, naming it the carro dell'ostro, or southern chariot, though his accompanying sketch was inaccurate. Credit for the first correct depiction usually goes to João Faras, who sketched and described the constellation, calling it las guardas, in a letter written from the beaches of Brazil on the 1st of May 1500 to the Portuguese monarch. Amerigo Vespucci appears to have observed both the cross and the neighboring Coalsack Nebula during his second voyage, from 1501 to 1502.

  Italian navigator Andrea Corsali, sailing to China and the East Indies between 1515 and 1517 under sponsorship from King Manuel I, wrote a letter in 1516 describing the southern sky in more explicit detail, including a crude map of the stars around the south celestial pole. Emery Molyneux and Petrus Plancius both get credit as the first sky-mappers to treat Crux as its own constellation, in depictions dating to 1592, though both relied on unreliable sources and placed it in the wrong position. Crux finally appeared correctly placed on the celestial globes of Plancius and Jodocus Hondius in 1598 and 1600, and Frederick de Houtman catalogued its stars separately from Centaurus in 1603. Jakob Bartsch adopted the constellation in 1624 and Augustin Royer in 1679, though Royer is sometimes wrongly credited as the first to separate it out.

• 3. A Cross With Many Names

  ›

  In India, a story credits the sage Vishwamitra with creating Trishanku Swarga, meaning Cross, out of the stars now known as Crux. Chinese astronomers called the same four stars Shí Zì Jià, meaning Cross, referring to an asterism of Gamma, Alpha, Beta, and Delta Crucis. Several Australian Aboriginal cultures also read Crux together with the neighboring Coalsack as the head of a much larger shape called the Emu in the Sky, traced not through the stars themselves but through the dark, starless spaces between them. Read as individual stars rather than dark shapes, the same four points became a possum sitting in a tree to the Boorong people of northwestern Victoria, the sky deity Mirrabooka to the Quandamooka people of Stradbroke Island, a stingray to the Yolngu people of Arnhem Land, and an eagle to the Kaurna people of the Adelaide Plains.

  Peoples across the East Indies and Brazil pictured the four stars as the body of a ray, called Bintang Pari in Indonesia and Buruj Pari in Malaysia, while in Vietnam the constellation was once known as sao Cá Liệt, the ponyfish star. Filipino names for it include kasing, paglong, and pasil, and it is called butiti, or puffer fish, in Waray. Javanese people called it Gubug pèncèng, or raking hut, because of its shape.

  In Mapudungun, the language of the Patagonian Mapuche, the Southern Cross is Melipal, meaning four stars. The Quechua of the Inca civilization call it Chakana, or stair, a name carrying deep symbolism within Quechua mysticism. The Bororo of Brazil saw Alpha and Beta Crucis as one foot of a Great Rhea constellation whose head was marked by Gamma Centauri and whose feet fell among the stars of Musca, while the Mocoví people of Argentina saw that same rhea under attack by two dogs, their heads marked by Alpha and Beta Centauri. The Kalapalo people of Brazil's Mato Grosso region instead saw angry bees, called Aganagi, emerging from the Coalsack, which they pictured as the beehive itself, and the neighboring Bakairi people wove the same stars into a still larger constellation representing a bird snare, stretching across Centaurus, Circinus, Lupus, and Musca. The Tswana people of Botswana saw two giraffes, called Dithutlwa, with Alpha and Beta Crucis forming a male and Gamma and Delta forming a female.

• 4. Voyaging Names of the Pacific

  ›

  Torres Strait Islanders saw the star Gamma Centauri as the handle of a fishing spear, with the four Crux stars forming the left hand of a figure called Tagai, while the stars of neighboring Musca became the trident of the spear he holds. In the Aranda traditions of central Australia, the four Cross stars became the talon of an eagle, with Gamma Centauri marking its leg. Bugis sailors used the Southern Cross, together with the star Mu Crucis, as a navigation aid they called bintoéng bola képpang, meaning incomplete house star.

  The Māori call the Southern Cross Māhutonga and picture it as the anchor, Te Punga, of the canoe Tama-rereti steers across the sky, the Milky Way, with the Pointer stars serving as its rope. In Tonga the constellation is Toloa, a duck flying south with one wing wounded, said to have been struck by a stone thrown by two men represented by Alpha and Beta Centauri; the nearby Coalsack is Humu, the triggerfish, for its shape, a name echoed in Samoa as Sumu for the same reason. Peoples of the Solomon Islands saw a knee protector and a net used to catch Palolo worms in the same stars, the Marshall Islands saw a fish, and Peninsular Malays saw the likeness of the Scomberomorus, known locally as Tohok. Among the Tuareg, the four brightest stars of Crux are considered to be four Maerua crassifolia trees, called iggaren.

• 5. Anatomy of the Cross

  ›

  Crux is bordered by Centaurus on three sides and by Musca to the south, covering just 68.4 square degrees, or 0.166 percent of the night sky, which makes it the smallest of the 88 constellations. The International Astronomical Union adopted Cru as its three-letter abbreviation in 1922, and Belgian astronomer Eugène Delporte fixed its official boundaries, a four-sided polygon, in 1930, with the constellation's declination running between 55.68 and 64.70 degrees south.

  Observers in tropical regions can see Crux from April to June, and it sits exactly opposite Cassiopeia on the celestial sphere, so the two constellations never share the sky at the same time. South of the 34th parallel, around Cape Town, Adelaide, or Buenos Aires, Crux is circumpolar and stays visible year-round; from a latitude of 25 degrees north, such as Cancún, it can still be glimpsed near the horizon around 10 pm at the end of April. Stargazers sometimes mistake it for the nearby False Cross, an asterism formed from stars in Carina and Vela that is larger, dimmer, and missing both the fifth star and the two prominent Pointer stars that flank the real cross; between the real cross and its imitator sits a third, even larger and dimmer pattern known as the Diamond Cross.

  Over the next few thousand years, precession will push Crux closer to the South Pole, carrying its middle to a declination of 67 degrees south before the trend reverses. By the year 14,000, Crux will become visible again across most of Europe and the continental United States, and by 18,000 its visibility will stretch as far as Northern Europe.

• 6. Navigating by the Cross

  ›

  In the Southern Hemisphere, sailors and travelers use the Southern Cross for orientation much as Polaris is used in the north. Projecting a line from Gamma to Alpha Crucis, the foot of the crucifix shape, roughly four and a half times its own length lands close to the Southern Celestial Pole. That point also happens to be where a perpendicular line from Alpha and Beta Centauri, the two stars known as the Pointers, crosses it. Argentine gauchos are documented using Crux this way for night orientation across the Pampas and Patagonia.

  Very few bright stars occupy the empty stretch of sky between Crux and the pole itself, which is part of what makes the method reliable. The one exception nearby is Musca, a constellation faint enough to be unremarkable on its own but distinctive enough, sitting immediately south of Crux, to help confirm a navigator has found the right patch of sky before trusting the bearing. Alpha and Beta Centauri themselves sit at a declination similar to Crux and span a width comparable to the cross, but shine at a noticeably higher magnitude, which is precisely what makes them easy to single out as the Pointers rather than mistake for part of the cross.

• 7. What's Hidden Inside the Cross

  ›

  Nicolas-Louis de Lacaille assigned 13 stars the Bayer designations Alpha through Lambda in 1756, and in 1879 astronomer Benjamin Gould added Mu Crucis, judging its stars bright enough to deserve names. Within Crux's borders, 49 stars shine at magnitude 6.5 or brighter, bright enough to spot without a telescope under good conditions. Acrux, or Alpha Crucis, is a triple star 321 light-years away and the brightest point in the cross at magnitude 0.8; its two close components, of magnitude 1.3 and 1.8, are visible together in a small amateur telescope. Mimosa, or Beta Crucis, is a blue giant 353 light-years out that varies by less than 0.1 magnitudes. Gacrux, an optical double 88 light-years from Earth, ranks among the closest red giants to our solar system. A fifth star, Ginan, or Epsilon Crucis, sits within the cross itself, an orange-hued giant of magnitude 3.6 lying 228 light-years away.

  Crux holds four Cepheid variable stars bright enough to glimpse with the naked eye under good conditions, including BG Crucis, which swings between magnitude 5.34 and 5.58 roughly every three and a half days; T Crucis, ranging from 6.32 to 6.83 over about six and three-quarter days; S Crucis, from 6.22 to 6.92 over roughly four and two-thirds days; and R Crucis, from 6.4 to 7.23 over almost five and five-sixths days. Lambda Crucis and Theta2 Crucis add a second family of pulsating stars to the constellation, both classed as Beta Cephei-type variables. A star called BH Crucis, discovered in October 1969 and nicknamed Welch's Red Variable, ranges from magnitude 6.6 to 9.8 over a 530-day cycle, and its period stretched by 25 percent in the three decades after it was first spotted. The exoplanet HD 106906 b, orbiting a star inside the constellation, holds one of the widest orbits of any known planetary-mass companion.

  Crux itself is backlit by the dense star fields of the Scutum-Centaurus Arm, the Milky Way's main inner spiral arm in this part of the galaxy, and two objects within the constellation's own borders partly obscure that glow. The Jewel Box, an open star cluster also called NGC 4755, sits near Crux's eastern edge about one degree from Beta Crucis and appears as a single fuzzy star to the naked eye. John Herschel named it for the range of colors visible through his telescope, and at roughly seven million years old it ranks among the youngest open clusters in the Milky Way. Nearby, the Coalsack Nebula spans about seven degrees by five and forms the most prominent dark patch visible in the southern Milky Way, sitting some 180 parsecs from Earth; found roughly 6.5 degrees southeast of Crux's centre, or 3 degrees east of Alpha Crucis, its edges spill over into the neighboring constellations of Centaurus and Musca.

  Out of the 92 stars bright enough to rank among the sky's most visible, three sit inside Crux, a concentration about 19 times higher than a random, even scattering of stars across the sky's 88 constellations would predict. Fifteen of Crux's 23 brightest stars are hot, blue-white B-type stars, and several, including Delta and probably Alpha and Beta Crucis, belong to the Lower Centaurus-Crux subgroup of the Scorpius-Centaurus association, the nearest such grouping of young, co-moving stars to the Sun, with ages estimated between 10 and 20 million years. Iota Crucis, a visual double 125 light-years away, pairs an orange giant of magnitude 4.6 with a much fainter companion, while Mu Crucis splits into two blue-white stars of magnitude 4.0 and 5.1 easily divided in small binoculars.

• 8. Flags and Songs

  ›

  The four or five brightest stars of Crux appear on the national flags of Australia, Brazil, New Zealand, Papua New Guinea, and Samoa, along with the flags of Niue, Victoria, the Australian Capital Territory, and Chile's Magallanes Region. Brazil's currency was even named for it twice, called the cruzeiro from 1942 to 1986 and again from 1990 to 1994, and the constellation appears on the country's coat of arms and passport cover. A red flag flown by the German East Africa Company between 1885 and 1920, the Petersflagge, carried five white Crux stars and later supplied the symbolism used by German colonial-revival organisations.

  The Southern Cross gets a line in Australia's national anthem, describing hearts and hands toiling beneath it, and another in Brazil's 1909 anthem, describing the image of the cross as resplendent; Samoa's national anthem calls the same stars the symbol of Jesus, who died for Samoa. The Argentine epic poem Martín Fierro references the cross, and the Argentine singer Charly García claims to be from the Southern Cross in his song No voy en tren. Crosby, Stills and Nash released Southern Cross as a single in 1981, reaching number 18 on the Billboard Hot 100 the following year; Black Sabbath released The Sign of the Southern Cross on their 1981 album Mob Rules, and a 1952 to 1953 NBC television series, Victory at Sea, featured a number called Beneath the Southern Cross.

  A stylized Crux appears on Australia's Eureka Flag and on the patches worn by two American divisions that fought in the Pacific during the Second World War, the Americal Division and the 1st Marine Division. British officer William Birdwood, who commanded the Australian and New Zealand Army Corps at Gallipoli during the First World War, carried the Southern Cross on his own coat of arms, and Melbourne's major rail terminal, Southern Cross station, still carries the name today.

  The constellation lends its name to Brazilian institutions as readily as it does to Australian ones. The Order of the Southern Cross is a Brazilian order of chivalry awarded to those who have rendered significant service to the nation, and the cross also turns up in the poem O Sweet Saint Martin's Land, whose lyrics invoke thy Southern Cross the night. In Australia, the Knights of the Southern Cross form a Catholic fraternal order, while the Personal Ordinariate of Our Lady of the Southern Cross serves groups of Anglicans across Australia and Asia who have sought full communion with the Catholic Church.