Canopus


Canopus, also designated α Carinae, Latinised to Alpha Carinae, is the brightest star in the southern constellation of Carina and the second-brightest star in the night sky. With a visual apparent magnitude of −0.74, it is outshone only by Sirius. Its proper name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta.
Located around from the Sun, Canopus is a bright giant of spectral type A9 or F0, so it is essentially white when seen with the naked eye. It has a luminosity over 10,000 times the luminosity of the Sun, is eight times as massive, and has expanded to 71 times the Sun's radius. Its enlarged photosphere has an effective temperature of around. Canopus is undergoing core helium burning and is currently in the so-called blue loop phase of its evolution, having already passed through the red-giant branch after exhausting the hydrogen in its core. Canopus is a source of X-rays, which are likely being emitted from its corona.
The prominent appearance of Canopus means it has been the subject of mythological lore among many ancient peoples. The acronychal rising marked the date of the Ptolemaia festival in Egypt. In Hinduism, it was named Agastya after the revered Vedic sage. For Chinese astronomers, it was known as the Old Man of the South Pole.

Nomenclature

The name Canopus is a Latinisation of the Ancient Greek name Κάνωβος/Kanôbos, recorded in Claudius Ptolemy's Almagest. Eratosthenes used the same spelling. Hipparchos wrote it as Κάνωπος. John Flamsteed wrote Canobus, as did Edmond Halley in his 1679 Catalogus Stellarum Australium. The name has two possible derivations, both listed in Richard Hinckley Allen's seminal .
In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Canopus for this star. It is now so entered in the IAU Catalog of Star Names.
Canopus traditionally marked the rudder of the ship Argo Navis. German celestial cartographer Johann Bayer gave it—as the brightest star in the constellation—the designation of α Argus in 1603. In 1763, French astronomer Nicolas Louis de Lacaille divided the huge constellation into three smaller ones, and hence Canopus became α Carinae. It is also listed in the Bright Star Catalogue as HR 2326, the Henry Draper Catalogue as HD 45348, and the Hipparcos catalogue as HIP 30438. Flamsteed did not number this southern star, but Gould gave it the number 7 in his Uranometria Argentina.
An occasional name seen in English is Soheil, or the feminine Soheila; in Turkish is Süheyl, or the feminine Süheyla, from the Arabic name for several bright stars, سهيل suhayl, and Canopus was known as Suhel in medieval times. Alternative spellings include Suhail, Souhail, Suhilon, Suheyl, Sohayl, Suhayil, Shoel, Sohil, Soheil, Sahil, Suhayeel, Sohayil, Sihel, and Sihil. An alternative name was Wazn "weight" or Haḍar "ground", possibly related to its low position near the horizon. Hence comes its name in the Alphonsine Tables, Suhel ponderosus, a Latinization of Al Suhayl al Wazn. Its Greek name was revived during the Renaissance.

Observation

English explorer Robert Hues brought Canopus to the attention of European observers in his 1592 work Tractatus de Globis, along with Achernar and Alpha Centauri, noting:
"Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third is in the right foote of the Centaure."

In the Southern Hemisphere, Canopus and Sirius are both visible high in the sky simultaneously, and reach a meridian just apart. Brighter than first magnitude, Canopus can be seen by naked eye in the early twilight. Mostly visible in mid to late summer in the Southern Hemisphere, Canopus culminates at midnight on December 27, and at 9 PM on February 11.
When seen from latitudes south of S, Canopus is a circumpolar star. Since Canopus is so far south in the sky, it never rises in mid- to far-northern latitudes; in theory the northern limit of visibility is latitude north. This is just south of Athens, Richmond, Virginia, and San Francisco, and very close to Seville and Agrigento. It is almost exactly the latitude of Lick Observatory on Mt. Hamilton, California, from which it is readily visible because of the effects of elevation and atmospheric refraction, which add another degree to its apparent altitude. Under ideal conditions, it can be spotted as far north as latitude from the Pacific coast. Another northernmost record of visibility came from Mount Nemrut in Turkey, latitude. It is more easily visible in places such as the Gulf Coast and Florida, and the island of Crete where the best season for viewing it around 9 p.m. is during late January and early February.
Canopus has a B–V color index of +0.15 where 0 is a blue-white, indicating it is essentially white, although it has been described as yellow-white. Canopus' spectral type has been given as F0 and more recently A9. It is less yellow than Altair or Procyon, with indices measured as 0.22 and 0.42, respectively. It may be that some observers have perceived Canopus as yellow-tinged because it is low in the sky and hence subject to atmospheric effects. Patrick Moore said that it never appeared anything but white to him.
Canopus was previously proposed to be a member of the Scorpius–Centaurus Association, however it is not located near the subgroups of that association, and has not been included as a Sco-Cen member in kinematic studies that used Hipparcos astrometric data. At present, Canopus is not thought to be a member of any nearby young stellar groups. In 2014, astronomer Eric Mamajek reported that an extremely magnetically active M dwarf, 1.16 degrees south of Canopus, appears to share a common proper motion with Canopus. The projected separation of the M dwarf 2MASS J06234738-5351131 is approximately 1.9 parsecs, however, despite this large separation, it is still within the estimated tidal radius for the massive star Canopus.
No star closer than Canopus is more luminous than it, and it has been the brightest star in Earth's night sky during three epochs over the past four million years. Other stars appear brighter only during relatively temporary periods, during which they are passing the Solar System much closer than Canopus. About 90,000 years ago, Sirius moved close enough that it became brighter than Canopus, and that will remain so for another 210,000 years. But in 480,000 years, as Sirius moves further away and appears fainter, Canopus will once again be the brightest, and will remain so for a period of about 510,000 years.

Spectrum

Canopus was little-studied by western scientists before the 20th century. It was given a spectral class of F in 1897, an early use of this extension to Secchi class I, applied to those stars where the hydrogen lines are relatively weak and the calcium K line relatively strong. It was given as a standard star of F0 in the Henry Draper Catalogue, with the spectral type F0 described as having hydrogen lines half the strength of an A0 star and the calcium K line three times as strong as Hδ. American astronomer Jesse Greenstein was interested in stellar spectra and used the newly built Otto Struve Telescope at McDonald Observatory to analyze the star's spectrum in detail. In a 1942 paper, he reported that the spectrum is dominated by strong broad hydrogen lines. There are also absorption lines of carbon, nitrogen, oxygen, sulphur, iron, and many ionised metals. It was studied in the ultraviolet by an early astronomical satellite, Gemini XI in 1966. The UV spectra were considered to be consistent with an F0 supergiant having a temperature of, the accepted parameters for Canopus at the time. New Zealand-based astronomers John Hearnshaw and K-Desikachary examined the spectrum in greater detail, publishing their results in 1982.
When luminosity classes were added to the MK spectral classification scheme, Canopus was assigned class Iab indicating an intermediate luminosity supergiant. This was based on the relative strengths of certain spectral lines understood to be sensitive to the luminosity of a star. In the Bright Star Catalogue 5th edition it is given the spectral class F0II, the luminosity class indicating a bright giant. Balmer line profiles and oxygen line strengths indicate the size and luminosity of Canopus.
When the effects of stellar rotation speed on spectral lines are accounted for, the MK spectral class of Canopus is adjusted to A9II. Its spectrum consists mostly of absorption lines on a visible continuum, but some emission has been detected. For example, the calcium K line have weak emission wings on each side of the strong central absorption line, first observed in 1966. The emission line profiles are usually correlated with the luminosity of the star as described by the Wilson-Bappu effect, but in the case of Canopus they indicate a luminosity much lower than that calculated by other methods. More detailed observations have shown that the emission line profiles are variable and may be due to plage areas on the surface of the star. Emission can also be found in other lines such as the h and k lines of ionised magnesium.

Distance

Before the launch of the Hipparcos satellite telescope, distance estimates for Canopus varied widely, from 96 light-years to 1200 light-years. The closer distance was derived from parallax measurements of around. The larger distance derives from the assumption of a very bright absolute magnitude for Canopus.
Hipparcos established Canopus as being from the Solar System; this is based on its 2007 parallax measurement of. At 96 parsecs, the interstellar extinction for Canopus is very low at 0.08 magnitudes. Canopus is too bright to be included in the normal observation runs of the Gaia satellite and there is no published Gaia parallax for it.

Physical characteristics

The absorption lines in the spectrum of Canopus shift slightly with a period of. This was first detected in 1906 and the Doppler variations were interpreted as orbital motion. An orbit was even calculated, but no such companion exists and the small radial velocity changes are due to movements in the atmosphere of the star. The maximum observed radial velocities are only 0.7 to. Canopus also has a magnetic field that varies with the same period, detected by the Zeeman splitting of its spectral lines. Canopus is bright at microwave wavelengths, one of the few F-class stars to be detected by radio.
An early interferometric measurement of its angular diameter in 1968 gave a limb-darkened value of, close to the accepted modern value. Very-long-baseline interferometry has been used to calculate Canopus' angular diameter at. Combined with distance calculated from its Hipparcos parallax, this gives it a radius of 71 times that of the Sun. If it were at the centre of the Solar System, it would extend 90% of the way to the orbit of Mercury. The radius and temperature relative to the Sun means that it is 10,700 times more luminous than the Sun, and its position in the H-R diagram relative to theoretical evolutionary tracks means that it is times as massive as the Sun. Measurements of its shape find a 1.1° departure from spherical symmetry.
Canopus is a source of X-rays, which are probably produced by its corona, magnetically heated to several million Kelvin. The temperature has likely been stimulated by fast rotation combined with strong convection percolating through the star's outer layers. The soft X-ray sub-coronal X-ray emission is much weaker than the hard X-ray coronal emission. The same behaviour has been measured in other F-class supergiants such as α Persei and is now believed to be a normal property of such stars.

Evolution

The spectrum of Canopus indicates that it has exhausted its core hydrogen and evolved away from the main sequence, where it spent some 30 million years of its existence as a blue-white star of around 10 solar masses. It has been classified as a less luminous supergiant, although it is now considered to be a bright giant. The position of Canopus in the H–R diagram indicates that it is currently in the core-helium burning phase. It is an intermediate mass star that has left the red-giant branch before its core became degenerate and is now in a blue loop. It lies on the warm side of the instability strip and does not pulsate like Cepheid variables of a similar luminosity. Canopus may not be massive enough for its fusion chain to reach iron and trigger a core collapse and subsequent supernova, instead eventually becoming a neon-oxygen white dwarf.

Role in navigation

The south celestial pole can be approximately located using Canopus and another bright star, Achernar. Make an equilateral triangle using these stars for two of the corners. The third imaginary corner will be the south celestial pole. Canopus's brightness and location well off the ecliptic make it useful for space navigation. The effects of precession will take Canopus within 10° of the south celestial pole around the year 14,000 CE.
Many spacecraft carry a special camera known as a "Canopus Star Tracker" plus a Sun sensor for attitude determination. Mariner 4 used Canopus for second axis stabilisation in 1964, the first time a star had been used.

Cultural significance

Canopus was known to the ancient Mesopotamians and given the name NUN-ki and represented the city of Eridu in the Three Stars Each Babylonian star catalogues and later MUL.APIN around 1100 BC. Today, the star Sigma Sagittarii is known by the common name Nunki.
Canopus was not visible to the mainland ancient Greeks and Romans; it was, however, visible to the ancient Egyptians. Hence Aratus did not write of the star as it remained below the horizon, while Eratosthenes and Ptolemy—observing from Alexandria—did, calling it Kanōbos. An Egyptian priestly poet in the time of Thutmose III mentions the star as Karbana, "the star which pours his light in a glance of fire, When he disperses the morning dew." Under the Ptolemies, the star was known as Ptolemaion and its acronychal rising marked the date of the Ptolemaia festival, which was held every four years, from 262 to 145 BC.
, who used his 1153 observation of Canopus in Marrakesh while the star was invisible in his native Spain as an argument that the earth is round.

India

In Indian Vedic literature, Canopus is associated with the sage Agastya, one of the ancient siddhars and rishis. To Agastya, the star is said to be the 'cleanser of waters', and its rising coincides with the calming of the waters of the Indian Ocean. It is thus considered the son of Pulastya, son of Brahma. Canopus is described by Pliny the Elder and Gaius Julius Solinus as the largest, brightest and only source of starlight for navigators near Tamraparni island during many nights.
Kalīla o Damna, an influential Pahlavi book of animal fables was later known as Anvar-i-Suhayli.
The southeastern wall of the Kaaba in Mecca is aligned with the rising point of Canopus, and is also named Janūb.
The Bedouin people of the Negev and Sinai also knew Canopus as Suhayl, and used it and Polaris as the two principal stars for navigation at night. Because it disappears below the horizon in those regions, it became associated with a changeable nature, as opposed to always-visible Polaris, which was circumpolar and hence 'steadfast'. It is also referred to by its Arabic name: سهيل, given by Islamic scientists in the 7th century AD. The Spanish Muslim astronomer Ibn Rushd went to Marrakesh to observe the star in 1153, which is invisible in his native Córdoba, Al-Andalus. He used the different visibility in different latitudes to argue that the earth is round, following Aristotle's argument which held that such an observation was only possible if the earth was a relatively small sphere.

China

Canopus was described as Shou Xing, the Star of Longevity, in the Shiji completed in 94 BC by Chinese historian Sima Qian. Drawing on sources from the Warring States period, he noted it to be the southern counterpart of Sirius, and wrote of a sanctuary dedicated to it established by Emperor Qin Shi Huang between 221 and 210 BC. During the Han Dynasty, the star was auspicious, its appearance in the southern sky heralding peace and absence war. From the imperial capital Chang'an, the star made a low transit across the southern sky, indicating true south to observers, and was often obscured by clouds. During this time it was also equated with Old Man of the South Pole Under this name, Canopus appears on the medieval Chinese manuscript the Dunhuang Star Chart, although it cannot be seen from the Chinese capital of Chang'an. The Chinese astronomer Yi Xing had journeyed south to chart Canopus and other far southern stars in 724 AD. Its personification as the Old Man Star was popularised in the Tang Dynasty, where it appeared often in poetry and memorials. Later still, during the Ming Dynasty, the star was established as one of the Three Stars, appearing frequently in art and literature of the time. This symbolism spread into neighbouring cultures in Asia.
In Japan, Canopus is known as Mera-boshi and Roujin-sei, and in Mongolia, it was personified as the White Old Man. Although the link was known in Tibet, with names such as Genpo karpo or Genkar "White Old Man", the symbolism was not popular. Instead, Canopus was more commonly named Karma Rishi སྐར་མ་རི་ཥི།, derived from Indian mythology. Tibetans celebrated the star's heliacal rising with ritual bathing and associated it with morning dew.

Polynesia

Bright stars were important to the ancient Polynesians for navigation between the many islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses to assist mariners in charting courses to particular destinations. Canopus served as the southern wingtip of a "Great Bird" constellation called Manu, with Sirius as the body and Procyon the northern wingtip, which divided the Polynesian night sky into two hemispheres. The Hawaiian people called Canopus Ke Alii-o-kona-i-ka-lewa, "The chief of the southern expanse"; it was one of the stars used by Hawaiʻiloa and Ki when they traveled to the Southern Ocean. The Māori people of New Zealand/Aotearoa had several names for Canopus. Ariki, was known as a solitary star that appeared in the east, prompting people to weep and chant. They also named it Atutahi, Aotahi or Atuatahi, "Stand Alone". Its solitary nature indicates it is a tapu star, as tapu people are often solitary. Its appearance at the beginning of the Maruaroa season foretells the coming winter; light rays to the south indicate a cold wet winter, and to the north foretell a mild winter. Food was offered to the star on its appearance. This name has several mythologies attached to it. One story tells of how Atutahi was left outside the basket representing the Milky Way when Tāne wove it. Another related myth about the star says that Atutahi was the first-born child of Rangi, who refused to enter the Milky Way and so turned it sideways and rose before it. The same name is used for other stars and constellations throughout Polynesia. Kapae-poto, "Short horizon", referred to it rarely setting as seen in New Zealand; Kauanga was the name for Canopus only when it was the last star visible before sunrise. The people of the Society Islands had two names for Canopus, as did the Tuamotu people. The Society Islanders called Canopus Taurua-e-tupu-tai-nanu, "Festivity-whence-comes-the-flux-of-the-sea", and Taurua-nui-o-te-hiti-apatoa "Great-festivity-of-the-border-of-the-south", and the Tuamotu people called the star Te Tau-rari and Marere-te-tavahi, the latter said to be the true name for the former, "He-who-stands-alone".

Africa

In the Guanche mythology of the island of Tenerife, the star Canopus was linked with the goddess Chaxiraxi.
The Tswana people of Botswana knew Canopus as Naka. Appearing late in winter skies, it heralded increasing winds and a time when trees lose their leaves. Stock owners knew it was time to put their sheep with rams. In southern Africa, the Sotho, Tswana and Venda people called Canopus Naka or Nanga, “the Horn Star”, while the Zulu and Swazi called it inKhwenkwezi "Brilliant star". It appears in the predawn sky in the third week of May. According to the Venda, the first person to see Canopus would blow a phalaphala horn from the top of a hill, getting a cow for a reward. The Sotho chiefs also awarded a cow, and ordered their medicine men to roll bone dice and read the fortune for the coming year. To the ǀXam-speaking Bushmen of South Africa, Canopus and Sirius signalled the appearance of termites and flying ants. They also believed that stars had the power to cause death and misfortune, and they would pray to Sirius and Canopus in particular to impart good fortune or skill. The ǃKung people people of the Kalahari Desert in Botswana held Canopus and Capella to be the horns of tshxum, the appearance of all three marking the end of the dry season and start of the rainy season.
The Kalapalo people of Mato Grosso state in Brazil saw Canopus and Procyon as Kofongo "Duck", with Castor and Pollux representing his hands. The asterism's appearance signified the coming of the rainy season and increase in manioc, a food staple fed to guests at feasts.

Americas

The Navajo observed the star and named it Maʼii Bizòʼ, the “Coyote Star”. According to legend, Maʼii took part in the naming and placing of the star constellations during
the creation of the universe. He placed Canopus directly south, naming it after himself.

Australia

Canopus was identified as the moiety ancestor Waa "Crow" to some Koori people in southeastern Australia. The Boorong people of northwestern Victoria recalled that War was the brother of Warepil, and that he brought fire from the heavens and introduced it to mankind. His wife was Collowgullouric War. The Pirt-Kopan-noot people of western Victoria told of Waa "Crow" falling in love with a queen, Gneeanggar "Wedge-tailed Eagle" and her six attendants. His advances spurned, he hears that the women are foraging for grubs and so transforms himself into a grub. When the women dig him out, he changes into a giant and carries her off.
The Kulin people knew Canopus as Lo-an-tuka. Objects in the sky were also associated with states of being for some tribes; the Wailwun of northern New South Wales knew Canopus as Wumba "deaf", alongside Mars as Gumba "fat" and Venus as Ngindigindoer "you are laughing". Tasmanian aboriginal lore held that Canopus was Dromerdene, the brother of Moinee; the two fought and fell out of the sky, with Dromerdene falling into Louisa Bay in southwest Tasmania.

Legacy

Canopus appears on the flag of Brazil, symbolising the state of Goiás.
Two U.S. Navy submarine tenders have been named after Canopus, the first serving from 1922 to 1942 and the second serving from 1965 to 1994.
The Royal Navy built six s which entered services between 1899 and 1902, and nine Canopus-class ships of the line in the early 19th century.
There are at least two mountains named after the star: Mount Canopus in Antarctica; and Mount Canopus or Canopus Hill in Tasmania, the location of the Canopus Hill astronomical observatory.