Canopus is the brightest star in the southern constellation of Carina and the second-brightest star in the night sky. It is designated α Carinae, which is romanized (transliterated) to Alpha Carinae. With a visual apparent magnitude of −0.74, it is outshone only by Sirius.
Located around from the Sun, Canopus is a bright giant of spectral type A9, 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 nine to ten 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. Its proper name is believed to originate from the mythological Canopus, a navigator for Menelaus, the king of Sparta. The acronycal rising marked the date of the Ptolemaia festival in Egypt. In ancient India, it was named Agastya after the revered Vedic sage. For Chinese astronomers, it was known as the Old Man of the South Pole. In Islamic astronomy, it is Suhail or Suhayl, a name that is also commonly used to imply rareness of appearance (as Canopus infrequently appeared to a gazer at Middle Eastern latitudes).
Nomenclature The name Canopus is a Latinisation of the Ancient Greek name Κάνωβος/Kanôbos, recorded in Claudius Ptolemy's Almagest (c.150 AD). 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 Star Names: Their Lore and Meaning.
The brightest star in the obsolete constellation of Argo Navis, which represented the ship used by Jason and the Argonauts, was given the name of a ship's pilot from another Greek legend: Canopus, pilot of Menelaus's ship on his quest to retrieve Helen of Troy after she was taken by Paris. A ruined ancient Egyptian port named Canopus lies near the mouth of the Nile, site of the Battle of the Nile. It is speculated that its name is derived from the Egyptian Coptic Kahi Nub ("Golden Earth"), which refers to how Canopus would have appeared near the horizon in ancient Egypt, reddened by atmospheric extinction from that position.
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) 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. Canopus is now included in the IAU Catalog of Star Names.
Canopus traditionally marked the steering oar of the ship Argo Navis. German celestial cartographer Johann Bayer gave it—as the brightest star in the constellation—the designation of α Argus (Latinised to Alpha 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 (Latinised to Alpha Carinae). It is 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 Benjamin Apthorp Gould gave it the number 7 (7 G. Carinae) 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, Sohail, Suhayil, Shoel, Sohil, Soheil, Sahil, Suhayeel, Sohayil, Sihel, and Sihil. An alternative name was Wazn "weight" or Haḍar "ground", implying the anchor stone used by ship, rather than being related to its low position near the horizon. Hence comes its name in the Alfonsine tables, Suhel ponderosus, a Latinization of Al Suhayl al Wazn. Its Greek name was revived during the Renaissance.
Observation
The Muslim astronomer Ibn Rushd went to Marrakesh, Morocco to observe the star in 1153, as it was 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.
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."Knobel, p. 416.
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.Schaaf, p. 257.
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, San Francisco, and Seoul, 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.D. Gieringer, "Exploring the Tropic of Canopus", Astronomy, December 1985, p.24. 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 (Greece) 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's spectral type has been given as F0 and the incrementally warmer A9. It is less yellow than Altair or Procyon, with indices measured as 0.22 and 0.42, respectively. Some observers may have perceived Canopus as yellow-tinged because it is low in the sky and hence subject to atmospheric effects.Schaaf, pp. 112–13. Patrick Moore said that it never appeared anything but white to him. The bolometric correction for Canopus is 0.00, indicating that the visual absolute magnitude and bolometric absolute magnitude are equal.
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. 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 (having strong coronal X-ray emission), 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 ("Canopus B") is approximately 1.9 parsecs. However, despite this large separation, it is still within the estimated tidal radius (2.9 parsecs) for the massive star Canopus. A 2022 study, using data from Gaia Early Data Release 3, found a candidate companion star with a similar parallax and proper motion to that of Canopus. With a mass of 0.2 times that of the Sun, Gaia EDR3 5500822971164705792 has a projected separation of 220.1 arcseconds, or 20,870 astronomical units at the distance of Canopus.
Since it is more luminous than any star closer to Earth, Canopus has been the brightest star in the 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.
Role in navigation
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 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'.
The south celestial pole can be approximately located using Canopus and two different bright stars. The first, Achernar, makes an equilateral triangle between the stars and the south pole. One can also locate the pole more roughly using an imaginary line between Sirius and Canopus; Canopus will be approximately at the midpoint, being one way to Sirius and to the pole.
Canopus's brightness and location well off the ecliptic make it useful for space navigation. 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 (after locking on the Sun) in 1964, the first time a star had been used.
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 crewed space mission, 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 Krishna 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.