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Canopus

[kuh-noh-puhs]
Canopus (α Car / α Carinae / Alpha Carinae) is the brightest star in the southern constellation of Carina, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.

Canopus is a rare example of a supergiant of spectral type F. Canopus is essentially white when seen with the naked eye (though F-type stars are sometimes listed as "yellowish-white"). It is located in the far southern sky, at a declination of −52° 42' (2000) and a right ascension of 06h24.0m.

Visibility

Since Canopus is so far south in the sky, it never rises in mid- or far-northern latitudes; it cannot be seen north of latitude 38°N. In the southern hemisphere, Canopus and Sirius are both visible high in the sky simultaneously, and reach the meridian just 21 minutes apart. It is a circumpolar star when seen from points south of latitude 38°S: this includes Melbourne, Australia; Wellington, New Zealand; Bahía Blanca, Argentina; and Valdivia, Chile.

Physical properties

Before the launching of the Hipparcos satellite telescope, distance estimates for the star varied widely, from 96 light years to 1200 light years. Had the latter distance been correct, Canopus would have been one of the most powerful stars in our galaxy. Hipparcos established Canopus as lying 310 light years (96 parsecs) from our solar system; this is based on a parallax measurement of 10.43 ± 0.53 mas. The difficulty in measuring Canopus' distance stemmed from its unusual nature. The spectral classification for Canopus is F0 Ia (Ia referring to "bright supergiant"), and such stars are rare and poorly understood; they are stars that can be either in the process of evolving to or away from red giant status. This in turn made it difficult to know how intrinsically bright Canopus is, and therefore how far away it might be. Direct measurement was the only way to solve the problem. Canopus is too far away for Earth-based parallax observations to be made, so the star's distance was not known with certainty until the early 1990s.

Canopus is 15,000 times more luminous than the Sun and the most intrinsically bright star within approximately 700 light years. For most stars in the local stellar neighborhood, Canopus would appear to be one of the brightest stars in the sky. Canopus is out-shone by Sirius in our sky only because Sirius is far closer to the Earth (8 light years).

Its surface temperature has been estimated at 7350 ± 30 K. Its diameter has been measured at 0.6 astronomical units (the measured angular diameter being 0.006 arcseconds), 65 times that of the sun. If it were placed at the centre of the solar system, it would extend three-quarters of the way to Mercury. An Earth-like planet would have to lie three times the distance of Pluto.

Canopus is part of the Scorpius-Centaurus Association, a group of stars which share similar origins.

Etymology and cultural significance

The name "Canopus" has two common derivations, both listed in Richard Hinckley Allen's touchstone of stellar mythology, Star Names: Their Lore and Meaning; which may be a matter of conjecture. One comes from the legend of the Trojan War. The constellation Carina was once part of the now-obsolete constellation of Argo Navis, which represented the ship used by Jason and the Argonauts. The brightest star in the constellation was given the name of a ship's pilot from another Greek legend: Canopus was the pilot of Menelaus' ship on his quest to retrieve Helen of Troy after she was taken by Paris.

The other etymology of the name is that it comes from the Egyptian Coptic Kahi Nub ("Golden Earth"), which refers to the way it would appear near the horizon in Egypt and be correspondingly reddened by atmospheric extinction from that position. There is also a ruined ancient Egyptian port, Canopus, apparently specifically named for the star, near the mouth of the Nile; its site was the location of the Battle of the Nile. (Or it could be that Menelaus's legendary pilot was named after the port, and the port was named the "Golden Floor" because of the many valuable cargoes that passed across its quays and the profits made by merchants there.)

A third possibility is its origin from the Semitic root G(C)-N-B (Gimal-Nuun-Beth), which in Arabic is Janub (جنوب ). The southeastern wall of the Muslim Ka'bah points to Canopus, and is named Janub as well.

It is known as 老人星(Lǎorénxīng, the Star of the Old) in Chinese, and سهيل (Suhayl) in Arabic.

In Ancient Hindu astronomy and astrolog, Canopus is named Agasti or Agastiya.

Kalīla o Damna, an influential Pahlavi (Middle Persian) book of animal fables was later known as Anvar-i-Suhaili or The Lights of Canopus.

Role in navigation

To anyone living in the northern hemisphere, but far enough south to see the star, it served as a southern pole star. This lasted only until magnetic compasses became common.

In modern times, Canopus serves another navigational use. Canopus' brightness and position away from the orbital plane of our solar system means it is often used by American space probes for navigational purposes, using a special camera known as a "Canopus Star Tracker" in combination with a "Sun Tracker".

The effects of precession will take Canopus within 10° of the south celestial pole around the year 14,000 AD.

To the Bedouin people of the Negev and Sinai, Canopus is known as Suhayl. It and Polaris are the two principal stars used for navigation at night. Due to the fact that it disappears below the horizon, it became associated with a cowardly or changeable nature, as opposed to always-visible Polaris, which was circumpolar and hence 'steadfast'.

See also

References

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