Canopus

Canopus

An image of Canopus by Expedition 6
Observation data Epoch J2000      Equinox J2000
Constellation	Carina
Pronunciation	/kəˈnoʊpəs/
Right ascension	06h 23m 57.1099s[1]
Declination	-52° 41′ 44.378″
Apparent magnitude (V)	−0.72
Characteristics
Spectral type	F0 Ia
U−B color index	0.04
B−V color index	0.15
Variable type	None
Astrometry
Radial velocity (Rv)	20.5[1] km/s
Proper motion (μ)	RA: 19.99 [1] mas/yr Dec.: 23.67 [1] mas/yr
Parallax (π)	10.43 ± 0.53[1] mas
Distance	310 ± 20 ly (96 ± 5 pc)
Absolute magnitude (MV)	−5.53
Details
Mass	8.5 M☉
Radius	65 R☉
Surface gravity (log g)	1.50
Luminosity	13600 L☉
Temperature	7,350[2] K
Metallicity	90% Sun
Other designations
Suhel, Suhail, Suhayl, Alpha Carinae, HD 45348, HR 2326, CD−52°914, FK5 245, SAO 234480, HIP 30438, GC 8302
Database references
SIMBAD	data

Canopus seen from Tokyo, Japan. The latitude is 35°38′N.

Canopus (α Car / α Carinae / Alpha Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, 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 (although 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.^[1]

Visibility

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 that have latitude south of 37°18' south; for example, Victoria and Tasmania, Australia; Auckland and south of it, New Zealand; Bahía Blanca, Argentina; and Valdivia, Chile and south of these cities in South America. Since Canopus is so far south in the sky, it never rises in mid- or far-northern latitudes; in theory the northern limit of visibility is latitude 37°18' north. This is just south of Athens and San Francisco and very close to Seville; it should just be visible under ideal conditions from Gibraltar or Los Angeles.

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 luminous 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.^[1] 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.^[3] 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 appears less bright than Sirius in our sky only because Sirius is much closer to the Earth (8.6 light years).

Its surface temperature has been estimated at 7350 ± 30 K.^[2] 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 for its star to appear the same size in the sky as our own sun.^[3]

Canopus is part of the Scorpius-Centaurus Association, a group of stars which share similar origins.^[3]

Canopus is a strong source of X-rays, which are likely produced by its corona, magnetically heated to around 15 million K.^[3]

Etymology and cultural significance

The name "Canopus" is a Latinisation of the Greek name "Kanôbos", recorded in Claudius Ptolemy's Almagest (c150 AD). The name has two common derivations, both listed in Richard Hinckley Allen's seminal Star Names: Their Lore and Meaning;^[4] and one less so. All are matters of conjecture:

• One from the legend of the Trojan War, where 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, pilot of Menelaus' ship on his quest to retrieve Helen of Troy after she was taken by Paris.

• A second from the Egyptian Coptic Kahi Nub ("Golden Earth"), which refers how Canopus would have appeared near the horizon in ancient Egypt, reddened by atmospheric extinction from that position.^[4] A ruined ancient Egyptian port named Canopus lies near the mouth of the Nile, site of the Battle of the Nile.

• A third is its possible origin from the Semitic root G(C)-N-B (Gimmel-Nun-Beth), from which the Arabic word for south, janūb ( جنوب ), is derived. The southeastern wall of the Muslim Ka'bah points to Canopus, and is named Janūb ("south") as well.^[5]

Other names

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.^[4] including Canopus and Regor.

It is known as the Star of the Old Man (in Chinese: 老人星 or in Chinese: Lǎorénxīng) in Chinese.

In Ancient Hindu astronomy and astrology, Canopus is named Agasti or Agastya.

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

Canopus was identified as the moiety ancestor Waa "Crow" to some Koori people in southeastern Australia.^[6]

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 location well off the ecliptic makes it popular for space navigation. Many spacecraft carry a special camera known as a "Canopus Star Tracker" plus a sun sensor for altitude determination.

The effects of precession will take Canopus within 10° of the south celestial pole around the year 14,000 AD.^[7]

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'.^[8]

See also

• List of brightest stars
• Canopus in fiction

References

External links

• Kaler, Jim. "Canopus". Stars. http://www.astro.uiuc.edu/~kaler/sow/canopus.html. Retrieved 2007-06-06. 
• Darling, David. "Canopus (Alpha Carinae)". The Encyclopedia of Astrobiology, Astronomy and Space Flight. http://www.daviddarling.info/encyclopedia/C/Canopus.html. Retrieved 2007-06-06. 
• Star Tales – Carina

Coordinates: 06^h 23^m 57.1099^s, −52° 41′ 44.378″