Canopus (disambiguation)

Canopus, Alpha Carinae, is the second brightest star in the sky at magnitude -0.72. It is located well into the southern hemisphere, at -52° 42' (2000) and is barely visible on the southern horizon of even the southern US States or the African coast of the Mediterranean Sea.

Canopus is, according to the Hipparcos satellite, 310 light years (96 parsecs) from Earth. Before Hipparcos, distance measurements for the star varied very widely, up to as much as 1200 light years; had the latter been correct Canopus would have been one of the most powerful stars in our galaxy. As is, it is still at least 30,000 times brighter than the sun and the most powerful star within 700 light years or so. It is much more luminous, intrinsically, than the sole star that appears brighter than it from Earth -- Sirius is a mere 22 times more luminous than our sun, and depends on being much closer to us to beat its rival. In fact, for a large fraction of stars in the local stellar neighbourhood, Canopus is the "brightest star in the sky".

The difficulty in measuring Canopus' distance stemmed from its unusual nature. The usual classification for Canopus is F0 IA-II, and F-class bright supergiants are rare and poorly understood; they may be stars in the process of evolving to or away from red giant status. This in turn made it difficult to guess how intrinsically bright it is and so how far away it might be. Direct measurement was the only way to solve the problem, and as it was too far away for earth-based parallax observations to be made, a precise distance had to wait until the Space Age.

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 is correct is a matter of conjecture. One comes from the legend of the Trojan War. As the constellation Carina is part of the now-obsolete, gigantic Argo Navis constellation, 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, apparently specifically named for the star, near the mouth of the Nile (in fact, its site was the location of the Battle of the Nile).

Due to its brightness and position away from the orbital plane of our solar system (the latter being in contrast to Sirius' position), Canopus 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".