Canopus, in the constellation Carina, is the second-brightest star in the sky. It is a yellowish star of magnitude -0.72, about 36 degrees south of Sirius in the sky. Being so far south, Canopus is best seen by observers near or south of the Earth's equator. It is visible from the southern parts of the United States near the southern horizon during the winter and spring months.
History and Mythology
Canopus was originally the Alpha star of the ancient constellation Argo Navis, the ship on which Jason sailed to find the golden fleece. In more modern times, Argo Navis was broken into three parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sails). Canopus fell into Carina, and is therefore now Alpha Carinae. Its traditional name, "Canopus", refers a ship's pilot from another Greek legend: the pilot of Menelaus' ship on his quest to retrieve Helen of Troy after she was taken by Paris.
Canopus was worshipped in ancient Egypt, and temples were built to mark the rising point of Canopus on the autumnal equinox. A temple built by Rameses III at Thebes marked the setting point of Canopus. There is also a ruined ancient Egyptian port near the mouth of the Nile, Canopus, apparently specifically named for the star.
Canopus is often used by interplanetary spacecraft as a reference point for navigation. Since this star lies away from the plane of the solar system (the Ecliptic) where the bright planets are found, Canopus serves as a convenient direction marker for long interplanetary missions. The effects of precession will take Canopus within 10° of the south celestial pole around the year 14,000 AD.
In science fiction, the planet Arrakis, upon which Frank Herbert's "Dune" takes place, is the third orbiting Canopus.
Properties
Although Canopus appears about half as bright as Sirius in our sky, Canopus is actually a much grander star. It is a rare class F0 Ia supergiant, with a yellow-white surface at 7350 K. Such stars are rare and poorly understood; they can either be in the process of evolving to or away from red giant status. This in turn makes it difficult to know how intrinsically bright Canopus is, and therefore how far away it might be. Before the launching of the Hipparcos satellite, distance estimates for Canopus varied widely, from 96 light years to 1200 light years.
Direct measurement was the only way to solve the problem, and Hipparcos established Canopus as lying 310 light years from our solar system. At this distance, the calculated luminosity of Canopus is 15,000 times the Sun's. Its diameter has been measured at 0.6 AU, from an angular diameter of 0.006" - 65 times the size of the sun. Canopus is large enough to stretch three-fourths of the way across Mercury's orbit. Canopus possesses a magnetically heated corona some 10 times hotter than the Sun's, which produces both observable X-rays and radio waves.
Evolution
Canopus is part of the Scorpius-Centaurus Association, a group of stars which share similar origins. As a supergiant, Canopus has ceased fusing hydrogen in its core, and is in the process of dying. Its luminosity suggests a birth mass 8 or 9 times solar. It may once have been a red giant like Betelgeuse, or it may become one yet; its exact status is unknown.
Not quite massive enough to explode as a supernova, Canopus will eventually become a massive white dwarf like Sirius B. Most white dwarfs are made of carbon and oxygen. But Canopus is massive enough that fusion reactions may proceed farther to produce a much rarer neon-oxygen white dwarf.