Canopus | Universe Today

Written by Jerry Coffey

Canopus is the 2nd brightest star in the night sky and the brightest star in the southern constellation of Carina, which used to be part of Ptolemy's constellation Argo Navis. Argo Navis has been broken up into three smaller constellations: Carina, Puppis, and Vella. It is also the only one of the original 48 constellations that is no longer recognized. Now back to Canopus. The star has a visual magnitude of -0.72 and an absolute magnitude of -5.53. This star is a rare example of a supergiant of spectral type F. Canopus is essentially white when seen with the unaided eye. Most F-type stars are 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.

Canopus is so far south that it never is seen in the northern latitudes. The farthest north that it is usually seen is Los Angeles, Athens, and cities along that line. 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 are south of latitude 37°18' south; for example: Australia and New Zealand, Argentina, Chile. Canopus is visible to the naked eye in Afghanistan.

The spectral classification for Canopus is F0 Ia (Ia referring to "bright supergiant"). These stars are rare and poorly understood. They 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. The launch of the Hipparcos satellite telescope cleared things up. Before that the best estimates for the distance to Canopus were 96 to 1200 light years away. The new telescope fixed the distance at 310 light years from our solar system. This is based on a parallax measurement of 10.43 ± 0.53 minutes of arc.

Canopus is 15,000 times more luminous than Sol and the most intrinsically bright star within 700 light years from us. Canopus appears less bright than Sirius in our sky only because Sirius is much closer to the Earth. Its surface temperature has been estimated at 7350 kelvin. Its diameter has been measured at 0.6 AU, 65 times that of the sun. If it were placed at the center of the solar system, it would extend three-quarters of the way to Mercury.

Before the magnetic compass anyone living in the northern hemisphere, but far enough south to see the star, Canopus served as a southern pole star. 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 attitude determination.

There is a nice article about Canopus here. We have a great article about the Carina constellation here on Universe Today. Astronomy Cast offers a good episode about the most famous stars.

Filed under: Astronomy

Tags: Argo Navis, Canopus, Carina