Credit: NASA. Image of Canopus
by Expedition 6
by Expedition 6
Carina, Puppis and Vela originally formed the constellation Argo.
A really interesting example is Canopus. It is incredibly brilliant, and the second brightest star after Sirius. it is 13,000 to 15,000 times brighter, between 65 and 73 times larger, and about 8.5 times heavier than the sun, and is around 310 light-years away. It is visible in the southern hemisphere, and cannot be seen in most of Europe or North America except parts of the southern US. Canopus was originally the main star in the ancient constellation of Argo, the ship on which Jason was said to have sailed in search of the golden fleece. Argo was separated into three smaller constellations, with Canopus in Carina, the Keel, thus its name Alpha Carinae. There is some debate over whether Canopus has been through the red giant stage already, and has shrunk and moved back to being a type F star, or whether it has yet to become a red giant. If the former is correct, as seems likely, then it is fusing helium, and will continue to fuse carbon. It may stop before producing the iron core neede for it to collapse and go supernova and become a rare neon-oxygen white dwarf.
Another example is Procyon A (alpha Canis Minoris). It is in the constellation Canis Minor, the smaller or lesser dog. It appears as the eighth brightest star in our skies due partly to being only 11.4 light years away. It is about 7 times brighter than the sun, 2.1 times larger, and about 1.4 solar masses. Being a little larger and heavier than the sun, it has gone through the hydrogen burning phase more quickly. At about three billion years old, it is moving into helium burning which the sun will not do for another five billion years or so when it is ten billion years old. It has a small, dim white dwarf binary companion, Procyon B, which would have started out at around 50% heavier than Procyon, and completed its journey to white dwarf status rather more quickly, loosing 75% of its mass in the process. Procyon A will also end up as a white dwarf.
Type F Stars
Polaris, the current Northern Pole Star, is about 430 light-years away, and is part of a multiple star system comprising five stars. It is about 30 times the Sun's diameter, and weighs about 7.5 solar masses. A type F7, it is borderline between bright giant or supergiant, and is the closest Cepheid variable to us. Over the last hundred years or so of detailed observations, the period of its variability has slowed by about 8 seconds every year from about 3.97 days in c. 1900 to around 3.98 days today. The amplitude of its variability has also decreased while its average brightness has increased. In fact, it appears that in the last 2,000 years or so, its visual luminosity has increased by a factor of 2.5 from a third magnitude to its current second magnitude. This is a phenomenal rate of increase if proven correct. Polaris is slowly moving away from its position over the North Pole due to the Earth's precession on a period of around 25,800 years. It is about ¾º away now, and in about 12,000 years time, the Northern Pole Star will be Vega again, as it was about 14,000 years ago.
Astronomy & Cosmology
-
Stars - Stellar Classes
Class F main-sequence stars are yellow-white dwarfs, usually weighing between 1.0 and 1.4 times the mass of the Sun but, again, there is great variability. Stars are yellow/white with surface temperatures between about 6,000K and 7,600K. The best known example, at least in the Northern Hemisphere, is the Pole Star (Polaris). Canopus, possibly a rare Class F giant, is the brightest Southern Hemisphere star.