Canopus
Published on Feb 15, 2007 at 5:32 pm.
2 Comments.
Filed under stars.
A little after sunset this time of year, Orion is at its hightest point in the sky. For those of us in the northern hemisphere, this puts it due south. Down and to the left of Orion is the star Sirius. Sirius is the brightest star in the sky, other than the Sun. Then, farther south from Sirius, right near the horizon, is the star Canopus. Canopus never gets high in the sky seen from here. In fact, it doesn’t get more than about 5° above the southern horizon from where I live (as seen in the above view). It rises and then sets not long after. In fact, here in North America, only Mexico and the southern states in the United States can ever see Canopus well, and it can not be seen at all north of 37° latitude. In the southern hemisphere, though, Canopus is easy to find, as it is high in the sky, as seen in the next sky view that I created to show the view from Australia.
Canopus is currently part of the constellation Carina, the Keel. However, Carina used to be part of Argo Navis, until that constellation was broken into several others in 1930 when the International Astronomical Union came up with its official list of 88 constellations. The Bayer designation for Canopus is α Car.
Allen’s book on star names gives two possible sources of the name Canopus for this star. One name comes from the Egyptian for “Golden Earth”. The star, always low in the sky seen from Egypt, would have been reddened by the atmosphere, appearing more golden than it really was. The other possible source of the name is that it may have been derived from the name of the chief pilot for Menelaus’ ship sailing back from the destruction of Troy. As with many stars, though, Canopus has many other names. It has also been called Suhail (or Suhel), “the plain”, though this name has also been associated with wisdom.
At magnitude -0.7, Canopus is the brightest star in the sky other than the Sun or Sirius. It doesn’t always look that way, though, because for us in the northern hemisphere we always see Canopus through much more atmosphere. Sirius appears almost twice as bright as Canopus, though, if you take away atmospheric effects (as in the southern hemisphere when you can see both stars high in the sky at the same time). But, Sirius is only about 8.6 lightyears away. Canpus is over 310 lightyears away! That means that even though it is 36 times farther away than Sirius, it is only half as bright! That means that Canopus must be a really intrinsically bright star. In fact, Canopus shines over 15,000 times brighter than our own Sun. It’s no wonder that it is still the second brightest star in the night sky for us even though it is so far away.
But, what type of star is Canopus that it shines so brightly? Long time readers will recall that I normally post some physical data about these stars when I do a star post. However, reliable data is tough to get on Canopus. You’d think that such a bright star would be exceedingly well studied. Well, it is well studied, but that doesn’t mean that we understand it well! Canopus is a distended star that has evolved off of the main sequence. Normally such stars are cool red giants or supergiants. But, Canopus is hot — nearly 7500K, in fact. Stars of this type are very rare, and very poorly understood. Looking through tables of data on Canopus I find is classified in many different ways, from spectral type A9 to F2. F0 seems about the most common. I have also seen it categorized as a bright giant (luminosity class II), a low luminosity class supergiant (Ib), an intermediate luminosity supergiant (Iab), and even a bright supergiant (Ia). What gives? Well, it turns out that at this brightness and temperature, there is very little distinguishing these different types of luminosity classes, so there is a lot of interpretation that goes on in classifying them. For cooler stars, spectral types K and M, the distinctions are far greater, and we can generally agree on them. But, Canopus is at at temperature where all the types sort of run together, and Canopus is such a rare type of star that we really don’t know how to tell the different types apart well in this temperature and luminosity range. This type of star is perhaps evolving either towards or away from being a red giant (or red supergiant). About all we can be sure of is that Canopus is not on the main sequence, so it is a dying star of some sort, but we can’t even tell at what stage in its dying it happens to be. We can most all also agree, though, that Canopus is much hotter than the Sun, vastly brighter, and much larger. It is likely about 75 times the diameter of the Sun. We really don’t know much about this type of star, so it is hard to say what its mass is, but I’d hazard a guess that it may be only a few times the mass of the Sun. Canopus does not have enough mass to become a supernova. It will still die and yield a white dwarf, but it will likely produce a very massive white dwarf. Canopus likely has sufficient mass to begin to fuse carbon into oxygen and neon (Main sequence stars fuse hydrogen into helium, and they begin to die when they start to run out of hydrogen to fuse. They then fuse helium into carbon. Most stars stop with fusing helium into carbon, but the really big ones can fuse the carbon into heavier elements).
So, if you live far enough south, then go out and look for Canopus. Here in North America, that means going out at just about the right time, because the star isn’t up for long. The best time to observe it is when Orion is highest in the sky.
-Astroprof
(Sky views created using Starry Night Pro)
aeresol on February 19, 2007 at 2:56 pm: 1
Interesting, is Canopus a variable ? I recall that reasonably massive stars often find themselves on an instability strip when they leave the main sequence.
Astroprof on February 19, 2007 at 4:34 pm: 2
Interesting question. I’d imagine that it is at least a little variable, but I don’t know of any references to its variability.