Object 74: Canopus

Podcast release date: 30 May 2022

Right ascension: 06:23:57.1


Epoch: ICRS

Constellation: Carina

Corresponding Earth location: Slightly less than 1500 km east of Heard Island in the Indian Ocean

Canopus, or Alpha Carinae, is the second brightest star in the night sky. It has a magnitude of -0.7 [1]. However, a lot of people have probably never seen it before, and that is because it is located very far south. It's too far south to be seen from most of Europe, the northern half of Asia, Canada, or the northern two-thirds of the United States. Even in some places like Northern Africa or the southern part of the United States, if you wanted to see Canopus, you would need a view of the southern horizon that is clear of any obstructions whatsoever, like trees, or buildings, or small hills, or even large clumps of grass. On the other hand, if you are in the tropics or the southern hemisphere in places like Sub-Saharan Africa, South Asia, Australia, New Zealand, or South America, you've probably seen the star before. It's even one of the stars featured on the globe on the flag for Brazil [2].

Canopus is located in the constellation Carina, which is also known as the keel, as in the bottom ridgeline of a boat. Along with Puppis and Vela, Carina was once part of an ancient mega-constellation called Argo Navis [3]. This represented the mythological ship that the Greek hero Jason used to fetch the Golden Fleece, although the constellation also appears as a ship in other Middle Eastern and North African myths as well as even in ancient Hindu stories.

Even though Canopus itself is extremely difficult to see from Greece, some Greek astronomers were aware of the star, possibly because some of them liked to vacation in more southern locations like Egypt and possibly because some of them enjoyed places like Egypt so much that they decided to permanently relocate to those locations. The star's name might have originated from a story about the Trojan wars [3, 4]. The Greek king Menelaus was supposedly returning to Greece with Helen when his fleet was blown off course and landed in Egypt. (Given that Troy was probably in modern-day western Turkey, this seems like it would have been extremely off-course.) Anyway, the fleet landed in Egypt, and while the crew were on land, one of the men, named Canopus, was bitten by a snake and died. Helen then killed the snake, and she and Menelaus buried Canopus at a location near modern-day Alexandria. A town was later built around this area and named after Canopus. If you don't believe this story for the origin of the name Canopus, the alternative explanation is that the name might have originated from the ancient Egyptian name for the star, Kahi Nub, which translates into English as "golden earth" and would have referred to the color of the star as it appears near the southern horizon as seen from Egypt [4].

If you look at the Wikipedia page on Canopus or do a Google search on the star, you can see that just about any other culture or civilization that either lived in or visited the tropics could see the star and had a name for it. I won't try to repeat everything listed on these webpages, but I thought I would mention a few highlights. First of all, the Arabic name is Suhail, although the origin of that name is not really clear [3, 4]. Given how many of the bright stars in the night sky have Arabic names, it's a little surprising that the name Suhail did not stick. Indian astronomers named it after Agastya [4], who was a sage who founded a tradition of southern Indian mysticism and who, according to legend, lived for 4000 years [5]. In China, the star name also referred to an old man, and could be called either Laoren ("old man") or Nanji Laoren ("old man of the south pole"), and it was also associated with Shouxing, the Chinese god of longevity [4]. In Hawaii, the star was called Ke Ali‘i o Kona i ka Lewa or "the chief of the southern heavens", and Hawaiian navigators used a line drawn from Mirzam (which is the second brightest star in the constellation Canis Major) to Canopus to determine which direction is south [6].

So that covers what makes Canopus interesting from a historical standpoint, but it is also an interesting object scientifically. At a distance of 309 light years (95 pc) [7], it is the closest massive yellow supergiant to Earth. Yellow supergiants like Canopus are stars several times the mass of the Sun that are passing through a brief phase in their evolution. Before these stars had become yellow supergiants, they would have started out as smaller, hotter stars that are blue or white in color and that fuse hydrogen into helium in their cores like the Sun. When these stars' cores filled up with helium, they would have expanded to become red supergiants, with hydrogen continuing to fuse into helium in shells around the stars' cores. Later, these stars' cores would start fusing helium into carbon, and when the cores filled with carbon, they would create oxygen, neon, or magnesium in their cores, with helium fusing into carbon in shells around that.

While these stars are at the red supergiant stage in their evolution, they might go through a phase where their outer gas layers warm up a little, and they change from a red color to a yellow color. When this happens, the stars become yellow supergiants, and Canopus is the best example of a star currently in this stage. After a short period of time, though, the stars will transform back into red supergiants. It's not entirely clear why red supergiants briefly become yellow supergiants and why they transform back into red supergiants.

Because Canopus is so close to Earth, it is the best object for astronomers to look at to learn about the yellow supergiant phase in stellar evolution. However, Canopus's close proximity is a good news/bad news type of thing. First the bad news. The star is so bright that it will blind many of the detectors that astronomers normally use when looking at astronomical objects more generally. This would be like taking your everyday camera or smartphone and pointing it at the Sun, but much worse. The good news is that it's possible to perform observations of Canopus that aren't feasible with most other stars because they are too faint. Two specific types of observations stood out to me.

First of all, the star has been a good target for extreme spectroscopy observations. Spectroscopy involves creating spectra or rainbows of starlight. However, this also involves spreading out the starlight, so most spectra will look relatively faint compared to the images of those stars. Since Canopus is extremely bright, making a bright spectrum is not much of a problem.

Various elements and compounds in stars' outer gas layers will absorb light at very specific wavelengths, creating what look like dark lines in these rainbow-like spectra, and astronomers often search for these dark lines to identify which elements are present within stars. In observations of most stars, astronomers can often find a few relatively basic elements like hydrogen, helium, nitrogen, oxygen, sodium, calcium, and iron, as well as some basic compounds like titanium oxide, but finding other things can be harder. In Canopus, astronomers have been able to find over 30 elements, including weird stuff like strontium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, erbium, thulium, and hafnium [8]. While this sounds like part of a fun competition where some astronomers are trying to find elements in Canopus that their colleagues in the chemistry department can't find in their stockroom, these observations actually have a very practical application, and that is to test whether the relative proportion of elements found in the Sun's outer gas layers are the same as the elements found in other stars' outer gas layers. In the case of Canopus, at least, it does look chemically very similar to the Sun. If Canopus had different amounts of these elements, that would have implied that the Sun and Canopus formed out of nebulae with different chemical compositions, and that would have brought up some rather complex questions about the origin of the Sun or Canopus or both stars, but since this is not the case, we don't need to worry about it.

The other unusual thing that astronomers can do specifically with Canopus is measure its diameter. This might sound relatively trivial, but the distances between stars are really large compared to the diameters of stars, so even when the most advanced telescopes look at most individual stars, the stars normally seem like infinitesimally small points of light. However, astronomers were actually able to measure the diameter of Canopus by applying advanced techniques to use the four 1.8 m Auxiliary Telescopes at the Very Large Telescope site as a single really wide telescope. They determined that the star has a width of about 100 million kilometers, or about 70 times the width of the Sun [9]. This might sound really large and therefore seem easy to measure, but measuring the diameter of Canopus is actually equivalent to measuring the width of a human hair from a distance of about a couple of kilometers. While this sounds like a fun technical achievement, this measurement is actually very important for accurately modelling the evolution of yellow supergiants like Canopus. You can expect astronomers to continue to try to image Canopus in even more detail in the future.


[1] Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog

[2] Macdonald, Ian, Astronomy of the Brazilian Flag, 2019, Flags of the World

[3] Ridpath, Ian, Star tales, 2018

[4] Allen, Richard Hinckley, Star names. Their lore and meaning, 1963

[5] Isha, Agastya Muni - The Father of Southern Indian Mysticism, 2021

[6] Hawaiian Voyaging Traditions, Hawaiian Star Lines and Names for Stars, 2022

[7] van Leeuwen, F., Validation of the new Hipparcos reduction, 2007, Astronomy & Astrophysics, 474, 653

[8] Reynolds, S. E. et al., The spectrum of Canopus. III. Abundances of r- and s-process elements., 1988, Monthly Notices of the Royal Astronomical Society, 235, 1423

[9] Domiciano de Souza, A. et al., Refined fundamental parameters of Canopus from combined near-IR interferometry and spectral energy distribution, 2021, Astronomy & Astrophysics, 654, A19


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: cmorris035, dronemachine, gibarroule, ivolipa, jameswrowles, Kagateni, modularsamples, newagesoup, qubodup, simon.danielsson, and tm1000 at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)