Going on a vacation to the southern U.S., Costa Rica or Mexico soon? If so you have a chance to see a sight denied to everyone living north of a line from Las Vegas to Oklahoma City to Nashville — Canopus! I’ve written many words about Sirius, the brightest star in the sky, but we rarely hear from about Canopus, the sky’s second brightest star. That’s largely because it’s only easily visible from the Gulf Coast and Florida.
Whenever I travel south in winter I make sure to look for it. You first find Sirius by shooting a line through the three belt stars of Orion toward the horizon left toward the horizon. Sirius shines pure white and brightly. Then drop a little more than three fists (36°) below and a little to the right of Sirius to find Canopus. As we learned a few days ago, a star apparent brightness is classified by magnitude with the larger the negative number the brighter the star. Sirius boasts a magnitude of –1.5 with Canopus not far behind at –0.7.
The difference is not quite a magnitude, making Sirius about twice as bright as Canopus. But Canopus is almost twice as bright as Arcturus, the next brightest star for us boreal skywatchers.
It might appear dimmer depending on how far south you travel. From Las Vegas, where Canopus is only 1.5° (three full moon diameters) high in the south at peak altitude around 8:30 p.m. local time in mid-March, the great density of air (plus water, dust, etc.) at that elevation dims the star to magnitude 3.8. But if you’re out in the desert and look just above to the southern horizon at that time, you’ll just be able to see it.
Go to Tucson, and Canopus climbs to 5°. It’s much more obvious there as it moves upward into thinner air but not until you get to Honolulu and points south does it shine at peak brilliance. I first saw Canopus when I was 16 years old on a clouded-out trip to Savannah, Georgia to see a total solar eclipse. The night was clear, so all was not lost. I’m itching to see it again, though I’ve no plans to fly south anytime soon. I hope you do.
Despite its second place ranking Canopus is a far brighter and bigger star than Sirius. Much of that star’s brilliance is due to proximity; Canopus is 73 times larger than the sun and 13,000 times brighter — a true beacon in the galaxy — but 36 times further away at 310 light years. It’s just not fair!
The magnitude scale I usually talk about describes a star’s apparent brightness. It’s true luminosity is given by its absolute magnitude or how bright the star would appear at a standard distance of 32.6 light years (equal to 10 parsecs). Placing all the stars at the same distance gives us a much more realistic picture a star’s true brightness unaffected by their varied distances. Sirius fades to magnitude +1.4 while our friend Canopus pours out its heart at magnitude –5.7, more than twice as bright as Venus!
Stars burn different fuels as they age. For the last 4.5 billion years, the sun’s been burning hydrogen gas only, converting it through fusion into helium. As it ages, its core will eventually grow hot and dense enough to fuse helium into carbon. Canopus is 8 to 9 times as massive as the sun and a further along in its evolution, burning helium in its core. It almost but probably not massive enough to explode as a supernova one day. Instead, it will evolve into a white dwarf, similar to the sun’s fate.
Canopus is the brightest star in the constellation Carina the Keel, part of the once-upon-a-time giant constellation Argo Navis, a large ship afloat in the southern Milky Way. 18th century astronomers found the ancient figure too large and cumbersome and disassembled it into three separate smaller groups: Carina, Puppis (the stern) and Vela (the sails).
Because of its brilliance, both apparent and absolute, Canopus has another fame. It’s a bright star located well away from the glare of the sun and planets (far from the plane of the solar system). That makes it an ideal navigation star for spacecraft. A space probe determines its exact location with a star tracker that uses the fixed stars as references.
I hope you get to see Canopus one day. Next time you’re headed south in winter or spring, add it on your itinerary.