Canopus is a brilliant white giant star in the southern constellation Carina. It is the second-brightest star in the night sky, yet lies more than 300 light-years away — a remarkable distance for a star that appears so bright from Earth. Here are some interesting facts about Canopus.
The Second-Brightest Star in the Night Sky
With an apparent magnitude of about −0.74, Canopus is outshone only by Sirius in the night sky. But while Sirius is just 8.6 light-years away, Canopus lies around 310 light-years from Earth — roughly 36 times farther away. For Canopus to appear almost as bright from such an enormous distance, it must produce vastly more light than Sirius.
Find it Below Sirius — If You Live Far Enough South
Canopus is relatively easy to find once you know where to look. First locate Sirius, the brightest star in the night sky. Canopus lies about 36° away in the direction of the southern horizon — a little more than three fist-widths at arm’s length — and is the brightest star in the constellation Carina. There is one catch, however: the star lies so far south in the sky that it never rises for observers north of about 37° N latitude, which puts it beyond the horizon from most of Europe and much of the northern United States. Even close to this northern limit it barely clears the southern horizon, while observers farther south can see it much higher in the sky.
It Can Flash Different Colors Near the Horizon
Canopus is a white star, but observers sometimes see it flickering through shades of red, orange, blue and green when it is low in the sky. The star itself isn’t rapidly changing color. Instead, its light is being bent and disturbed as it passes through turbulent layers of Earth’s atmosphere, an effect known as scintillation. The effect becomes stronger near the horizon because the starlight has to travel through more atmosphere before reaching your eyes. This can make Canopus particularly striking for northern observers, where the star never climbs very high above the southern horizon.
Enormous and Exceptionally Luminous
Canopus has a radius around 70 times that of the Sun. If it replaced the Sun at the center of our Solar System, its surface would extend roughly 90% of the way to Mercury’s orbit. Its surface temperature is around 7,400 K (12,900°F / 7,100°C). Combined with its enormous size, this allows Canopus to produce more than 10,000 times as much light as the Sun — explaining how it can appear so bright in our night sky from more than 300 light-years away.
Young — But Already an Evolved Giant
Canopus is thought to be only around 30 million years old, making it extraordinarily young compared with our 4.6-billion-year-old Sun. Yet with roughly eight to nine times the Sun’s mass, Canopus has already exhausted the hydrogen at its core and evolved away from the main sequence. Massive stars progress through their stellar life cycle much more rapidly than smaller stars, consuming their nuclear fuel faster and living dramatically shorter lives.
Canopus lies close to the mass range where a star’s eventual fate becomes difficult to predict. One possibility is that it will eventually shed its outer layers and leave behind an unusually massive oxygen-neon white dwarf, rather than exploding as a supernova.
A Navigation Star for Spacecraft
Canopus has proved useful for navigation far beyond Earth. Its exceptional brightness and position away from the crowded plane of the Milky Way make it an excellent reference point for spacecraft star trackers. Voyager 2, for example, used Canopus as a celestial reference to help determine its orientation during its journey through the Solar System. In effect, spacecraft could use the distant star as a fixed point in the sky to help work out which way they were pointing — a modern role for a star that humans had used for navigation for centuries.