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Canopus II

February 23, 2015

Until the last couple of decades, the second-brightest star in the night sky was a bit of a mystery. Astronomers couldn’t get a good handle on its distance, so they couldn’t tell whether it was simply big and bright, or super big and bright.

Canopus is the leading light of the constellation Carina, the keel. From the southern U.S., it’s quite low above the southern horizon this evening, well below Sirius, the only star that outshines it.

Canopus is far enough away that it’s difficult to measure its distance from the ground. Estimates ranged from a couple of hundred light-years to more than a thousand. But if you don’t know a star’s distance, you can’t tell that much about it. It might look bright because it really is bright, or just because it’s close by.

In the early ’90s, though, a space telescope measured the distance to Canopus with unprecedented accuracy: it’s about 300 light-years away. That yielded the star’s true brightness — about 13,000 times brighter than the Sun.

And that allowed astronomers to compile a better profile of the star. It’s about 8 to 10 times as massive as the Sun, and more than 70 times the Sun’s diameter. The great size tells us that Canopus is nearing the end of its life, so it’s puffed outward. Over the next million years or so it may get even bigger and brighter. And while the star is already a good beacon for interplanetary travel, the extra brilliance would enhance that role. More about that tomorrow.


Script by Damond Benningfield, Copyright 2014

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