Pole Stars

The Big Dipper is plunging toward the northern horizon as night falls now, as if it’s about to dip into a pail of cool water.

Line up the stars at the leading edge of the dipper’s bowl, and follow that line to the upper right. The first moderately bright star you come to is Polaris, the Pole Star or North Star. Earth’s north pole aims toward it, so Polaris forms the hub of the northern sky — all the other stars appear to rotate around it. And it’s always at the same point above the horizon — night and day, all year long.

A star marks the south pole, too. It’s not nearly as prominent as Polaris, though. In fact, it’s barely visible.

The southern pole star is Polaris Australis. It’s also known as Sigma Octantis because it’s in the constellation Octans, which depicts a navigational instrument known as an octant.

Polaris Australis isn’t as impressive as Polaris mainly because Polaris is huge and brilliant — an especially impressive specimen.

Compared to most stars, though, it’s impressive, too. It’s more than half again the mass of the Sun. It’s expanding as it nears the end of its “normal” lifetime, so it’s several times wider than the Sun. And its outer layers puff in and out, so it brightens and fades a bit every couple of hours; on average, it’s about 40 times brighter than the Sun. But it’s also 280 light-years away. That distance keeps Polaris Australis from being a good pointer to the celestial south pole.

Script by Damond Benningfield