Sigma Octantis

Sigma Octantis is a bright star in the Octans constellation. It acts like the pole star for the Southern Hemisphere. It is also called Polaris Australis.

This star is only about one degree from the southern celestial pole. That point is right above the Earth's South Pole. If you are in the southern part of the world and look up, Sigma Octantis helps you find true south. It works like the North Star does for finding north.

Sigma Octantis is about 294 light-years from Earth. It is a subgiant star with a spectral type of F0 IV. Its brightness changes a little over time, making it a Delta Scuti variable. It is not as bright as the North Star, so it can be harder to see without special tools.

Nomenclature

σ Octantis (Latinised to Sigma Octantis) is the star's Bayer designation.

It is called Polaris Australis because it is the southern hemisphere's pole star. This name was first used in the 1700s. In 2016, a group called the Working Group on Star Names decided to make star names standard. They approved the name Polaris Australis for this star in 2017. Now this name is used officially. It is the southernmost named star.

Properties

Sigma Octantis is a star that is a subgiant. It is thought to be about 900 million years old. The star is 4.4 times the size of the Sun and shines 44 times brighter.

This star changes its brightness a little—about 0.03 magnitudes—every 2.33 hours. Scientists believe it pulses in a simple way.

Southern pole star

Sigma Octantis is the current southern pole star, with Polaris being its northern counterpart. In the southern hemisphere, Sigma Octantis looks almost still while the other stars move around it. It is part of a small shape that looks like half a hexagon. It is just a little more than one degree from the true south pole, and this position changes slowly over time.

Because Sigma Octantis is not very bright, it is hard to see without special tools, unlike the brighter Polaris. Instead, many people use the constellation Crux to find the South Celestial Pole. Once you know where Sigma Octantis is roughly, you can check it using a group of stars that form a special shape called a trapezoid.

In astrometrics

Sigma Octantis helped scientists measure how bright stars are in the southern sky. They made a special list in 1908. They used a special star in the north to measure stars there. They found that these stars did not seem to change in brightness. This did not change the results of their measurements.

In culture

Sigma Octantis is the dimmest star shown on a national flag. You can see it on the flag of Brazil, where it stands for the Brazilian Federal District.