Sigma Octantis

Sigma Octantis is a bright star in the Octans constellation that serves as the pole star for the Southern Hemisphere. It is also called Polaris Australis. This star is found just one degree away from the southern celestial pole, which is the point in the sky directly above the Earth's South Pole. If you were looking up at the night sky from the southern part of the world, Sigma Octantis would help you find the direction of true south, much like the North Star helps find north in the northern part of the world.

Sigma Octantis is located about 294 light-years from Earth. It is a type of star known as a subgiant and has a spectral type of F0 IV. This star's brightness changes slightly over time, making it a Delta Scuti variable. Even though it is an important guide for the southern sky, it is not as bright as the North Star, so it can be a little harder to spot 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 on September 5, 2017. Now this name is officially used. It is the southernmost named star.

Properties

Sigma Octantis is a star that belongs to the F0IV class, which means it is a subgiant. It is thought to be near the end of its main life stage, being about 900 million years old. The star has grown to be 4.4 times the size of the Sun and shines 44 times brighter.

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

Southern pole star

Sigma Octantis is the current southern pole star, with Polaris being its northern counterpart. In the southern hemisphere, Sigma Octantis seems 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 for a special list made in 1908. They used a special star in the north for measuring stars there. They noticed that these stars did not seem to change in brightness, and even if they did, it would 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.