# Polaris Australis (Sigma Octantis)

Polaris Australis (Sigma Octantis) is a star in the constellation Octans, magnitude 5.4.

**The southern pole star — so faint it is no use for navigation, yet it flies on Brazil’s flag.**

The north has Polaris, bright enough to steer by. The south has Sigma Octantis — Polaris Australis — barely visible to the naked eye. It sits about a degree from the south celestial pole, so as the sky turns, every other southern star appears to wheel around it. But it is so faint that navigators use the Southern Cross to find the pole instead.

Up close it is a white subgiant, about 4.4 times the Sun’s radius and 44 times as luminous, near the end of its hydrogen-burning life at around 900 million years old. It also pulses as a Delta Scuti [variable star](https://astropilot.space/glossary/variable-star), brightening and fading by about three hundredths of a magnitude every 2.33 hours.

Its reign is temporary. Precession — the slow wobble of Earth’s axis — is carrying the pole away from it. The name Polaris Australis was first used in the 1700s, and it is the southernmost named star.

Polaris Australis (Sigma Octantis) is 280 light-years away: the light we see from it set off around 1745, 25 years before Charles Messier published his famous catalogue.

## Did you know?

- **On Brazil’s flag.** Sigma Octantis is the faintest star shown on any national flag. On Brazil’s flag it stands for the Federal District, home of the capital.
- **A yardstick for brightness.** For the Revised Harvard Photometry of 1908, astronomers used Sigma Octantis as a reference for measuring southern stars, while Polaris and Lambda Ursae Minoris served in the north.

A faint 5th-magnitude star about a degree from the south celestial pole — you need a dark sky to see it at all. Confirm it with binoculars: it forms a small trapezoid with three similar stars, Chi, Tau and Upsilon Octantis.

## When and where to see it

It is best placed in the evening sky around September, when it crosses the meridian at about 10 pm local time (around midnight in August). It can be seen from everywhere south of 1°N and never sets south of 1°S.

At its highest it stands below the horizon from 50°N, below the horizon from 35°N, 1° up from the equator, 36° up from 35°S.

## Facts

- Type: Star
- Constellation: Octans
- Designations: Sigma Octantis, σ Oct, HR 7228, HD 177482, HIP 104382
- Magnitude: 5.4
- Position: RA 21h 8.8m, Dec −88° 57′ (J2000)
- Spectral type: F0IV
- Colour: yellow-white (B−V 0.32)
- Distance: 280 light-years
- Discovered: Called Polaris Australis since the 1700s

Open AstroPilot to see when Polaris Australis (Sigma Octantis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view. https://astropilot.space/object/polaris-australis