Star Constellation Facts: Octans

Star Constellation Facts: Octans

Octans is the southernmost of all the 88 recognized constellations, and was named in the 18th century by the French astronomer Nicolas Louis de Lacaille after the now obsolete navigational instrument known as an octant. The brightest star in this southern sky constellation is Nu Octantis, an orange giant located 69 light years from Earth with an apparent magnitude of 3.76. Another one of its stars, Polaris Australis, marks the south Celestial Pole, but unfortunately, the whole constellation is rather faint and so, unlike its northern counterpart Polaris in Ursa Major, is not very useful for navigational purposes. There are no known meteor showers associated with this constellation.


Octans is the night sky’s 50th largest constellation, and can be seen by observers situated between +0° and -90° of latitude, although it is best seen in October. The constellation is circumpolar, or visible all year round from southern hemisphere locations, and also contains the southern pole star, known alternatively as Sigma Octantis, or Polaris Australis. The neighboring constellations surrounding Octans includes Apus, Chamaeleon, Hydrus, Indus, Mensa, Pavo and Tucana.


While the northern constellations were familiar to ancient civilizations, those of the southern sky remained largely unknown to northern hemisphere explorers until roughly the 1600s when Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman discovered and invented 12 new constellations. In the 1750s, French astronomer Nicolas Louis de Lacaille subsequently added a further 14 southern sky constellations to the list during his expedition to South Africa. One such constellation was Octans, with Lacaille having borrowed a number of stars from the nearby constellation of Hydrus to form his new star arrangement.

Lacaille Constellation Family

Octans is a member of the Lacaille family of constellations, together with Norma, Circinus, Telescopium, Microscopium, Sculptor, Fornax, Caelum, Horologium, Mensa, Reticulum, Pictor and Antlia.

Principle Stars & Deep Sky Objects

Octans does not contain any notable space objects worth seeing outside of our solar system, with the exception of the open cluster Melotte 227, and a pair of overlapping galaxies called NGC 6438 and NGC 6438A. Nevertheless, these deep-sky objects are difficult to see, and even fainter to resolve is the galaxy NGC 2573, which is the NGC object closest to the southern celestial pole that was named ‘Nebula Polarissima Australis’ by Sir John Herschel. Furthermore, Octans does not have any stars brighter than 4th magnitude:

Constellation of Octans– Nu Octantis, an orange giant (K1III) situated 69 light years from the Sun, is the constellation’s brightest star, although with a visual magnitude of just 3.76 it still appears faint to the naked eye. It has a radius 5.9 times that of our sun, and is 1.4 times as massive. However, Nu Octantis is actually a binary system with its two stars separated by 2.55 AU, and orbiting each other once every 2.9 years.

– Beta Octantis, the constellation’s second brightest star, is a 4.14 magnitude white subgiant (A9IV-V) located 140 light years from our solar system.

– Delta Octantis, the constellation’s third brightest star, is a 4.31 magnitude orange giant (K2III) found around 279 light years away. It has a similar age to our sun at 4.3 billion years old, but has 25 times its radius, and 1.2 times its mass. Delta Octantis is also the southern pole star on the planet Saturn.

– Sigma Octantis (Polaris Australis) is yellow-white giant star (F0III) located 270 light years from Earth. It has about 2 solar masses, and shines with an apparent magnitude of 5.42, although this variable star’s magnitude varies by 0.03 every 2.3 hours. Despite being the southern pole star, however, its dimness makes it too faint to be used for navigational purposes.

Other stars of interest in Octans includes Theta Octantis, an orange giant (K3III) found 221 light years away with a magnitude of 4.79; Alpha Octantis, an eclipsing binary star of 5.15 magnitude situated 148 light years distant; as well as the other binary systems of Mu-2 Octantis, and HD 142022.

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