The Octans Constellation

Octans is a faint constellation located in the deep southern sky. Its name is Latin for the eighth part of a circle, but it is named after the octant, a navigational instrument. The constellation was devised by French astronomer Nicolas Louis de Lacaille in 1752, and it remains one of the 88 modern constellations. Octans is the 50th constellation in size, occupying an area of 291 square degrees. It lies in the fourth quadrant of the southern hemisphere (SQ4) and can be seen at latitudes between +0° and -90°. The neighbouring constellations are Apus, Chamaeleon, Hydrus, Indus, Mensa, Pavo and Tucana. Octans belongs to the Lacaille family of constellations, along with Antilla, Caelum, Circinus, Formax, Horologium, Mensa, Microscopium, Norma, Pictor, Reticulum, Sculptor, and Telescopium. Octans contains two stars with known planets and has no Messier objects.

Octans is a very faint constellation; its brightest member is Nu Octantis, a spectral class K1 III giant star with an apparent magnitude 3.73. It is approximately 69 light years distant from the solar system. The star is an orange giant with the stellar classification K1III. It is one of the least luminous giant stars known, with a luminosity only 16 times that of the Sun. It is also a relatively small giant, with a radius only 5.9 times solar and a mass 1.4 times that of the Sun. The star will eventually expand and, in about 100 million years, it will become 15 times larger and 60 times brighter than it is now. The star’s estimated age is about 12.1 billion years. Nu Octantis has a binary companion, a K7-M1 class dwarf with a mass 0.5 times solar. The two are separated by 2.55 astronomical units and orbit each other with a period of 2.9 years. The primary star, Nu Octantis A, has an unconfirmed planet with a mass at least 2.5 times that of Jupiter in its orbit. Beta Octantis is a white star halfway between the dwarf and subgiant stage of evolution. It has the stellar classification of A9IV-V. It has an apparent magnitude of 4.14 and is approximately 140 light years distant from the Sun. It is the second brightest star in Octans constellation. Delta Octantis is an orange giant star belonging to the stellar class K2III. It has an apparent magnitude of 4.31 and is approximately 279 light years distant from the solar system. The star has a mass 1.2 times that of the Sun and a radius 25 times solar. Its estimated age is the same as the Sun’s, about 4.3 billion years. Delta Octantis is the southern pole star on Saturn. Sigma Octantis, the southern pole star, is a magnitude 5.4 star just over 1 degree away from the true South Celestial Pole. The star is a yellow-white giant star belonging to the stellar class F0III. It has an apparent magnitude of 5.42 and is approximately 270 light years distant from the Sun. It has about two solar masses. Sigma Octantis is classified as a Delta Scuti variable, which is to say a variable star that exhibits variations in brightness as a result of both radial and non-radial pulsations of its surface. These stars are used as standard candles for determining the distances to deep sky objects such as globular and open star clusters and the Galactic centre. Sigma Octantis exhibits a variation of 0.03 magnitudes throughout 2.3 hours.

Conveniently for navigators, there are other, much easier methods for locating the southern celestial pole. For example, the constellation Crux, the Southern Cross, currently points toward the South Celestial Pole, if one draws a line from Gamma Crucis to Alpha Crucis. Another method includes an asterism made up of Sigma, Chi, Tau, and Upsilon Octantis, which form a distinctive trapezoid shape. In addition to having the current southern pole star of Earth, Octans also contains the southern pole star of the planet Saturn, which is the magnitude 4.3 Delta Octantis.

HD 142022 is another binary system in the constellation. It has an apparent magnitude of 7.69 and is approximately 117 light years distant from the Sun. The primary component belongs to the spectral class G9V and has a mass 1% less than the Sun. It is a very old star, only about 400 million years younger than the Universe itself. A planet was discovered orbiting it in 2005. HD 212301 has the stellar classification of F8V, which makes the star a yellow-white dwarf. It has an apparent magnitude of 7.77 and is 171.93 light years distant from the Sun. The star has a planet orbiting it with a period of 2.24572 days. Credits: Constellation Guide, Wikipedia.