A portion of Lacaille's 1752 star map [Credit: Linda Hall Library]

Octans was one of the southern constellations created by 18th-century French astronomer Nicolas-Louis de Lacaille. He used them to fill the gaps in the southern sky map, naming them for tools of the arts and sciences of his day. An octant was a navigation device that preceded the sextant.

History
The ancient constellations from classical Greek and Roman times are the best known ones, and are also the oldest ones still used by astronomers. They're generally what was visible from the Mediterranean region long ago. However, European exploration finally made it clear to astronomers that there was a lot of sky missing from their celestial globes.

Celestial cartographer Petrus Plancius (1552-1622) arranged for navigators Pieter Dirkszoon Keyser and Frederick de Houtman to bring back observations of the southern skies from their voyages. These observations became twelve new constellations.

Then in the middle of the 18th century, the French Academy of Sciences sent Lacaille to the tip of Africa to chart the southern sky. In his two years at the Cape of Good Hope, he observed several thousand stars and invented new constellations to fill the remaining gaps. Fourteen of those constellations are in the official list of the 88 constellations of the International Astronomical Union.

The ancient constellations tell tales of gods and mortals, and those of Plancius were inspired by natural history. The names of Lacaille's constellations are somewhat obscure to 21st-century people. Yet for Lacaille they were celebrations of the tools of the scientists, artists and explorers of the Age of Enlightenment.

The stars of Octans
Wrapped around the south celestial pole, Octans isn't visible in the northern hemisphere. It's so faint that it's not seen much in the southern hemisphere either.

On the magnitude scale that astronomers use for brightness, the dimmest stars we can see with our unaided eyes are of the sixth magnitude. The numbers get smaller as the stars get brighter. A very bright star can have zero magnitude or even a negative one, such as Sirius whose magnitude is -1.5.

The best known star in Octans is Sigma Octantis, the south polar star. Its official name is Polaris Australis, and it's one degree away from the south celestial pole. If it were a second-magnitude star like the northern star Polaris, it would be a navigation star. Unfortunately, its magnitude is a mere 5.5, and you need very good viewing conditions to see it.

The three brightest stars of Octans are the fourth-magnitude stars Nu (ν) Octantis, Beta (β) Octantis, and Delta (δ) Octantis. They form the green triangle on the sky map. Alpha Octantis, 7th in the order of brightness, is a fifth-magnitude star.

Nu Octantis is an orange giant that has just recently – in astronomical terms, of course – stopped burning hydrogen as its fuel. It's a small giant, only about six times the size of the Sun. However, it's still expanding, and in 100 million years or so will be much bigger and about sixty times brighter than it is now. Nu is a binary star with a red dwarf as a close companion. There's also evidence of a planet, but as of August 2019, it was still unconfirmed.

Delta Octantis is another orange giant, but older and bigger than Nu Octantis. Its claim to fame is an unusual one – it's the south polar star for Saturn.

Deep sky objects
Looking up at Octans, you're looking out of the plane of the Milky Way, so there aren't galaxies galore to be seen. Constellation fact sheets are rather dismissive of the constellation. Yet although they are dim, there are deep sky objects. In fact, John Herschel discovered several during his time at the Cape of Good Hope in the 1830s.

The best known of the deep sky objects is NGC 2573, which Herschel named Nebula Polarissima Australis. It isn't a nebula in the modern sense, but at the time, all of the nebulous objects in the sky were known as nebulae. NGC 2573 is a spiral galaxy whose name tells us that it's close to the pole.

But the real gem of Octans is another of Herschel's discoveries, a rare barred-ring spiral galaxy, NGC 7098. A barred spiral is a spiral galaxy with a central bar-shaped structure made up of stars. NGC 7098 has two bars in its bright central region, but the most prominent feature are the two rings. These are actually its spiral arms which are wound around the core.

Even brilliant polymath that he was, on that September night in 1835, when John Herschel discovered NGC 7098, he could never have imagined what he was looking at.