Sand and Stars

On the Way to Polarissima Australis

Hadley’s octant, 1760. Image credit Galata Maritime Museum, Genoa

15 Aug 2017

Last night I set out across Octans to observe Polarissima Australis, the galaxy closest to the south celestial pole, but on my way to it, I was waylaid by some other very nice observations. Octans is somewhat sparse in DSOs, but those it does have are well worth a visit: the four galaxies John Herschel discovered at the Cape of Good Hope with his 18-inch f/13 speculum telescope, among them a lovely interacting pair, Polarissima Australis’ close neighbours, which are also an interacting pair of galaxies; the Index Catalogue’s sole Octans object; Octans’ two open clusters (rather odd things to find down around our southern polar region) and, of all odd things, Saturn’s southern pole star. 

The reflecting octant was invented by the English mathematician John Hadley in 1730, and it was the instrument that revolutionised maritime navigation for seafarers.

John Hadley (1682-1744)

Until its invention, mariners used back-staffs – instruments that measure the altitude of the sun by the projection of a shadow. (Users kept the Sun to their back – hence the name – and observed the shadow cast by the upper vane on a horizon vane.) Their major problem was that it was impossible to sight the stars; a back-staff could only be used during the day. 

The octant changed that. It allowed direct observations of stars. The octant contained an arc of forty-five degrees, one eighth of a circle (hence its name). It had two-mirrors which reflected the light of a star and enabled the observer to measure altitudes (the angle between the star above the horizon) up to ninety degrees. The sight was easy to align because the horizon and the star seemed to move together as the ship pitched and rolled. (Prior instruments were also difficult to use on shipboard, except when the sea was calm.)

The inimitable Abbe Nicolas Louis de Lacaille, with his love of all things scientific, really got it right when he engraved an octant into the stars as our southern polar constellation. 

Three interesting stars

Delta Octantis, Saturn’s southern pole star. DSS2 image

Delta Octantis is a very unusual star. Not because it is an attractive orange star in the telescope (lots of orange stars in the sky), but because it has the distinction of being Saturn’s southern pole star. Saturn is the only other planet besides Earth that has its southern pole aimed at Octans.

And talking of stars, mag 5.15 Alpha Octantis is a palest yellowy-white star in the telescope, and it has the distinction of being the dimmest of all the stars named Alpha and, indeed, it isn’t the brightest star in the constellation, mag 3.73 Nu Octantis is, which itself is another pretty orangey star in the telescope.

After a quick look at the solitary mag 5.4 Sigma Octantis (a nice pale yellow in the telescope) that forms our southern pole star, it was time to head deeper into Octans…

NGC 2573 Polarissima Australis Galaxy  

RA 01 42 09.1   Dec -89 20 05   Mag 13.5   Size 2.0’x0.8’   SB 13.9

DSS image
Polarissima Australis Galaxy. DSS image

This galaxy shows as a small smudge of light, with low surface brightness, slightly elongated east-west. It brightens slightly toward the core, and has a surprisingly bright stellar nucleus. The western edge of the galaxy seems to be just slightly hazier. It is situated nearly halfway between a 10th mag star to its south, and a small triangle of stars 11th, 13th and 13th mag. to its north.

NGC 2573A Galaxy 

RA 23 12 04.5   Dec -89 07 35   Mag 14.3   Size 2.1’x0.6   SB 14.4

NGC 2573B Galaxy 

RA 23 07 32.6   Dec -89 07 00   Mag 14.6   Size 1.7’x0.6   SB 14.5

DSS image
NGC 2573A & NGC 2573B .DSS image

A pair of Interacting galaxies is always a treat to see, even if they are as faint as faint can be yet still be visible. NGC 2573A is the fainter of the pair, and is indeed very faint, a mere smudge of palest grey light, elongated north-south. NGC 2573B is also very faint – a small, thin streak of light; its major axis extended in the direction of NGC 2573A.

NGC 6438  Galaxy

RA 18 22 16.9   Dec -85 24 08   Mag 11.1   Size 1.6’x1.3’   SB 11.8

NGC 6438A  Galaxy 

RA 18 22 44.1   Dec -85 24 36   Mag 11.8   Size 2.7’x1.0’   SB 12.7

NGC 6438 & NGC 6438A. Image credit Hubble

This is another interacting pair of galaxies, and they present a very unusual sight in the eyepiece. They appear as a small, round, moderately bright glow with a faint, diffuse elongated arc-shaped glow attached to its eastern side. The round glow is NGC 6438; the faint arc-shaped glow is NGC 6438A. No nucleus seen in either glow, although NGC 64638’s round glow does brighten very slightly to the centre with averted vision, and NGC 6438A’s arc-shaped glow appears marginally brighter with averted vision where it attaches to its companion, but I am not sure if it actually is brighter or if it’s simply because of the relative brightness of NGC 6438.

NGC 7098 Galaxy 

RA 21 44 16.4   Dec -75 06 43   Mag 11.3   Size 4.1’x2.6’   SB 13.7

NGC 7098. Image credit ESO

This galaxy appears as a fairly faint glow elongated NE-SW. It has a broad brighter core that appears slightly bar-shaped. Averted vision reveals a very faint outer halo, a soft gossamer halo of slightly uneven brightness. The galaxy lies in a field with many bright stars; a very pretty sight.

NGC 6920 Galaxy 

RA 20 43 57.7   Dec -80 00 08   Mag 12.3   Size 1.8’x1.5’   SB 13.3

DSS image
NGC 6920. DSS image

This galaxy appears as a faint, small, round, glow with a very small but intense core. Averted vision reveals a very faint, even outer halo that fades away towards the periphery.

IC 4333 Galaxy 

RA 14 05 20.9   Dec -84 16 23   Mag 13.4   Size 1.6’xo.4’   SB 12.8

DSS image
IC 4333. DSS image

This galaxy appears as an extremely faint, and extremely thin streak of dim light, elongated NE-SW. The glow is even, with no hint of a nucleus.

Mel 227 Open Cluster

RA 20 15 18.9   Dec -79 17 00   Mag 5.3   Size 70’

DSS image
Mel 227. DSS image

This is a very large, very loose, very irregular collection of roughly 40 or so stars of mixed magnitude that stand out fairly well against the background stars, even though it lacks any sort of central concentration. The northern region appears to have the majority of the stars. Higher power adds more stars… but whether they are cluster members or not, it is impossible to tell.

ESO 8-6 Open Cluster

RA 14 55 12.0   Dec -83 25 00   Mag –   Size 10’

DSS image
ESO 8-6. DSS image

This open cluster lies in the western region of Octans, not too far from Pi Octantis, and it requires high power to pick it up. It consists of roughly mag 12 stars, with the western portion of the cluster appearing to be slightly denser. Surprisingly, although it is rather faint, it stands out quite well against the background.

Copyright © Susan Young 2017