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DEEP SKY SOUTH : Octans - The Octant


"...small telescope users can have fun testing out their optics on a range of double stars."

This article describes some of the Deep Sky objects in the constellation of the South Celestial Pole, Octans. All the objects mentioned I’ve observed personally. I’ve been a Deep Sky observer for about 30 years and have kept notes – sometimes very cursory and other times more detailed. The image at right is of the author with the telescope with which most of the objects in the article were observed.

l also like to refer to others’ observations, especially those of Australian amateur observer Ernst Hartung, a Professor of Chemistry at the University of Melbourne who, in his retirement, observed and compiled extensive notes on Deep Sky objects from rural Victoria. His 1968 book, Astronomical objects for southern telescopes, updated in 1995, was a seminal work for southern amateur astronomers.

Unlike most other Deep Sky observers, Hartung had an interest in double stars as well as the more ‘glamorous’ clusters, nebulae and galaxies. I’ve always been a keen double star observer as well.

Deep Sky to astronomers means everything outside of the Solar System: Double Stars, Open Star Clusters, Globular Star Clusters, Planetary Nebulae, Reflection, Emission and Dark Nebulae, and, of course, the Galaxies beyond our own Milky Way. Aiming a telescope at bodies within our home system, even those toward the outer limit such as Neptune or Pluto, we are observing light that has taken only a matter of hours on its journey to our optic nerve. As soon as we aim at even the nearest stars, by contrast, the light we see has been travelling for years, often hundreds or thousands of years. Observing clusters and nebulae within our Milky Way galaxy can extend this to tens of thousands of years. Galaxies beyond our own are another huge jump again to millions of years.

Large amateur instruments can observe faint galaxies hundreds of millions of light years away.

Faint galaxies just visible in a very large aperture telescope may not be as beautiful to look at as a cluster dense with bright and colourful stars, but can still cause a feeling of awe if we appreciate that that faint patch of light existed when dinosaurs were the dominant life on Earth. That is the beauty of the Deep Sky through a telescope. There are objects from very bright to extremely faint and whatever instrument you have there is always something to see and appreciate.

Octans’s ‘claim to fame’ is having the closest star to the South Celestial Pole. This ‘Pole Star’, sigma-Octans, is only just visible naked-eye in a non-light-polluted location. In fact Octans is faint in terms of naked-eye stars and also devoid of bright, showy, Deep Sky objects. Still, there are many interesting double stars and one open cluster observable to small telescope users observing even from suburban areas. If you like hunting down faint galaxies there are plenty of them, though you’ll need a large aperture telescope and be in a dark-sky location. The diagram of the Southe celestial Pole showing Octans is from the Herald-Bobroff ASTROATLAS, HB2000 Publications, 1994.

For general viewing from suburban locations with any size telescope though, there are the double stars. These vary greatly in the angular separation and the magnitude difference between the components. Stars very close together in the eyepiece also often display subtle differences in colour. When observing double stars, look straight at them in order to use the very centre of your retina where the colour defining cone cells are most concentrated. Star colour is determined by the surface temperature of the star.

Octans contains a range of double stars that will test the ability of your telescope and observing location. My own observations have been made with a 250mm aperture Newtonian reflector from suburban Wollongong.

  • h 5306 has components of magnitudes 6.0 and 10.5 separated by 35 arcseconds. This is a very wide separation, so resolution is extremely easy with any decent telescope; though using a small telescope under light pollution may make the fainter component difficult to detect. I saw the brighter (A) star as yellow. I could not make out any colour in the fainter (B) star.

  • h 4490 has components of magnitudes 6.2 and 9.0 separated by 25 arcseconds. I have two records of this pair. On both occasions I saw the A component as orange. On one occasion I saw the B star as red: on the other occasion blue. When looking at fainter stars, colour can be subtle.

  • ∆ 238 has components of magnitudes 6.2 and 8.9 separated by 21 arcseconds. I saw A as yellow, B red.

  • Mu2-Octans (also designated ∆ 232) has components of magnitudes 6.5 and 7.1 separated by 17 arcseconds. It was observed by Hartung who saw them as a yellow pair. I thought I could detect some red in the B star.

  • ∆ 82 has components of magnitudes 7.1 and 7.6 separated by 15 arcseconds. I saw the components as yellow and red.

  • Gli 263 has components of magnitudes 7.3 and 9.6 separated by 5 arcseconds. This should be resolved with most telescopes and will be a nice test for a small aperture. I saw A as yellow and B red.

  • Lamda-Octans has components of magnitudes 5.6 and 7.3 separated by 3 arcseconds. A nice ‘tester’ for small telescopes. Hartung split it with a 75mm telescope. He does not mention but I assume it was a refractor. He saw the components as yellow and white. On the night I observed the pair with my much larger aperture reflector I could only just split them. The A star was yellow.

  • R 38 is a double star with components of magnitude 6.6 and 8.1 separated by 1.9 arcseconds. When it comes to resolving double stars many factors need to be considered, including the telescope’s optical quality as well as its aperture. Refractors generally get closer to their theoretical resolving limit than reflectors, which usually have larger aperture for seeing fainter objects. R 38 theoretically could be split with a telescope of 60mm aperture although it would probably need to be a high optical quality refractor with good atmospheric seeing. (Nights when the stars twinkle is a sign of bad seeing. Close doubles are more easily split on nights when the stars don’t twinkle – good seeing). The magnitudes of the stars will present no problem to even small apertures so it’s worth a go to see if you can resolve them. With my 250mm reflector I have occasionally split stars closer than this. I haven’t been able to split this pair, however.

  • Iota-Octans has components of magnitudes 5.9 and 6.9 separated by 0.7 arcseconds. If you have a high-quality telescope you may split this pair under good seeing. I saw a single orange star.

  • Melotte 227 is the only Open Star Cluster in Octans. Open clusters are made up of stars that formed together from the same gas cloud. They can vary greatly in size and number of stars. Mel 227, is both large in size and sparse in stars. It is described in The Night Sky Observer’s Guide as, “a very large, loose, irregular cluster spanning 70’ that is difficult to distinguish as a cluster.” Still, it is a bright object and even a small telescope using lower giving a wide field of view will show it. Using an eyepiece giving a wide field of view in my 250mm telescope, I saw about 30 stars very spread out.

  • NGC 2573 (Polarissima Australis) is the closest Deep Sky object to the South Celestial Pole. Hartung mentioned it only as an object he could never see using his 300mm telescope. Observing with a 400mm telescope, I have seen it and noted it to be, “Surprisingly Bright. Possibly I was only seeing the nucleus”. Several references I’ve read note that it has a compact bright nucleus surrounded by a tenuous halo.

Despite Hartung not having seen this or recording having seen any other galaxies in Octans, the constellation does contain two dozen galaxies described in The Night Sky Observer’s Guide. In expeditions to dark sites over the years, my notes show that I have seen four of them, apart from Polarissima Australis:

  • NGC 6438: With a 400mm telescope, I saw it as faint but obvious and shapeless. [There are actually two galaxies 6438 A and B. I couldn't see the separate entities].

  • NGC 6920: With a 250mm telescope, I described it as round with a bright nucleus.

  • NGC 7098: With a 250mm telescope, I described it as large with a round nucleus.

  • NGC 7367: With a 250mm telescope, I saw a non-stellar, possibly round, haze.

So for Octans, small telescope users can have fun testing out their optics on a range of double stars. If you possess a large aperture instrument away from the light pollution of the city you can test its ability to see faint, far-away galaxies.

1 Stellarium Astronomy Software – stellarium.org

2 http://www.glyphweb.com/esky/clusters/mel227.html

3 https://telescopius.com/deep-sky/object/

References

  1. Astronomical objects for southern telescopes, E.J. Hartung, Cambridge University Press, 1968

  2. Hartung’s Astronomical Objects for Southern Telescopes, Revised by Malin and Frew, Melbourne University Press, 1995

  3. The Cambridge Double Star Atlas, Mullaney and Tirion, Cambridge University Press, 2009

  4. The Night Sky Observer’s Guide Volume 3 The Southern Skies, Cooper et al, William-Bell inc. 2008

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