DEEP SKY SOUTH: Tucana - The Toucan

The subject of this article by Andrew Wood, keen observer and Skywatcher's member, is the constellation Tucana. This constellation is close to the meridian directly above Octans as the sky becomes fully dark during November – about 10PM daylight saving time in the Sydney region. Below he describes some of the Deep Sky Objects he has personally observed in Tucana using a 250mm Newtonian reflector, complemented with observations recorded in E.J. Hartung’s Astronomical Objects for Southern Telescopes.

This is Part 2 of my challenge to observe Deep Sky objects in the Southern part of the celestial sphere. You can read about Octans (the subject of the first instalment of this series) via this link.

Unlike Octans which has no ‘showy’ Deep Sky Objects, Tucana has a range of observing opportunities from naked eye to large telescope. It contains the Small Magellanic Cloud (SMC), one of the two large naked eye satellite galaxies of our Milky Way galaxy. Close to the SMC is the large and bright showpiece globular cluster called 47 Tucanae. So to make a start on Tucana, if you have access to a fairly dark sky before midnight during November, both the SMC and 47 Tuc should be visible without optical aid. Using binoculars, these two objects should even be visible under suburban light pollution. The image below is one I took form my backyard.

Beta-Tuc (also designated Lac 119). A wide double star with nearly equal components (magnitudes 4.3 and 4.5; separation 27 arcseconds (“). Pairs like this are excellent for beginning observers. They are easy to separate in any-sized telescope and look stunning. My notes describe a pair of ‘headlights’, the brighter star white and the fainter possibly with a tinge of orange. Hartung described, “A beautiful field containing the very bright wide pale yellow pair”.

Delta-Tuc (also designated h 5334). The components of this double star have a large magnitude difference (4.5 and 8.7) but a relatively large separation of 7”. I saw the fainter star at 60x magnification. The pair have a notable colour difference. I saw them as white and red (quoting the brighter star first). Hartung also noted the same colours, describing a beautiful pair through a 75mm aperture telescope.

h 3416. Components close in brightness (mag 7.6, 7.7) separated by 5”. I was able to separate them using 60x and the split became more apparent at higher magnification. I saw the brighter star slightly yellow and the fainter component white.

Kappa-Tuc (also designated h 3423). Also separated by 5”, with mags of 5.0 and 7.0. I saw the brighter star coloured very strongly orange. Hartung described this pair as “particularly beautiful in a moonlit field, the stars yellow and orange”. (That is an advantage of double stars. Unlike other Deep Sky Objects they can sometimes be observed under moonlight or light pollution).

Gli 9. (In the same field as NGC 419 – see below). This is more challenging, particularly due to the magnitude difference (mag 7.2 and 10.2; 3“). I was able to see a clear split at 115x.

CorO 3. Components are magnitudes 6.3 and 8.0 separated by 2.3 arcseconds. I was able to split them at 200x.

NGC 104 (also designated as 47 Tucanae).

This incredible object mentioned early in this article is generally something you take a look at whenever you get the chance, in any-sized telescope; the bigger the aperture the even better. I’ll leave it to Hartung to describe: “This wonderful globular cluster…is crowded with innumerable stars steadily increasing to the dense, very bright centre, the scattered outliers reaching to 25’ diameter. Even 7.5cm shows granularity in the large bright haze and 10.5cm resolves it well, whilst larger apertures present a most impressive and beautiful sight”.

The mage right is 47 Tucanae Credit: ESO

NGC 121. While observing 47 Tuc at low power, which shows it in all its glory, within the same field you may see NGC 121, a much smaller globular cluster. (The best analogy I can think of size-wise is a ball bearing next to a shot-putt). Hartung describes “A small, remarkably elongated globular cluster”. Still a fairly bright object in larger apertures, NGC 121 is more representative of a typical globular; which really illustrates how spectacular 47 Tuc is.[Also within the same low power field as 47 Tuc are two clusters described by Hartung which I am yet to observe. They are Kron 3, according to Hartung visible in a 15cm telescope; and Kron 7, visible through a 30cm telescope on a very dark night. Having found out about them, I’m intrigued and intend to look for them at the next opportunity.]

NGC 362. Another large, very bright and well resolved (this describes that individual stars are visible) globular that is a great sight, once more overshadowed by its proximity to 47 Tuc. Hartung describes it as a beautiful object with a very bright, compressed centre.

A multitude of DSOs in Tucana are found within the Small Magellanic Cloud. [Hartung actually has a separate chapter for the SMC apart from Tucana itself]. Anyone with a large aperture telescope and a detailed atlas can spend many nights just on the objects within the SMC.

Here are a few objects I’ve observed that Hartung also described:

NGC 419. At magnitude 10.0 and with an angular size of 2.6’, I saw this as having a very bright, dense nucleus. Hartung saw a round, symmetrical, hazy object rising to a broad centre. (The same field also contains the double star Gli 9 mentioned above).

NGC 330 and NGC 346 are open clusters that I saw in the same low power field. NGC 346 is also listed as having associated nebulosity. Using a UHC filter at 55x, I was able to pick up the nebulous haze. I saw NGC 346 as very bright and slightly oval in shape. Hartung described its appearance similarly and noted that: “The stars must be very luminous to be evident in such a remote cluster”. He also noted the nebula of NGC 346 using an OIII filter.

Leaving the SMC and further into intergalactic space, there are two galaxies I have observed in Tucana, neither mentioned by Hartung. He noted one which I have not observed.

NGC 406. At mag 12.5, I saw it as fairly easy to see and quite obvious with prolonged viewing. Sits 1/3 of the way between two bright field stars.

NGC 7329. At mag 11.9, I saw it as faint with a distinct nucleus.

NGC 7408. Quoting Hartung: “It needs good aperture and 30cm shows a slightly elliptical faint haze about 70” wide, rising a little and broadly to the centre”. (Clearly he was more descriptive than I am).

There are many more Deep Sky Objects in Tucana. The Cambridge Double Star Atlas lists more doubles within the view of smaller telescopes. The Night Sky Observer’s Guide describe many more distant clusters and galaxies needing larger aperture. Apart from these, there are many more reference manuals available to amateur observers to aid their exploring the Deep Sky.

References

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

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

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

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