Object Of The Week July 5, 2020 – NGC 6752, the 3rd or 4th brightest globular cluster in the sky

[wvreeven]

NGC 6752, Caldwell 93, the Great Peacock Globular

Globular Cluster

Constellation: Pavo

RA: 19 10 52.11
DEC: -59 59 04.4

Mag: 5.40 (though Simbad says 6.28)

Size: 20.4'

First let's address a minor controversy: Is this the third or the fourth brightest globular cluster in the sky? For a very long time, Wikipedia claimed it was the third brightest, after Omega Centauri and 47 Tucanae. As a matter of fact, NGC 6752 was APOD on July 5, 2013 (https://apod.nasa.gov/apod/ap130705.html) (see the picture below), and the info there seems to have been copied straight from Wikipedia. Funny enough, Wikipedia has stated for a long time that the brightness of M 22 is +5.1 clearly making it brighter than NGC 6752. When I started writing this article I noticed that now the Wikipedia page for NGC 6752 says that it is the fourth brightest globular after Omega Centauri, 47 Tucanae and M 22 whew.

3945
NGC 6572 as imaged by Damien Peach.

NGC 6752 was discovered by James Dunlop on June 30, 1826, who was the first astronomer to systematically observe the southern hemisphere beating John Herschel by over 10 years. Dunlop did more than 40,000 observations from Paramatta, New South Wales, (now called Parramatta) and catalogued nearly 7400 stars between June 1823 and February 1826. He left the Paramatta observatory and started conducting private observations from his home in Paramatta from March 1826 until he returned to Scotland in 1827. Dunlop returned to Australia in 1831 and would stay there until his death in 1848.

The cluster itself is not a very special one. It lies at about 13,000 ly from Earth, making it one of the closer ones, and about 17,000 ly from the galactic center. It is of Shapley-Sawyer Concentration Class IV, meaning intermediate density. Dunlop could discern stars in the cluster, though I don't know what telescope he used to observe it.

So, why choose this cluster then? Well, simply because of it's beauty. I observed the cluster shortly after the solar eclipse in 2019 from Chile using a 12" dobsonian telescope. My notes read:

"Visible to the naked eye. At 28x clearly visible as a large, round glow. At 112x spectacular! A large cluster with no less than five streamers and an arc of stars and a very compact core. Fantastic! Beats M 13!!!"


As always,

"Give it a go and let us know!
Good luck and great viewing!"

[Ivan Maly]

My Australian notes simply say, "A beautiful globular." (D = 150 mm.) I am pretty certain Dunlop observed it with his homemade, approx. 9" speculum Newtonian. One cannot argue with the statement that he was the first to observe deep sky objects of the southern skies systematically (although Halley and Lacaille might disagree), and that is certainly what you meant. As far as simply being the first astronomers to systematically observe the sourthern skies, that must be Keyser and de Houtman in the 16th century (they weren't really astronomers when they left Europe, and Keyser did not get to return, but they certainly became accomplished astronomers on their voyage... both of them, that is, if we are to believe de Houtman's assertions as to his role in the work - which I personally have no reason to disbelieve).

[Steve Gottlieb]

I don't know if this makes it special (just being one of the top globulars in the entire sky is enough!), but an extremely dim dwarf spheroidal called Bedin 1 (https://en.wikipedia.org/wiki/Bedin_I) was discovered about a year and half back serendipitously during an HST study, hiding behind NGC 6752. Here's the discovery paper (https://academic.oup.com/mnrasl/article/484/1/L54/5288002).

Here's one of my observations from Australia --

18" (7/10/02): NGC 6752 is one of the top naked-eye globular clusters (4th brightest in integrated magnitude and 2nd in terms of brightest members) and was spectacular from Magellan Observatory. At 128x, it seemed fully resolved with an uncountable number of stars densely packed over a large region. Streamers in the halo greatly increased the diameter to roughly 20'. The central region was well-compressed to a very bright core! There was a strong 3-dimensional effect as the core was covered with scores of fairly bright stars seemingly superimposed over a rich mat of fainter stars and all set over a background glow. The nucleus was small and very bright. Many 11-12th magnitude stars in the halo formed complete loops and long chains. A mag 7.7 double star is superimposed on the SW edge of the halo (HJ 5085 = 7.7/9.2 at 2.8"). This was one of my favorite objects!

[IVAN]

This is also a favourite of mine, I usually visit it when Pavo is favourably placed. I grew up around Parramatta and first spotted NGC 6752 through a 6 inch F7 scope in the early 1980's as a teenager.

[Bertrand Laville]

Hi All,

Here is my drawing with a 80 mm scope, from Namibia.
This observation has brought to me a beautiful story of ... starfish!
But, as it is a bit too long to tell it, may I ask you to give it a go at http://www.deepsky-drawings.com/ngc-6752/dsdlang/fr

Thanks
Bertrand
http://www.deepsky-drawings.com/

3948