George's Random Astronomical Object

Object 53: NGC 6352

Podcast release date: 09 August 2021

Right ascension: 17:25:29.1

Declination: -48:25:20

Epoch: J2000

Constellation: Ara

Corresponding Earth location: Slightly less than 1700 km west of southern Chile in the Pacific Ocean

NGC 6352 is a globular cluster in the constellation Ara. Ara is another one of these types of constellations that almost no one has heard of before because it's located sufficiently far to the south that it's difficult to see from a lot of the Northern Hemisphere. Rather surprisingly, it does show up in ancient Greek star charts. It is supposed to represent an altar from an ancient Greek temple. However, the medium-brightness stars in this constellation could be construed to look like just about anything. Some of my references refer to it as looking like a distorted version of the letter H, so the ancient Greeks could have called this the constellation Eta (which is the Greek letter that looks like an H), and that would have made more sense to both the ancient Greeks and modern people.

In any case, NGC 6352 is a globular cluster found at a distance of about 22000 light years (6700 pc) [1] within this constellation. Globular clusters are giant gravitationally-bound spheres of stars that orbit outside of the plane of our galaxy in the galaxy's halo. A typical globular cluster could contain up to a million stars or so, or, to be more accurate, enough stars that they have a combined mass equivalent to a million times the mass of the Sun. NGC 6352 itself has a mass of about 100 thousand times the mass of the Sun [2].

Astronomers are very interested in looking at globular clusters because they typically contain the oldest stars within our galaxy, which means that, by studying these stars, we can understand what the early universe was like. Many globular clusters appear to have come from dwarf galaxies that have fallen into the Milky Way, which means that these clusters can provide some information on how the Milky Way has grown in size over time by absorbing smaller galaxies.

However, NGC 6352 is an abnormal globular cluster that isn't like other globular clusters. The key difference is that a the stars in NGC 6352 contain many more elements heavier than hydrogen and helium than what we find in typical globular clusters [3,4].

These heavy elements have slowly formed within stars in the 13.8 billion years since the Big Bang, and the heavy elements get ejected into space when stars die. Stars in our galaxy as old as the Sun, which is about 4.5 billion years old, would have formed out of interstellar gas that contained a relatively large amount of these heavy elements because stars would have been dying and ejecting these heavy elements into the interstellar medium for about 9 billion years or so. These heavy elements are still present in the outer gas layers of our Sun and other stars, and although the amounts are very small, astronomers can see that they are there. When the oldest stars in the universe formed, including the stars found in globular clusters, the interstellar gas that they formed out of would have contained hardly any heavy elements. For example, the ratio of iron to hydrogen in the stars in a typical globular cluster is less than 10% of the ratio found in the outer atmosphere of the Sun [3]. So, it's weird that the iron to hydrogen ratio for the stars in NGC 6352 is about 30% of the value for the Sun, which is really high compared to other globular clusters [4].

After rejecting the hypothesis that NGC 6352 had been buying iron and other dietary mineral supplements from its local drug store, astronomers came up with an explanation of why this globular cluster contained so many heavy elements. They determined that NGC 6352 is probably a couple of billion years younger than the typical globular cluster [3,5,6]. This would mean that NGC 6352 would have formed out of interstellar gas that had been accumulating heavy elements from dying stars for a couple of billion years rather than forming out of mostly pure hydrogen and helium like other globular clusters.

It also looks like the cluster formed within our galaxy, and what's really interesting is that it might have formed when gravitational interactions between the Milky Way and another galaxy caused interstellar gas clouds within our galaxy to collapse and form stars [4,7]. In other words, NGC 6352 is the relic of a major intergalactic event about 11 billion years ago that changed our galaxy.

So that is what professional astronomers find to be so interesting about NGC 6352. However, it also turns out to be a popular amateur astronomy target as well because it has been included in an amateur astronomy catalogue of objects called the Caldwell Catalogue [8]. It even has the alternate designation of Caldwell 81. (However, only amateur astronomers would understand this designation; professional astronomers would only recognize the name NGC 6352.) The Caldwell Catalogue was originally created in the 1990s as a list of 109 very bright, very interesting star clusters, nebulae, and galaxies that were easy to see with amateur telescopes but that did not make it into the Messier Catalogue originally created by Charles Messier in the eighteenth century [8]. Some of the Caldwell objects were simply overlooked by Messier when he made his catalogue despite being as bright as other things that did make it into his catalogue. Other objects were simply too far south in the sky for Charles Messier to see from France, which is where he did all of his work. NGC 6352 is one of these southern objects.

The globular cluster can be seen about 1.75 degrees to the northwest of the brightest star in the constellation Ara, which is the star Alpha Ara at the upper left end of the H shape. The cluster has a magnitude in visible light of about 8 [8], and most of the stars fall within a circular region with a diameter of about 7 arcminutes (or about one-quarter the diameter of the Moon) [8]. While it's possible to see this cluster in a 15 cm (6 inch) diameter telescope and see the individual stars in a 40 cm (16 inch) diameter telescope, it's not a terribly bright or exciting globular cluster [8]. In fact, the open cluster IC 4651 to the south of this is actually much brighter. However, because NGC 6352 made it into the Caldwell Catalogue and IC 4651 didn't, amateur astronomers pay much more attention to NGC 6352.


[1] Shao, Zhengyi and Li, Lu, Gaia parallax of Milky Way globular clusters - A solution of mixture model, 2019, Monthly Notices of the Royal Astronomical Society, 489, 3093

[2] Baumgardt, H. and Hilker, M., A catalogue of masses, structural parameters, and velocity dispersion profiles of 112 Milky Way globular clusters, 2018, Monthly Notices of the Royal Astronomical Society, 478, 1520

[3] Rosenberg, A. et al., Galactic Globular Cluster Relative Ages, 1999, Astronomical Journal, 118, 2306

[4] Feltzing, S. et al., Stellar abundances and ages for metal-rich Milky Way globular clusters. Stellar parameters and elemental abundances for 9 HB stars in NGC 6352, 2009, Astronomy & Astrophysics, 493, 913

[5] Faria, D. and Feltzing, S., The Age of the Metal-Rich Globular Cluster NGC 6352, 2002, in Observed HR Diagrams and Stellar Evolution, 274, 373

[6] Pulone, L. et al., HST observations of the metal rich globular clusters NGC 6496 and NGC 6352, 2003, Astronomy & Astrophysics, 399, 121

[7] Casuso, E. and Beckman, J. E., The bimodal metallicity distribution function of the globular clusters in the Galaxy: halo disc complementarity, 2006, Astronomy & Astrophysics, 448, 571

[8] Mobberley, Martin, The Caldwell Objects and How to Observe Them, 2009

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 8 August 2021