Object of the Week, April 20, 2025 - Southern Pleiades and ESO 92-18

Designations: Southern Pleiades, IC 2602, Collinder 229, Melotte 102, vdB-Ha 103, Theta Car cluster and ESO 92-18
Constellation: Carina
Type: Open star clusters (or a young globular cluster?)
RA, DEC: 10:42:57, -64°23'42" and 10:14:58, -64°36'48"
Mag (v): 1.6 and ~13.5
Size: 90' and 5'
Class: 1 3 r and I 2, 60 stars and 1100 stars


I recently made a three-week southern astro trip to Western Australia in March - April. Another Swedish amateur astronomer and I drove into the bush outside Geraldton, Western Australia, every clear night. We brought two telescopes - a 10-inch Dobsonian and a 5.5-inch Comet Catcher - plus two binoculars: a 25x100 and an 8x40. We observed hundreds of deep-sky objects during approximately 75 hours of observing time.

The far southern constellation Carina (The Keel) harbors two very different star clusters a few degrees apart. The larger and brighter is nicknamed the Southern Pleiades (IC 2602) and rivals its northern counterpart. The cluster is centered on the magnitude 2.7 Theta Carinae, a blue straggler, meaning it's brighter and bluer than expected. The cluster forms the northern end of the Diamond Cross, one of the three "southern crosses" along with the "true" Southern Cross in Crux and the False Cross in Vela - Carina. All three point in the same general direction.

IC 2602 is clearly visible naked eye, and its lucida is comparable in brightness to Alcyone in M45. Eight stars are of magnitude 5.8 or brighter can be seen, but they lack surrounding nebulosity.

In the southern part of the cluster lies another star cluster, Melotte 101 (Collinder 227), with a quite different character. It contains about 50 equally bright stars with a total magnitude of 8.0. Seen through my 25x100 binoculars, the two clusters offer a beautiful view, resembling M35 and NGC2158 in Gemini as seen through a telescope.

Another type of deep-sky object is found in the western part of IC 2602: the magnitude 14.4 planetary nebula Henize 2-51 (PK 288-5.1, ESO 92-23). Its tiny 10 arc-second wide disk can be discerned at high power among nearby stars.

Attachment 5695

Three degrees west of the Southern Pleiades lies an object that is special to me, ESO 92-18. On Boxing Day in 2009, I was mostly observing star clusters in the Large Magellanic cloud using my 10-inch Dob from near the centre of Geraldton. The sky was not especially dark (SQM-L only 20.1). In Uranometria 2000.0 Deep-Sky Atlas Volume 2 (2nd edition), on page 210, I found the symbol for a star cluster designated E92-18. ESO star clusters can vary greatly - many are bright but sparse - but this one turned out to be a challenge. ESO 92-18 looked like a faint, round nebula, and I was not sure whether it was the cluster at all.

Back home in Sweden, I checked the Digitized Sky Survey and saw a 5 arc-minute wide star cluster with very faint stars. According to Star Clusters by Archinal & Hynes, it contains a whopping 1100 stars from magnitude 15.7 at a distance of 9500 pc (31000 ly)! I asked about ESO 92-18 in the amastro forum, Observations of ESO 92-18? and in (the long thread) amastro@groups.io | Report from Geraldton, Western Australia , if anyone else had observed the cluster - but no one seemed to have seen it. My 2009 observation may very well have been the first visual sighting!

Dana de Zoysa from South Africa later described the cluster in detail in A tale of two clusters: ESO 92-18 - IceInSpace after I mentioned it on Cloudy Nights forum in 2016, Report(s) from Geraldton, Western Australia - Deep Sky Observing - Cloudy Nights

Attachment 5696

Revisiting ESO 92-18 from the bush outside Geraldton (SQM-L readings 21.8-ish), it was an intriguing sight. At 150x magnification, it really looked like a ghost globular cluster - I have always described it as "Palomar cluster-like". In fact, its true nature remains inconclusive! See Unveiling the nature of 12 new low-luminosity Galactic globular cluster candidates | Astronomy & Astrophysics (A&A) I could discern five or six 14th mag stars across its mottled surface, but they were likely just foreground stars. The cluster didn't seem to have any central brightening.

If you're ever lucky enough to be able to observe the entire southern skies, you might be among the first few to see ESO 92-18. And it that case:

Give it a go - and let us know!

--------------------------------------------------------------------------------------------------------

Timo Karhula
Virsbo, Sweden

(my most-used instruments)
12" f/5 Dob
10" f/5 Dob (in Geraldton, Western Australia)
25x100 binoculars

Often spotted naked eye and with binoculars from Chile bit never studied in detail. I'll put it and the other objects on my TODO list for next time I'm down there.

The first time I saw (and sketched) the Southern Pleiades was in 2022 with a 12" dobson and 31T5 Nagler. It was a nice view, but I knew I had to revisit it with a big bino for the best possible view. I went back to Namibia in 2024 and I sketched it again, this time together with Mel 101.

It was a sight to remember... IC 2602 is very large and loose with about a dozen bright blue stars. I did however notice one orange star in the field, can you find it in the sketch? The APM 100mm bino frames the cluster beautifully at 2.3° and indeed Mel 101 is included too (bottom left).

Attachment 5697

Martijn

When in Aruba for the 1998 solar eclipse, I observed the southern sky with 11x80 binoculars. IC-2602 was a very bright open cluster, almost an asterism. Mel 101 with patience, I could see a glow adjacent to a field star. Near murk.

Melotte 101 in a 14-inch at 73x and 147x on my birthday in 2016:

Very large beautiful cluster, centered 12' E of a mag 5.5 star. Roughly 120-150 stars resolved, depending on the assumed size. There is no distinct border so the size is pretty arbitrary but most of the brighter stars are within a 8'-10' circle. Many of the stars are arranged in long chains and loops. The brightest mag 9.7 star is at the west end, with the majority from 12th to 13th-magnitude.