Object of the Week July 31, 2022 - NGC 6624

NGC 6624, vdB-Ha 262, Mel 199, C1820-303, Bennett 109, GCL 93
Type: Globular Cluster
Constellation: Sagittarius
RA: 18h 23m 40.5s
DEC: -30° 21’ 40"
Mag(v): 7.6
Size: 8.8’

The constellation Sagittarius holds 6 Messier globular clusters ranging in magnitude from 5.2 to 8.6. This week's OOTW, NGC 6624, is another globular cluster in Sagittarius that at magnitude 7.6 is just as bright as its many of its neighboring globulars such as M54, M69, and M70, and a full magnitude brighter than M75. Why it didn't make Charles Messier list is not known. My hypothesis is that by being a only 47' away from the 2.7 magnitude star Delta Sagittarii (Kaus Media) Messier just plain missed it.

Attachment 4814

This globular cluster was discovered by William Herschel in June 1784 and cataloged as H I-50 with the following notation: “Considerably large, round, very bright middle, milky.” Nearly 63 years later in 1847, it was observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded his observation as "globular, vB, R, psvmbM, diam in RA = 5 seconds; all clearly resolved into stars barely discernible." On a second occasion he called it "globular, B, R, psmbM, diam 6 seconds in RA, barely resolved so as to be sure it consists of stars." His third observation was recorded as "globular, B, R, stars first g, then psvmbM, 3.5' or 4' diameter, clearly resolved into stars 16m, a fine object.”

NGC 6624 has been well studied. Astronomers have determined that it has resided in the galactic bulge for nearly the last 14 billion years, making it another extremely aged globular cluster, like M69 in Sagittarius and 47 Tucana. It is among the most metal rich globulars known, its members each contain, on average, about 1/2 to 1/3 as much iron (per unit of hydrogen) as does the Sun. Despite its extreme age, the Hubble Space Telescope (HST) has detected at least ten candidate blue straggler stars within five arc seconds of NGC 6624's center. Blue stragglers are formed either when the stars in double-star system slowly merge, or when two unrelated stars collide.

Six pulsars have been found in NGC 6624. The first of these to be discovered was PSR J1823-3021A. The HST has also found evidence for an unusual double-star system at the very heart of the cluster - consisting of a neutron star and a white-dwarf companion, which orbit one another every 11.5 minutes; it is the fastest binary system known. This system, cataloged as 4U 1820-30, is an X-ray source. The X-rays are generated by the gravitational pull of the neutron star ripping away gas from the white dwarf. The gas forms a swirling disk around the neutron star, which heat up to temperatures greater than 100,000°F. The gas falling onto the star supplies fuel for sporadic, intense X-ray bursts created by nuclear fusion explosions.

Two observational points I'll share with you. NGC 6624 shares a field of view with Delta Sagittarii and the yellow and gold optical double Rosseau 31 (RSS 31), be sure to check out RSS 31. NGC 6624's core is one of, if not the most, highly condensed globular core that I have observed.

Now here are few observation reports to whet your appetite.

"A very bright and small globular which lies in a nice rich field. Two stars lie immediately to the east. At 139x, this shows a very bright compact core. With averted vision it is partly resolved, especially the outer areas. A nice object." (Faith Jordan)

"Bright, pretty large, much compressed and round. It has a very bright middle and is resolved into 18 stars against a grainy background at 165X. This a nice globular that doesn't get observed often." (Steve Coe, observing with a 13" f/5.6)

"This is a moderately bright cluster 25cm. It is 2' diameter and the small core appears granular at 200x. A mag 11.5, 13 pair with 10" separation in pa 270° lies 1.7' WSW of center. 30cm shows it to 2.2' with hints of granulation in the core. The halo shows a fair degree of concentration to an occasionally visible nonstellar nucleus." (Christian B. Luginbuhl and Brian A. Skiff in their Observing Handbook and Catalog of Deep-Sky Objects)

Attachment 4813
Image by Gary Imm
Field radius: 0.437 degrees
"This object, when combined with the dense star field background of Sagittarius, make for a beautiful pairing. I especially like how the larger bright blue stars in the star field offset the many small orange ones."


Attachment 4815
Sketch by Iiro Sairanen
Instrument: Newton 110/805 mm
Magnification: 161x
Field: 25'

Now it is your turn.
Give it a go and let us know!

Hi, here's my observation from 7/24/2009: Ngc 6624 globular cluster in Sagittarius ; magnitude:7.6 ; size: 8' ; bright but small,resolved only stars around periphery; core is bright and mottled ; I used a 8mm Ethos at 238x with my 14.5 Starstructure f/4.3Attachment 4816

I have only seen this globular cluster with my old 6" newt from southern France. I logged it as an unresolved, round glow with a bright center. I'll have to revisit it with my 20" some day!

Likewise, have seen this once with a 10" at 86x: "Very bright circular globular. Almost resolvable. Same field as NGC-6629."

I too have only observation with a smaller scope - this brief observation note is from 1993 with my 8-inch f/4:

"Bright globular with a bright center but no resolution at 105x"

Looks like I'm overdue for another look!

Maybe my first post on this forum. I searched my records and found I'd observed this globular from Foothills Park in Palo Alto CA, a bright observing site for sure. Like others here, my view was in an 8". Here's the note including other objects:

"I finished the night turning the 8" (f/6.9) toward the small globulars in Sagittarius that I had been hunting with 10x50 binoculars a few night before. Of course they showed much more easy in the 8". I picked off NGC 6638, NGC 6642, NGC 6624, NGC 6659, NGC 6528 which shared the FOV with NGC 6522, and NGC 6652. They were all obvious, without the need for charts (I'd spent enough time with binos that I knew where to look)."

Here's another note I had from a site above Palo Alto in the Santa Cruz Mountains, two nights earlier (July 3, 2003):

"I kept at it until about 1 a.m., picking up other globs - NGC 6624 (5.9' & 8.1m), NGC 6652 (3.5' & 8.9m) and NGC 6638, which was fairly tough (5' & 9.2m). The most difficult were NGC 6642 (4.5' & 9.4) above M22, NGC 6717 (7' & 9.2m).

Observing all these NGCs in binoculars surprised me."

Still surprising 20 years later.

I had a go at NGC 6624 from my backyard with the 30-inch a couple nights ago. It's in one of the brightest parts of my sky, but it still stood out well at low power. The seeing was terrible this low, and it took a magnification of 506x to finally see a few individual, but mushy, stars across the face of the cluster. I'll bet 6624 will look terrific under better conditions, but I'll have to wait until later this month to find out.

I managed to observe this globular cluster from Cerro Pachon in Chile last night using my 12" dob. Cerro Pachon is the mountain that hosts the SOAR and Gemini South observatories as well as the construction site for Vera C. Rubin Observatory and I am lucky enough to work on that project. I assisted in some testing after night fall and took the opportunity to bring up my 12" dob with me. I set up the telescope around 10 PM local time next to Hotel Pachon, where I spent the night.

The globular cluster was easily located using a 31 mm Nagler (48x, 1º48'). It showed Delta Sgr and the cluster together in the same field of view. The globular cluster appeared as a fuzzy ball of light. Bumping up the magnification to 150, 188 and 250x increasingly revealed more details in this cluster. At 250x I noticed the very compact core which was surrounded by a beautiful halo of small stars. At this magnification it very much resembled 47 Tuc at 48x.