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.

NGC_6624_map.png

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)

ngc 6624 Gary Imm.jpeg
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."


iisai_ngc_6624.gif
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!