NGC 6441, sometimes also known as the Silver Nugget Cluster,[9] is a
globular cluster in the southern
constellation of
Scorpius. It was discovered by the Scottish astronomer
James Dunlop on May 13, 1826, who described it as "a small, well-defined rather bright nebula, about 20″ in diameter". The cluster is located 5
arc minutes east-northeast of the star
G Scorpii,[3] and is some 43,000
light-years from the Sun.[2]
This is one of the most massive and luminous globular clusters in the Milky Way, with an estimated 1.6 million
solar masses of stars. It is located in the
bulge of the galaxy at a distance of 13 kilolight-years (3.9 kpc) from the
core,[6] and is considered metal "rich". That is, it has a relatively high abundance of elements with higher mass than helium.[4] The core region of the cluster subtends an angle of 0.11
arc minutes, compared to the half-mass radius of 0.64 arc minutes. The density of stars in the core region is indicated by the luminosity density: 5.25
L⊙ pc−3.[5] The cluster has a half-light radius of 7.1 ly (2.18 pc).[6]
This cluster has an abnormally large number of
RR Lyrae variables—68 candidates as of 2006, and their periods are longer than is typical for their respective
metallicities. (The mean period for the cluster's RRab stars is 0.759 day.) There are also several
type II Cepheid stars, which is unusual given the high metallicity of this cluster.[10] Examination of the
red giant branch section of the
color-magnitude diagram suggests that there are at least two and possibly three distinct populations in the cluster. The brightest and higher temperature members of the
red clump stars are more concentrated toward the center of the cluster. This group may be a helium-enriched second generation of stars.[11]
The cluster contains at least four
millisecond pulsars, of which two are in
binary systems. One of these binaries, PSR J1750−37A, is in a highly eccentric orbit with an
eccentricity of 0.71.[5] The cluster has an
X-ray burster, X1746-370, which has the longest period known in any globular cluster and is consistent with the galaxy as a whole.[12] Finally, there is a
planetary nebula, JaFu 2,[13] one of only four planetary nebulas known to inhabit globular clusters in the Milky Way.[3]
^
abOliveira, R. A. P.; Ortolani, S.; Barbuy, B.; Kerber, L. O.; Maia, F. F. S.; Bica, E.; Cassisi, S.; Souza, S. O.; Pérez-Villegas, A. (2022). "Precise distances from OGLE-IV member RR Lyrae stars in six bulge globular clusters". Astronomy & Astrophysics. 657: A123.
arXiv:2110.13943.
Bibcode:
2022A&A...657A.123O.
doi:
10.1051/0004-6361/202141596.
S2CID239998638.
^Jacoby, George H.; et al. (December 1997), "Planetary Nebulae in the Globular Cluster PAL 6 and NGC 6441", Astronomical Journal, 114: 2611,
Bibcode:
1997AJ....114.2611J,
doi:
10.1086/118671.