NGC 6441

NGC 6441
Observation data (J2000 epoch)
ClassIII
ConstellationScorpius
Right ascension17h 50m 13.06s[1]
Declination−37° 03′ 05.2″[1]
Distance42.7 ± 2.3 kly (13.1 ± 0.7 kpc)[2]
Apparent magnitude (V)7.2[3]
Apparent dimensions (V)9.6′[3]
Physical characteristics
Mass1.6×106[4] M☉
Radius4.8[5]
Tidal radius88.8 ly (27.23 pc)[6]
Metallicity = −0.53[7] dex
Estimated age13−13.7[5] Gyr
Other designationsGCl 78, CD−37° 11916, CPD−37° 7530, HD 161968[8]
See also: Globular cluster, List of globular clusters

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]

  1. ^ a b Cite error: The named reference aj140_6_1830 was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference Oliveira was invoked but never defined (see the help page).
  3. ^ a b c d Cite error: The named reference omeara2013 was invoked but never defined (see the help page).
  4. ^ a b Cite error: The named reference apj765_1_32 was invoked but never defined (see the help page).
  5. ^ a b c d Cite error: The named reference apj675_1_670 was invoked but never defined (see the help page).
  6. ^ a b c Cite error: The named reference mnras360_2_631 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference aj144_6_183 was invoked but never defined (see the help page).
  8. ^ "NGC 6441". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-02-19.
  9. ^ Stoyan, Ronald; Schurig, Stephan (2014). interstellarum Deep Sky Atlas. Erlangen: Cambridge University Press; Oculum-Verlag GmbH. ISBN 978-1-107-50338-0. OCLC 920437579.
  10. ^ Cite error: The named reference aj132_3_1014 was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference ajl767_2_L27 was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference aj123_6_3255 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference aj114_2611 was invoked but never defined (see the help page).