Messier 71

Messier 71 (also known as M71 or NGC 6838) is a globular cluster in the very mildly northern constellation Sagitta. It was discovered by Philippe Loys de Chéseaux in 1745 and included by Charles Messier in his catalog of non-comet-like objects in 1780. It was also noted by Koehler at Dresden around 1775.

Messier 71
M71 from Hubble Space Telescope; 3.35′ view
Credit: NASA/STScI/WikiSky
Observation data (J2000 epoch)
ClassX-XI
ConstellationSagitta
Right ascension19h 53m 46.49s[1]
Declination+18° 46′ 45.1″[1]
Distance13.0 kly (4.0 kpc)[2]
Apparent magnitude (V)+6.1[3]
Apparent dimensions (V)7′.2
Physical characteristics
Mass1.7×104[4] M☉
Radius13 ly[5]
Metallicity = –0.78[6] dex
Estimated age9-10 Gyr
Other designationsM71, NGC 6838, Cr 409, GCl 115[3]
M 71 from Calar Alto Observatory 1995, T. Credner and S. Kohle

This star cluster is about 12,000 light years away from Earth and spans 27 light-years (8 pc). The irregular variable star Z Sagittae is a member.

M71 was for many decades thought (until the 1970s) to be a densely packed open cluster and was classified as such by leading astronomers in the field of star cluster research due to its lacking a dense central compression, and to its stars having more "metals" than is usual for an ancient globular cluster; furthermore, it lacks the RR Lyrae "cluster" variable stars that are common in most globulars. However, modern photometric photometry has detected a short "horizontal branch" in the H-R diagram (chart of temperature versus luminosity) which is characteristic of a globular cluster. The shortness of the branch explains the lacking of the RR Lyrae variables and is due to the globular's relatively young age of 9–10 billion years. Taking in many or only late series (Population I) stars explains relatively its stars. Hence today M71 is designated as a very loosely concentrated globular cluster, much like M68 in Hydra. M71 has a luminosity of around 13,200 Suns (L☉).

Map showing location of M71

References
  1. Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830.
  2. Hessels, J. W. T.; et al. (November 2007), "A 1.4 GHz Arecibo Survey for Pulsars in Globular Clusters", The Astrophysical Journal, 670 (1): 363–378, arXiv:0707.1602, Bibcode:2007ApJ...670..363H, doi:10.1086/521780.
  3. "M 71". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-17.
  4. Marks, Michael; Kroupa, Pavel (August 2010), "Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way", Monthly Notices of the Royal Astronomical Society, 406 (3): 2000–2012, arXiv:1004.2255, Bibcode:2010MNRAS.406.2000M, doi:10.1111/j.1365-2966.2010.16813.x. Mass is from MPD on Table 1.
  5. distance × sin( diameter_angle / 2 ) = 13 ly. radius
  6. Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51.

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