Object Name: Messier 71
Alternative Designations: M71, NGC 6838
Object Type: Class X-XI Globular Cluster
Right Ascension: 19 : 53.8 (h:m)
Declination: +18 : 47 (deg:m)
Distance: 13.0 (kly)
Visual Brightness: 8.2 (mag)
Apparent Dimension: 7.2 (arc min)
Locating Messier 71: This wonderful mass of stars is fairly easy to locate – if you can see the arrow shape of Sagitta! Just use the chart and look for the 6th magnitude star about halfway between Gamma and Delta Sagittae.
In average binoculars it will show as a fairly large fuzzy patch that almost seems to come to resolution, and begin to reveal individual stars to a small aperture telescope. Larger telescopes can and will full resolve this unusual globular cluster. M71 makes a fine target for urban skies and does well under even partially moonlit conditions.
What You Are Looking At: Messier 71 is unusual because it was a difficult classification – condensed star cluster or globular? It began its tour classified as a dense galactic cluster, the work of Harlow Shapely. In 1943, James Cuffey became of a mind that it was probably a loose globular cluster, so he also began some studies… only to find them inconclusive. “In NGC 6838 the giant stars are red and become bluer with decreasing brightness. In this respect, as well as in structure and richness of faint stars, NGC 6838 bears a striking resemblance to some of the globular clusters.” said Cuffey, “The tendency of the color magnitude diagrams in the very rich galactic clusters to resemble the diagrams observed in the globular clusters emphasizes again the poorly defined boundaries between the two classes of objects.”
Poorly known radial velocity, metallicity… Not many clues were around to help us understand what we were looking at until around 2008. “Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed. Stars from the turn-off up to the Red Giant Branch observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample. Previous findings related to the CN bimodality and CN-CH anticorrelation in stars of M71 are confirmed.” says A. Alves-Brito (et al), “We also find a CN-Na correlation, and Al-Na, as well as an anticorrelation. A combination of convective mixing and a primordial pollution by AG or massive stars in the early stages of globular cluster formation is required to explain the observations.”
So what else does 13,000 light years distant Messier 71 hold secret inside of its 27 light year light year diameter influence? Try millisecond pulsars… “We observe the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources are found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A.” said Ronald F. Elsner (et al), “The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources are found within the half-mass radius, reaching to a limiting luminosity of 1.5 × 1030 ergs s?1 (0.3-8 keV). From a radial distribution analysis, we find that 18 ± 6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11 ± 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range LX = 1030–1031 ergs s?1 than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches.”
Optically speaking, there’s alot to be found inside M71, too… Just ask the Hubble Team! “Clusters such as NGC6638 are known to have significant and difficult-to-model differential reddening. We have deliberately included a significant number of metal rich clusters, a sample that has been somewhat problematic in ground based surveys (due to differential reddening and/or field star contamination).” says Ata Sarajedini (et al), “This will allow us to use positional information about stars measured on multiple dithered exposures to improve the precision and depth of the photometry. The calibration to the standard Cousins VI system will be performed via comparisons with ground-based observations, most of which are already in hand. The ground-based data will also provide many more stars to populate the brighter portions of the clusters.”
Still need more? Then check this out. “We report the discovery of an SX Phoenicis type pulsating component in the Algol-type semidetached eclipsing binary QU Sge, in the metal-rich globular cluster M71. QU Sge is only about 80″ from the center of M71 and is located in the blue straggler region in the color-magnitude diagram of M71. It is considered to be a probable member of M71, with a membership probability greater than 60% deduced from a proper-motion study in the literature.” says Jeon Young-Beom (et al), “From time-series CCD photometry, we find that QU Sge has an orbital period of 3.790818 days and a primary minimum depth of 1.333 mag. The eclipsing light curve solution shows that QU Sge has a semidetached binary configuration with the secondary component fully filling its Roche lobe. After subtracting the eclipses from the light curve, we discover an SX Phoenicis type pulsation feature. It is found to have a short period of about 0.03 days and a small amplitude of about 0.024 mag. This is the first eclipsing binary system in a globular cluster to exhibit a pulsating feature. This result supports the model in which the origin of some blue stragglers in globular clusters is mass transfer between two components in the primordial binary systems.”
Would you like to know what else they’ve found out about Messier 71? How about the fact that it sometimes kicks out its white dwarf stars? According to the work of Jeremy Heyl, “The wind of an asymptotic giant branch star is sufficiently strong that if it is slightly asymmetric, it can propel the star outside the open cluster of its birth or significantly alter its trajectory through a globular cluster. Therefore, if these stellar winds are asymmetric, one would expect a deficit of white dwarfs of all ages in open clusters and for young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Hence detailed studies of the radial distribution of young white dwarfs in globular clusters could provide a unique probe of mass loss on the asymptotic giant branch and during the formation of planetary nebulae both as a function of metallicity and for a limited range of stellar mass.”
History: The discovery of this most unusual gathering of stars is first credited to De Cheseaux as lucky #13. Of it, he said; “Two others of which I didn’t yet determine the positions, one above the northern feet of Gemini, and the other above and very near to Sagitta.” Well, in my book, if you don’t position it, you might have seen it – but can’t quite take credit for it. Next up? A much smarter Johann Gottfried Koehler, who labeled it his #7 and positioned it as “”A very pale nebula in the Arrow [Sagitta] at 1deg 50′ [Aqr] [301d 50'] and 39d northern latitude.” however, he didn’t date it and his observations occured somewhere between 1772 and 1779!
So who should the credit really go to? Try Pierre Mechain on June 28, 1780 and dutifully and correctly logged by Charles Messier on October 4, 1780. “71. 19h 43m 57s (295d 59′ 06″) +18d 13′ 00″ – Mechain: (296d 00′ 04″) +18d 14′ 21″ – Nebula discovered by M. Mechain on June 28, 1780, between the stars Gamma and Delta Sagittae. On October 4 following, M. Messier looked for it: its light is very faint and it contains no star; the least light makes it disappear. It is situated about 4 degrees below [south of] that which M. Messier discovered in Vulpecula. See No. 27. He reported it on the Chart of the Comet of 1779.”
It was first shattered into resolution by Sir William Herschel in 1783, observed by his sister Caroline and cataloged by his son John… However, it was best described by Admiral Smyth who said; “A rich compressed Milky-Way cluster on the shaft of the arrow, and 10deg north-a quarter-east from Altair. It was discovered by Mechain in 1781, and described by Messier as a nebula unaccompanied by stars, and of a very feeble light. Piazzi seems to have observed it meridionally as a star of the 8th magnitude, by admitting the light of a lamp upon it, but his darkened field ought to have shown that it is flanked with four telescopic stars, besides other larger companions in view.”
Enjoy your own observations of this highly unusual “star cluster”!
Top M71 image credit, Palomar Observatory courtesy of Caltech, Messier 71 2MASS image, M71 courtesy of Chandra, M71 Hubble image, M71 by DSS (probably by Evered Kreimer) and M71 color image courtesy of REU program/NOAO/AURA/NSF.