Object Name: Messier 12
Alternative Designations: M12, NGC 6218
Object Type: Class IX Globular Cluster
Right Ascension: 16 : 47.2 (h:m)
Declination: -01 : 57 (deg:m)
Distance: 16.0 (kly)
Visual Brightness: 6.7 (mag)
Apparent Dimension: 16.0 (arc min)
Locating Messier 12: In binoculars, M12 is a same-field binocular pair with globular cluster M10 located about half a fistwidth west of Beta Ophiuchi, M12 is the northernmost of this pair and will appear slightly fainter. To help orient yourself to the correct area, identify Beta Scorpii as your first starhop marker. Slightly more than a fistwidth north, you will see the twin Yed stars (Delta and Epsilon). To the northeast are another close, bright pairing – Beta and Gamma Ophiuchi. M10 and M12 are about 1/3 the distance between the twin Yeds and the Beta/Gamma pair. Both are bright enough to be seen as a small, fuzzy patch in the finderscope.
What You Are Looking At: Influencing about 75 light years of space in a spherical area, this loose globular cluster was once believed to be a tightly concentrated open cluster. With most of its low mass stars carried away by the gravitational influence of the Milky Way, M12 contains on 13 variable stars. “In the solar neighbourhood and in most stellar clusters, the least massive stars are the most common, and by far”, said Guido De Marchi (ESA), lead author of the study. “Our observations with ESO’s VLT show this is not the case for Messier 12.”
The studies which were carried out by Luigi Pulone and Francesco Paresce (INAF, Italy), measured the brightness and colors of more than 16,000 stars, many 40 million times fainter than human detection, within the globular cluster Messier 12 with the VLT at Cerro Paranal. “It is however clear that Messier 12 is surprisingly devoid of low-mass stars”, said De Marchi. “For each solar-like star, we would expect roughly four times as many stars with half that mass. Our VLT observations only show an equal number of stars of different masses.”
Since globular clusters travel in extended elliptical orbits which pass through the dense regions of the galactic plane, smaller stars can literally be ripped away from one form of gravitational attraction towards a greater one. “We estimate that Messier 12 lost four times as many stars as it still has”, said Francesco Paresce. “That is, roughly one million stars must have been ejected into the halo of our Milky Way.” But how many more trips can it make around our galaxy before Messier 12 is gone? Perhaps another 4.5 billion years – about a third its current age. Not long considering the typical life expectancy of a globular cluster is about 20 billion years!
Has it evolved? You bet. Messier 12 is home to a host of red giant stars. According to studies done by E. Carretta (et al), this is one heavy metal cluster: “Our results indicate that NGC 6218 is very homogeneous as far as heavy elements are concerned. On the other hand, light elements involved in the well known proton-capture reactions of H-burning at high temperature, such as O and Na, show large variations, anticorrelated with each other, at all luminosities along the red giant branch. The conclusion is that the Na-O anticorrelation must be established in early times at the cluster formation.”
History: M12 is one of Charles Messier’s original discoveries, found on May 30, 1764. From his notes of that night: “In the same night of [May] 30 to 31, 1764, I have discovered a nebula in Serpens, between the arm and left side of Ophiuchus, according to the charts of Flamsteed: That nebula doesn’t contain any star; it is round, its diameter can be 3 minutes of arc, its light is faint; on sees it very well with an ordinary [non-achromatic] refractor of 3 feet [FL]. I have determined its position, by comparing with the star Delta Ophiuchi; its right ascension has been concluded at 248d 42′ 10″, and its declination at 1d 30′ 28″ south. I have marked it in the chart of the apparent path of the Comet which I have observed last year [the comet of 1769].”
Although Sir William Herschel would be the first to resolve Messier 12 into stars in 1785, his son John would later describe it best in his own notes of 1828 by saying: “…has stragglers in lines and branches extending some distance from the most condensed part, which is 3′ diam. Comes almost up to a blaze in the middle, has a * 10.11 m in the centre. Very rich globular cluster. The stars are of 10th to 16th magnitude; very gradually much brighter toward the middle, but has stragglers in lines and branches extending some distance from the most condensed part, which is 3′ in diameter. Comes almost up to a blaze in the middle, has a star of 10th or 11th magnitude in the centre.”
In binoculars, M12 will forever remain a hazy patch, but intermediate sized telescopes will reveal this beauty looking almost like a “gone to seed” dandelion set against the dark night sky. Enjoy!
B&W image thanks to Palomar Observatory, courtesy of Caltech, M12 core region by ESO and color image thanks to REU program/NOAO/AURA/NSF.