Messier 12

M12, also known as the Gumball Globular, NGC 6218, C 1644-018 and GCI 46, is a globular star cluster located in the constellation Ophiuchus. The Gumball Globular has an apparent magnitude of 7.68 and lies at a distance of 15,700 light-years from Earth at right ascension 16h 47m 14.18s and declination -01°56’54.7”. Messier 12 has a diameter of 75 light-years and lies about 3 degrees in the sky from Messier 10, a slightly brighter globular cluster in Ophiuchus. Messier 12 has a Shapley-Sawyer classification of IX, which means that its member stars are concentrated relatively loosely toward the centre for a globular cluster. Compared to its neighbour Messier 10 (class VII), M12 is notably less dense toward the central region. It is about 12.67 billion years of age. Messier 12 is best seen in the months of May, June and July.

The Gumball Globular contains about 200,000 stars, with a radius of 37.2 light-years. The brightest ones are of 12th magnitude and the mean visual magnitude of the 25 brightest stars in M12 is 13.97. American astronomer Allan Sandage found 13 variable stars in the cluster. In 2006, M12 was discovered to contain a surprisingly low number of low mass stars. Scientists believe that these stars were stripped from M12 by the gravitational pull of the Milky Way Galaxy and that the cluster lost four times as many members as it still has over its lifetime.  In other words, as the cluster’s orbit took it through the denser regions of the Milky Way plane, M12 ejected about a million stars into the galaxy’s halo. This explains why there are hardly any M-class (red) dwarfs in the cluster. But how much longer can M12 make it around our galaxy before it is stripped of all of its stars? Estimates indicate that it has 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. And in the course of its lifetime so far, the cluster has certainly evolved and is presently home to a host of red giant stars. According to studies performed 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 temperatures, 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.”

The cluster was discovered by Charles Messier on May 30, 1764. Messier added the object to his catalogue, describing it as a “nebula without stars.” William Herschel was the first to resolve the cluster into individual stars.

Using binoculars, M12 is can be seen in the same field as the globular cluster M10. It can be located by finding the Ophiuchi constellation, then looking about half a fist-width 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 star-hop marker. Slightly more than a fist-width north, you will see the twin Yed stars of 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 finder-scope. Using 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. Credits: Messier Objects, NASA, Universe Today, Wikipedia.

The high concentration of stars within globular clusters, like Messier 12, shown here in an image from the NASA/ESA Hubble Space Telescope, makes them beautiful photographic targets. But the cramped living quarters in these clusters also makes them home to exotic binary star systems where two stars are locked in tight orbits around each other and matter from one is gobbled up by its companion, releasing X-rays. It is thought that such X-ray binaries form from very close encounters between stars in crowded regions, such as globular clusters, and even though Messier 12 is fairly diffuse by globular cluster standards, such X-ray sources have been spotted there. Astronomers have also discovered that Messier 12 is home to far fewer low-mass stars than was previously expected. In a recent study, astronomers used the European Southern Observatory’s Very Large Telescope at Cerro Paranal, Chile, to measure the brightness and colours of more than 16 000 of the globular’s 200 000 stars. They speculate that nearly one million low-mass stars have been ripped away from Messier 12 as the globular has passed through the densest regions of the Milky Way during its orbit around the galactic centre. It seems that the serenity of this view of Messier 12 is misleading and the object has had a violent and disturbing past.
Messier 12 lies about 23 000 light-years away in the constellation of Ophiuchus (The Serpent Bearer). This image was taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. The colour image was created from exposures through a blue filter (F435W, coloured blue), a red filter (F625W, coloured green) and a filter that passes near-infrared light (F814W coloured red). The total exposure times were 1360 s, 200 s and 364 s, respectively. The field of view is about 3.2 x 3.1 arcminutes. Image: ESA/Hubble & NASA