"In the solar neighborhood and in most stellar clusters, the least massive stars are the most common, and by far," said team leader Guido De Marchi. "Our observations with ESO's VLT show this is not the case for Messier 12."
De Marchi and his team measured the brightness and colors of more than 16,000 stars within Messier 12, using the FORS1 multi-mode instrument attached to one of the VLT unit telescopes at Cerro Paranal. It enabled astronomers to study stars up to 40 million times fainter than what the unaided eye can see – at magnitude 25.
Located about 23,000 light-years away in the constellation Ophiuchus, or the Serpent-holder, Messier 12 got its name by being the 12th entry in the catalog of nebulous objects compiled in 1774 by French astronomer and comet chaser Charles Messier.
Astronomers know the formation as NGC 6218. It contains about 200,000 stars, most with a mass between 20 and 80 percent of the Sun.
"It is however clear that Messier 12 is surprisingly devoid of low-mass stars," De Marchi said. "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."
Globular clusters move in extended elliptical orbits that periodically take them through the densely populated regions of the Milky Way – in the plane, then high above and below in the halo. When venturing too close to the galaxy's innermost and denser regions, called the bulge, a globular cluster can be perturbed, and its smallest stars can be ripped away.
"We estimate that Messier 12 lost four times as many stars as it still has," said Francesco Paresce. "That is, roughly 1 million stars must have been ejected into the halo of our Milky Way."
Astronomers predict the total remaining lifetime of Messier 12 to be about 4.5 billion years, or about one-third of its present age. This is very short compared to the typical expected globular cluster's lifetime – about 30 billion years.
In 1999, the same team of astronomers found another example of a globular cluster that had lost a large fraction of its original content. They said they hope to discover other Messier 12s, because observing clusters being disrupted should help clarify the dynamics of the process that shaped the Milky Way's halo.
