* Astronomy

Based on observations with ESO's Very Large Telescope, a team of Italian astronomers reports that the stellar cluster Messier 12 must have lost to our Milky Way galaxy close to one million low-mass stars.

"In the solar neighbourhood and in most stellar clusters, the least massive stars are the most common, and by far. Our observations with ESO's VLT show this is not the case for Messier 12" - Guido De Marchi (ESA), lead author of the study.

The team, which also includes Luigi Pulone and Francesco Paresce (INAF, Italy), measured the brightness and colours of more than 16,000 stars within the globular cluster Messier 12 [2] with the FORS1 multi-mode instrument attached to one of the Unit Telescopes of ESO's VLT at Cerro Paranal (Chile). The astronomers could study stars that are 40 million times fainter than what the unaided eye can see (magnitude 25).
Located at a distance of 23,000 light years in the constellation Ophiuchus (The Serpent-holder), Messier 12 got its name by being the 12th entry in the catalogue of nebulous objects compiled in 1774 by French astronomer and comet chaser Charles Messier. It is also known to astronomers as NGC 6218 and contains about 200,000 stars, most of them having a mass between 20 and 80 percent of the mass of the Sun.


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This image shows the centre of the globular cluster Messier 12 as observed with the FORS-1 multi-mode instrument on ESO's Very Large Telescope (Cerro Paranal, Chile). The picture covers a region of about 3.5 arcmin on a side, corresponding to about 23 light years at the distance of Messier 12. It is based on data in five different filters: U, B, V, R and H-alpha. Here only the short exposures were used while for their scientific analysis, the authors used much longer exposures. Guido De Marchi (ESA) reduced the data and Kristina Boneva and Haennes Heyer (ESO) did the final image processing. The observations were obtained with very good conditions, the image quality ('seeing') being around 0.6 arcsec.

"It is however clear that Messier 12 is surprisingly devoid of low-mass stars. 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"- Guido De Marchi.

Globular clusters move in extended elliptical orbits that periodically take them through the densely populated regions of our Galaxy, the plane, then high above and below, in the 'halo'. When venturing too close to the innermost and denser regions of the Milky Way, the 'bulge', a globular cluster can be perturbed, the smallest stars being ripped away.

"We estimate that Messier 12 lost four times as many stars as it still has. That is, roughly one million stars must have been ejected into the halo of our Milky Way" - Francesco Paresce.

The total remaining lifetime of Messier 12 is predicted to be about 4.5 billion years, i.e. about a third of its present age. This is very short compared to the typical expected globular cluster's lifetime, which is about 20 billion years.
The same team of astronomers had found in 1999, another example of a globular cluster that lost a large fraction of its original content.

The scientists hope to discover and study many more clusters like these, since catching clusters while being disrupted should clarify the dynamics of the process that shaped the halo of our home galaxy, the Milky Way.