A Stripped-Down Globular

Center of M12
The Very Large Telescope (VLT) captures the central region of globular cluster M12 (also known as NGC 6218). VLT observations show that the cluster is extremely deficient in low-mass stars. The image covers an area of sky 3.5 arcminutes on each side, which translates to a physical size of 23 light-years at M12's estimated 23,000-light-year distance. Click on the image to view a larger version.
Courtesy European Southern Observatory.
Pity the globular cluster M12 in the constellation Ophiuchus. Like many of the small dwarf galaxies that surround our Milky Way, it has been pillaged and plundered by its much larger neighbor — losing nearly 1 million stars.

That's the message coming from astronomers at the European Southern Observatory's Very Large Telescope (VLT) in Chile. Using Antu, one of the VLT's four 8.2-meter reflectors, Guido De Marchi (European Space Agency) and two colleagues measured the brightnesses and colors of more than 16,000 of M12's 200,000 stars, down to magnitude 25. "It is, however, clear that M12 is surprising devoid of low-mass stars," says 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 low-mass stars dominate the populations of almost all galaxies and star clusters, something must have stripped M12 of its least-massive stars. And that "something" is the neighborhood bully: the Milky Way.

M12 orbits the Milky Way in a highly extended oval that periodically swings it through the galactic plane near the core. When the cluster ventures into this densely populated region, the lowest-mass stars are literally stolen by the gravitational pull of the galaxy's stars, gas clouds, and dark matter. The tremendous gravity of this huge quantity of mass slingshots the stars into orbits that extend high above and below the plane, into the galaxy's halo.

"We estimate that M12 has lost four times as many stars as it still has," says team member Francesco Paresce (National Astrophysics Institute, Italy). "That is, roughly 1 million stars must have been ejected into the halo of our Milky Way."

The group predicts that M12 has another 4.5 billion years of life before the stripping process reduces it to nothingness. The cluster itself probably formed about 13 billion years ago, when our home galaxy first began assembling itself.