INT+PFC, IDS

Globular clusters are well known as the oldest conglomerations of stars in the Milky Way. Once thought all to have formed at roughly the same time, a small number of these clusters were recently found to have ages at least 3 Gyr younger that their siblings.

According to deep V and I CCD images of the loosely populated galactic globular cluster Palomar 1 and the surrounding field obtained with the Isaac Newton Telescope, an estimated age at 6.3 to 8 Gyr was derived. That makes Palomar 1 just over half as old as typical globulars and the youngest Galactic globular cluster identified so far. Also surprising are its comparatively low luminosity and uncrowded population of stars — unusual traits for a globular.
 

Central 4.3' × 4.3' image of Palomar 1, taken with the I filter and an exposure time of 600 s. [ TIFF ]

The astronomers discuss the possibility that Palomar 1 is in fact a very old open cluster. But that would be an even worse fit for its properties. Furthermore, Palomar 1's location in the outer halo, about 55,000 light-years from the Galaxy's centre, would be difficult to reconcile with an open-cluster classification. An alternate explanation, which may also account for the other young globular clusters, could be a different formation process. Most globulars are thought to have coalesced at the same time as the Galaxy itself. The younger ones, on the other hand, may have come in three other ways: as gas clouds that survived in the halo after the Milky Way's formation, later to form stars; as captured intergalactic star groups; or as cannibalised dwarf galaxies.

References:
 

• A Rosenberg et al, 1998, "Palomar 1: Another Young Galactic Halo Globular Cluster?", Astron J, 115, 648.
• A Rosenberg et al, 1998, "The Metallicity of Palomar 1", Astron J, 115, 658.