Observations of star clusters in the Milky Way defy the view that the constituents of these systems are almost invariably chemically alike. The outlying clusters could be the tattered relics of once larger systems.
In its halo of dark matter, our Galaxy hosts a family of about 150 globular star clusters (GCs). Conventional wisdom holds that they are compact, roughly spherical systems of high stellar density, each containing about 5 million stars held together by gravity. Undergraduates are taught that these classic laboratories for studying stellar evolution each contain a single population of stars of uniform age and chemical composition. More than 30 years ago, it became clear that the most luminous of these clusters, ω Centauri, was the exception to the rule: the system contains stars with a range of iron abundances (Fe metallicity) that vary by more than a factor of 30 (refs 1, 2). In this issue, Lee et al.3 (page 480) and Ferraro et al.4 (page 483) report the discovery of two other GCs that harbour stars containing different proportions of iron and other heavy elements.
Variations among the light elements within individual GCs were also discovered several decades ago. But, unlike heavy elements, light elements can be made during the course of normal stellar evolution in intermediate-mass stars through the fusion of hydrogen at high temperatures5. The resultant 'ash' could be mixed into the surfaces of these evolved stars, ejected by gentle winds and then mixed into the gas in the young cluster. A second generation of stars could then be formed, giving rise to the observed variation in light-element content within a GC.
Heavier elements — including calcium, iron and beyond — are mostly produced in stellar explosions known as supernovae. Because material is violently ejected from supernovae at a very high velocity and the gravitational binding energy of present-day GCs is low, in the current conditions supernova gas ejecta would escape from the cluster. The only way in which such energetic gas, rich in heavy elements, could have been retained would be if the mass of the GC was much higher in the past than is typical today. If we find a GC showing variations in those heavy elements, suspicion naturally arises that it is the remnant of a formerly accreted small galaxy, as was suggested for ω Centauri. This is not wild speculation; there are indications that the GC called M54 will probably be the only remnant structure from the Sagittarius dwarf galaxy to survive the galaxy's ongoing violent disruption by the Milky Way.
We now have much better tools with which to search for variations in age and elemental abundance within individual Galactic GCs. These tools operate at a level of accuracy that we could only dream of a decade ago. Lee et al.3 demonstrate definitively that there is a spread in the abundance of calcium within the massive GC M22, which has been a suspect for many years. They find that the population of red-giant branch stars — stars in which the core has ceased to burn hydrogen but the outer shell is still doing so — in the system splits into two subpopulations of different calcium abundances. Two very recently completed spectroscopic studies6,7 detect star-to-star variations in iron abundance in M22 for smaller samples of red giants. It seems that M22 will join M54 as the only remnant of the disruption of an entire dwarf galaxy in the halo of the Milky Way.
Lee and colleagues3 go further, claiming that they can detect multiple stellar populations with smaller but still statistically significant variations in calcium abundance in more than half of the systems in their sample of 37 GCs. This is the most interesting and controversial part of their paper because, if they are correct, many GCs — not just a few outliers — must be pathetic remnants of much more massive systems that were accreted by the Milky Way halo during its formation. Although the authors' case for the system NGC 1851 seems reasonably secure, their claims for other GCs seem to be only marginally significant, and will require further confirmation. A previous investigation8 has already ruled out variations exceeding 12% in Fe metallicity for the majority of the eight GCs that have been studied in detail by Lee et al., demonstrating yet again that there is a high degree of uniformity in the abundance of Fe in most GCs throughout the stellar population.
Analysis of the current generation of high-quality images of GCs, whether taken by the Hubble Space Telescope or with ground-based telescopes equipped with adaptive-optics systems, has allowed exquisite data to be gathered for thousands of stars, and has enabled the discovery in GCs of subtle phenomena that previous studies missed. The GC NGC 1851 was found to have two branches of subgiant stars where there should just have been one9. And Piotto and colleagues10 found that main-sequence stars — those in which energy is created through the fusion of hydrogen in the star's core — in the GC NGC 2808 are divided into three separate branches.
To this collection of abnormalities we can now add the discovery of two subgroups of horizontal branch stars (those that are powered by the fusion of helium in the core) in the GC Terzan 5 that is presented by Ferraro and colleagues4. This particular anomaly has never previously been seen in a Galactic GC. The authors4 have also obtained spectra of a few horizontal branch stars in Terzan 5 that demonstrate that Fe metallicity varies by about a factor of three within this GC. So Terzan 5 must be yet another tattered remnant of a once much more massive system.
Potential causes for the bizarre behaviour of these GCs include helium-content variations (which must exist as a result of the same hydrogen-burning process that gives rise to variation among the observed light elements, but helium is extremely difficult to detect), age differences, and variations among the heavy elements. Another possibility, which was previously suggested11 to explain the peculiar case of NGC 1851, is extremely large variations among the light elements (particularly carbon, nitrogen and oxygen, the most abundant of these). All of these possibilities can also occur in combination, adding to the confusion. We know that age variations within GC systems are small, but of the order of 10%12. D'Antona and Ventura13 suspect that, in some cases, very high helium abundances (up to 40%) are required to reproduce some of the observed irregularities. This is almost twice the primordial abundance of helium produced in the Big Bang, the relic of which is found in present-day, metal-poor stars, and there is no direct observational evidence to support such a high helium abundance in any GC.
As we look closer and with more precision, the model of the GCs in the Milky Way as simple, single stellar population systems is being severely challenged. Are the anomalies, which seem to be turning up with increasing frequency, confined only to the most massive of the Galaxy's GCs? Exactly how common and how big such deviations from uniformity are among the Milky Way's GCs, and how they relate to stellar streams in the halo, is a hot topic.
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