The Young, the Old, and the Globular

Abstract

They hover in the haloes and bulges of galaxies: globular clusters, gravitationally bound collections of 100,000 to 1 million stars following highly eccentric, elliptical orbits that drift across the galactic plane with very long periods. Among the first to recognize that these enigmatic objects were made of stars was the German-born astronomer William Herschel, who coined the term “globular cluster” in 1789. For the next three centuries, globular clusters remained fuzzy in detail but bright enough to be used as standard candles in cosmology. In the past three decades, major advances in observing tools and computing prowess have led to significant improvements in our understanding of their evolution, structure, dynamics, and perhaps most critically, ages. Age is central to globulars' fascination. Thought to be among the oldest stellar systems, they can provide valuable information about the first population of stars and the age of the universe. That status caused some vexation in the past, when astronomers pegged cluster ages between 15 billion and 20 billion years at a time when cosmologists calculated that the universe itself was only 10 billion to 15 billion years old. Nowadays, however, harmony reigns. In their review of age estimates for globulars in the Milky Way, Krauss and Chaboyer (p. 65) suggest that astronomers now have the lower limit about right, making the universe at least 11.2 billion years old. That cosmic youthfulness, coupled with evidence that the universe is geometrically flat and with current estimates of the Hubble constant, implies that the universe must be dominated by the mysterious antigravity-producing stuff called dark energy. In a new twist, astronomers have recently started estimating the ages of globular clusters from the decay of long-lived radioactive elements such as thorium (Th) and uranium (U) inside their stars. They have calculated stellar ages at 11 billion to 15 billion years. Although those numbers are consistent with estimates of the lower age limit of the universe, the “old” end of the range makes theorists uncomfortable, and more work is needed. To get more reliable ages, astronomers need to know how much U and Th the stars started with. That requires modeling the sites of nucleosynthesis and the masses of the neutron-rich nuclei that create the heavier elements. Sneden and Cowan (p. [70][1]) review these issues as well as other discoveries about the nucleosynthesis of heavy elements in the Milky Way. Globular clusters are also interesting for the light they shed on stellar processes and dynamics and, of course, as objects in their own right. Recent work has turned up some surprises: for example, evidence that hitherto unexpected star formation in old clusters may be confusing age estimates, and that a black hole or a concentration of neutron stars lurks in the core of some clusters. Irion (p. 60) summarizes these observations and describes how interactions between binary stars can turn placid-seeming clusters into stellar combat zones. Globulars also flourish in other galaxies. Schilling (p. 63) describes evidence that some extragalactic clusters, unlike those in the Milky Way, harbor two different generations of stars, one younger, redder, and more metal-rich than the other. Some theorists suggest that the younger clusters form in starbursts that ignite when galaxies collide—a challenge to the standard view that all clusters form in the collapse of molecular clouds during galaxy formation. We've come a long way since Herschel's insight that globulars consist of stars. A little more time should resolve what kinds of stars they are: a bit too old, a bit too young, or just right. [1]: /lookup/doi/10.1126/science.1077506