The Dynamic Lives Globularof Clusters These ancient stellar systems continue to surprise astronomers with a host of fascinating phenomena. By S. George Djorgovski 38 October 1998 Sky & Telescope ©1998 Sky Publishing Corp. All rights reserved. lobular star clusters are the delight of amateur and professional astronomers alike. The oldest star systems known, globulars witnessed the birth of our These models were spectacularly galaxy. They now serve as laboratories successful. They predicted cluster for theories of stellar dynamics and structures in excellent agreement with evolution. Extensively studied since the the observations available at the time, G and they still provide good fits with the birth of modern astronomy, globulars roughly like those between molecules in continue to yield surprising results and greatly superior data we have today. a uniform-temperature gas. The stars However, cluster models developed by challenge our understanding. are not physically colliding in this celes- The simple beauty of globular clusters Hénon at about the same time did tial pinball game, just deflecting each something entirely different: his simulat- is that they combine an elegant spherical other gravitationally. This energy ex- symmetry with the glittering richness of ed clusters didn’t stay in a slowly chang- change eventually leads to thermal equi- ing equilibrium. Instead, the models pre- thousands of individual stars. This ap- librium. In a typical cluster this takes parent simplicity has enticed astrono- dicted that globulars would fall in upon about a hundred million years, during themselves in a process called the gravo- mers to construct mathematical mod- which an individual star may cross the els of them. Early attempts arrived at a thermal catastrophe, or core collapse. cluster a hundred times. Thus there has Hénon reasoned like this: at any basically correct picture: collections of been plenty of time for globular clusters stars held together by their aggregate given moment the random motions of to reach relaxed equilibrium states, since stars in a cluster’s core can be consid- gravity, with members that move ran- they formed at least 10 billion years ago. domly like molecules of gas, interacting ered to balance the gravitational poten- The King-Michie simulations pro- tial of the stars’ mutual attractions. with Newtonian gravity. In simple sys- duced globular clusters with cores of tems with a limited number of essen- When a fast-moving star escapes the nearly uniform stellar density. The con- gravitational pull of the cluster core, tially pointlike masses (such as our centration of stars then gradually de- solar system or a widely separated bi- taking with it some kinetic energy, the clines out to an invisible boundary core must shrink a little. The remain- nary) the dynamics are mostly regular, called the cluster’s tidal radius. Here the predictable, and even boring. However, ing stars have to move a bit faster to gravitational attraction of the Milky compensate for the increase in the putamillionorsostars together in sev- Way is stronger than that of the cluster eral cubic light-years of space, let them gravitational binding energy of a itself, and stars become unbound and denser core. More of them can then es- orbit a sizable disk galaxy, and many join the galaxy’s stellar halo. Globular interesting new phenomena result. clusters were thus predicted to evapo- rate slowly, with galactic tides constant- Competing Theories for ly stripping away their stars. Collapsing Clusters The first successful dynamical models of globular clusters were developed in the Facing page: One of the southern sky’s showpieces, 47 Tucanae (NGC 104) glimmers from early 1960s by Richard Michie, Ivan 15,000 light-years away with the combined light of perhaps a million stars. Courtesy the Eu- King, and Michel Hénon, each working ropean Southern Observatory. Above: An easily located jewel in Pegasus, M15 is a prime ex- independently. The starting point for ample of a highly evolved cluster that has undergone core collapse. This color-composite Michie and King was the ongoing ran- image was taken with the Canada-France-Hawaii Telescope. Courtesy Peter B. Stetson, Do- dom encounters between cluster stars, minion Astrophysical Observatory. Below: Many amateurs’ first acquaintance with globular clusters came courtesy the Messier catalog, named for its 18th-century author, Charles Messier.The Messier globulars depicted here and on the following page were photographed 1 by Evered Kreimer with a 12 ⁄2-inch reflector. North is up with east to the left in each photo. M M M ©1998 Sky Publishing Corp. All rights reserved. cape, and so the process repeats itself of a collapsing globular cluster? of the cluster as a whole! Here was the and accelerates. This eventually leads to Such an energy source does exist, in resolution of Hénon’s paradox: binaries a rapid shrinking of the cluster core. the form of binary stars. Binaries can can serve as energy sources that stabilize The density profile of such a cluster is exchange energy during close encoun- globular-cluster cores against collapse. much steeper than the uniform cores ters with single stars, thereby becoming And thus both King and Michie on of the King-Michie models, declining either more or less tightly bound. one side and Hénon on the other were approximately as the inverse of the ra- These interactions can be very com- right: globular cluster cores do col- dius squared. plex, resulting in the formation of a lapse, but the collapse is arrested by At first glance, there doesn’t seem to temporary triple-star system, the dis- the presence of one or more hard bi- be any way to stop the collapse and ruption of the binary, or the exchange naries. Precollapse clusters can look stabilize the cluster. But since globular of partners. When all is said and done, just like the King-Michie models, and clusters abound, and none seem to however, in most cases the interaction some stabilized postcollapse ones can have little quasars within them, ends with a binary flying off in one di- too. But clusters that have undergone a Hénon’s models were all but disregard- rection and a single star flying off in recent collapse and recovery still retain ed for more than 20 years. Yet subse- another. And this is where the miracle the characteristic Hénon density pro- quent theoretical investigations in the happens. As it turns out, so-called hard file, with its very small core. In the 1980s by Donald Lynden-Bell, Haldan binaries — those with binding energies mid-1980s, in the first systematic sur- Cohn, Jeremy Goodman, and many larger than the average among stars in veys of globular-cluster structure, others have essentially confirmed and the cluster — tend to lose energy in about one-fifth of all galactic globulars extended Hénon’s ideas. such encounters. They become ever showed the characteristic post-core- Virtually the same process of core more tightly bound and more likely to collapse morphology — a cusplike collapse happens in protostars, but provide energy to the next star that density distribution near their centers. with a crucial difference: in protostars passes by. Examples include M15 and M30. On the core becomes hot and dense Extensive numerical simulations of the other hand, M5 and M13 are enough for thermonuclear reactions to binary-star/single-star interactions per- King-Michie type clusters. We now be- start. These reactions provide energy formed in the 1980s have confirmed lieve that most globulars evolve only that compensates for thermal losses, this prediction. Binaries provide a huge gradually toward core collapse, unless preventing collapse. But what can sub- reservoir of energy; a single hard binary they evaporate first. stitute for nuclear reactions in the core can have a binding energy equal to that Cluster cores may never settle down Left: This simulation shows the interaction of a single star with a “hard” (tightly bound) binary in the core of a globular cluster. Such interactions typically provide kinetic energy to passing stars at the expense of the binary’s orbit, which tightens. Eventually, the binding energy of a frequent- ly star-crossed binary can rival that of the entire cluster. Below: From the relatively diffuse exem- plars on the bottom of the previous page to the more condensed ones shown here, globulars present a wide range of morphologies to eyepiece observers. However, quantitative bright- ness measurements are required to tell which clusters’ cores have actually collapsed. Opposite page, bottom: The origin of a globular cluster’s core collapse. As more stars are lost the process accelerates, until it is halted by energy pro- vided by “hard” binary stars. Opposite page, top: At any given time, the core of a collapsing globular cluster has a uniform density, while just outside the core the density sharply decreases (left side of diagram). As time passes the core shrinks and the density increases, but the E. STERL PHINNEY, CALTECH / STEIN SIGURDSSON, UNIVERSITY OF CAMBRIDGE overall shape of the density profile remains roughly constant. M M M 40 October 1998 Sky & Telescope ©1998 Sky Publishing Corp. All rights reserved. completely: core collapses and bounces integrated star clusters along with ters undergo core collapse every billion can recur in a cycle called gravothermal some dwarf satellite galaxies. The 150 years. In that same period, several clus- oscillation. Obviously, a collapse-and- or so globulars surviving today are ters evaporate under the influence of recovery sequence cannot be observed probably just a small fraction of those tidal shocks. Eventually there will be directly, as it may take tens or hundreds that once populated the galactic halo. none left! of millions of years to complete, but Tidal shocks can also accelerate the Everything said so far presumes that the cycle’s existence seems convincingly evolution of clusters toward core col- stars can be treated as point masses in- established in numerical experiments.