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Imagine trying to understand how a football game works based on just a few fuzzy snapshots of the game in play. Astronomers have faced this challenge when it comes to understanding the dynamics of the beehive swarm of stars in the globular star clusters that orbit our Milky Way Galaxy. Now, NASA's Hubble Space Telescope has provided astronomers with the best observational evidence to date that globular clusters sort out stars according to their mass, governed by a gravitational billiard ball game between stars. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its periphery. This process, called "mass segregation," has long been suspected for globular star clusters, but has never before been directly seen in action.

A typical globular cluster contains several hundred thousand stars. Although the density of stars is very small in the outskirts of such stellar systems, the stellar density near the center can be more than 10,000 times higher than in the local vicinity of our Sun. If we lived in such a region of space, the night sky would be ablaze with 10,000 stars that would be closer to us than the nearest star to the Sun, Alpha Centauri, which is 4.3 light-years away (or approximately 272,000 times the distance between Earth and the Sun). Like a subway car crowded with commuters, this stellar crowding strongly increases the probability of encounters among stars, even collisions and mergers. The cumulative result of many such encounters is the theoretically expected mass segregation. But at the same time such crowded conditions make it extremely difficult to accurately identify individual stars.

Astronomers had to wait for the extreme sharpness of Hubble's vision to trace the motions of many thousands of stars in a single globular cluster. Now highly accurate speeds have been measured for 15,000 stars at the very center of the nearby globular cluster 47 Tucanae - one of the densest globular clusters in the Southern hemisphere. A small number of these stars are of a very rare type known as "blue stragglers": unusually hot and bright stars long thought to be the product of collisions between two normal stars.

The velocities of the blue straggler stars agree with the predictions of mass segregation. In particular, a comparison between blue stragglers (that have twice the mass of the average star) and other stars shows that, as expected, they do move more slowly than average stars.

Using the Wide Field and Planetary Camera 2 and the newer Advanced Camera for Surveys instrument on Hubble, Georges Meylan of Ecole Polytechnique Federale de Lausanne (EPFL) in Sauverny, Switzerland and collaborators took ten sets of multiple images of the central region (within a distance of about 6 light-years of the center) of 47 Tucanae. Images were taken at regular intervals over nearly seven years. By carefully measuring the positions of as many as 130,000 stars in every one of these "snapshots," extremely small position changes could be measured over time, betraying the motions of the stars across the sky. Precise velocities were obtained for nearly 15,000 stars in this cluster. Of these 15,000, 23 were blue stragglers.

This is the largest sample of velocities ever gathered, by any technique with any instrument, for a globular cluster in the Milky Way. The results were also used to check whether a black hole exists in the cluster's core, by looking for its gravitational pull. But the measured stellar motions rule out a very massive black hole.

With these observations, Hubble accomplished in less than a decade what would have taken ground-based telescopes as long as nearly a century because of poorer observing conditions from the ground. The study would have been impossible without Hubble's sharp vision. From the ground, the smearing effect of the Earth's atmosphere blurs the image of the numerous stars in the crowded cluster core. The typical angular motion of even the normal stars in the center of 47 Tucanae was found to be just over one ten millionth of a degree per year. This means the angular motion of a star in one year is equivalent to the angular size of a dime seen as if it were 4,500 miles away.

To take full advantage of these exquisite Hubble images, astronomers developed entirely new data analysis methods, which eventually provided measurements of proper motions (velocities) that corresponded to changes in the positions of stars at the level of about 1/100th of a pixel (picture-element) on the Hubble's digital cameras.

The results were published in the September Astrophysical Journal Supplement Series.

The international team was made of the following scientists: D.E. McLaughlin (University of Leicester), J. Anderson (Rice University), G. Meylan (Ecole Polytechnique Federale de Lausanne), K. Gebhardt (University of Texas at Austin), C. Pryor (Rutgers University), D. Minniti (Pontifica Universidad Catolica), and S. Phinney (Caltech).