Globular clusters sort their stars

November 10, 2006
Small changes in the positions of nearly 130,000 stars tracked over a 7-year period reveal that globular clusters do, in fact, take part in mass segregation. Mass segregation is the process of heavier stars slowing and sinking to the cluster’s core while lighter stars pick up speed and move toward the cluster’s edge.

Using the Hubble Space Telescope’s (HST) Wide Field and Planetary Camera 2 and the Advanced Camera for Surveys, astronomer Georges Meylan (École Polytechnique Federale de Laussane) led researchers in taking 10 sets of multiple images of the center of globular cluster 47 Tucanae (47 Tuc). HST accomplished in 7 years what might have taken ground-based telescopes nearly a century because of poorer observing conditions. Earth’s atmosphere tends to blur individual stars in images from the ground. Images from the orbiting Hubble are precise enough to show the annual movements of stars in the globular cluster accurate to one ten-millionth of a degree.

Unlike previous studies that concentrated on two or three hundred stars, Meylan told Astronomy, “We were able to measure 130,000 stars, and out of these were able to keep the 13,000 best ones. From those, we were able to prove that what was expected is exactly what is happening.”

Globular clusters are massive, spherical systems containing roughly 10,000 to 1 million stars in a diameter of up to roughly 20 light years. Their densities makes them prone to stellar encounters, which makes the theory of mass segregation plausible. However, the density of these areas also makes identifying individual stars nearly impossible – until we had HST’s precision.

Hubble’s resolution allowed astronomers to follow the motions of thousands of stars in 47 Tuc that included 23 rare “blue stragglers,” unusually heavy stars that likely form when two normal stars collide. The team measured the velocities of blue stragglers in relation to the velocities of smaller, lighter stars. Scientists observed that the heavier stars did, in fact, move more slowly than the lighter ones. Overall, the team measured the velocities of approximately 13,000 stars — the largest sample of velocities ever gathered for a globular cluster by any technique with any instrument.

Besides their findings on mass segregation, astronomers also found the possible presence of a black hole. “If there is a black hole, it is not more massive than 1,000 to 1,500 solar- masses, but we cannot reject that there may be a black hole [in 47 Tucanae],” said Meylan.

These findings are useful in estimating star ages, which, in turn, allows scientists a more complete view of the history of star formation.

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