ESA Science & Technology - Hubble Space Telescope Finds Blue Straggler Stars in the Core of a Globular Cluster

The discovery was made by Francesco Paresce, Michael Shara, Georges Meylan of the Space Telescope Science Institute, and the FOC team (Drs. Paresce and Meylan are also astronomers with the European Space Agency, a joint partner with NASA on the Hubble Space Telescope project). Their findings will be published in the 25 July issue of Nature magazine.

Utilizing HST's high resolution and ultraviolet sensitivity to probe the dense center of the cluster, the astronomers resolved 21 blue stragglers. "The 47 Tucanae blue stragglers are extraordinarily centrally concentrated, supportive of the view that they are the product of collisions, mergers or close encounters among stars," says Dr. Paresce, "This surprising result explains why blue stragglers had, up to now, eluded detection with instruments that do not have the required high spatial resolution to resolve the dense core of 47 Tucanae."

Globular clusters are beehive swarm agglomerations of several hundred thousand stars each. Globular clusters are among the earliest inhabitants of our Milky Way Galaxy. They formed in the vast halo of our Galaxy before it flattened to form a pancake-shaped spiral disk. Star formation essentially stopped in globular clusters 15 billion years ago, so astronomers expect to find only old stars. In fact astronomers use globular cluster ages as a benchmark for estimating the age of the universe.

Observations with ground-based telescopes confirm that the clusters' stars have quickly evolved to red giant, old age stars. However in 1953, astronomer Allan Sandage found a puzzling new population of stars which seemed to go against the rules of stellar evolution in globular clusters. Sandage detected hot young blue stars in the globular cluster M3, and subsequently in other globulars. He dubbed them stragglers because they looked like they were trailing or left behind by other blue stars which long ago evolved to the red giant stage.

The simplest explanation for blue stragglers is that they formed much later in the cluster's life. This is the least likely explanation though, because globular clusters were stripped of residual dust and gas which is needed for new star formation many billions of years ago. So all stars should have formed at about the same time.

Another idea is that blue stragglers are somehow far more efficient at mixing their internal hydrogen supply so that they can burn (by nuclear fusion reactions) much longer than blue stars normally do. A typical star consumes only about 10 percent of its hydrogen fuel through nuclear fusion. Stars like our Sun puff off much of the rest of the remaining hydrogen late in their lives. Blue stragglers would need to approach an extraordinary efficiency of 100 percent, to essentially stretch out their youth, and burn brightly for billions of years longer than normal.

In 1964 astronomers Fred Hoyle and W.H. McCrea independently suggested that blue stragglers result when two stars capture each other and form a tight binary system. This would stir up the hydrogen within each star to provide fresh fuel for their respective nuclear furnaces.

This scenario is feasible because stellar close encounters must occur relatively frequently at the tightly packed cores of globular clusters. Astronomers estimate that one out of every one hundred stars in a globular cluster has had a close encounter with another star. If our own local stellar neighborhood were so crowded, more than a million individual stars could be seen in the night sky with the unaided eye (in reality about 3500 stars can be seen on a moonless night). The nearest star to our Sun would lie only twenty times farther away than the outermost planet, Pluto.

More than simply passing in the night, the stars are likely to capture each other as well. That's because most stars in globular clusters drift at relatively slow velocities of 10 000 miles per hour (by contrast our Sun travels through space at 40 000 mph). A pair of bypassing stars are more likely to "feel" each other's gravitational tug for a period of time long enough for the stars to capture each other.

In such a pair the less massive star would siphon fresh hydrogen from its more massive and hence faster evolving companion star. With the new fuel supply the smaller star heats up, growing bluer and hotter. In stellar encounters which are more nearly head-on collisions, the stars might actually merge, mixing their nuclear fuel and "re-stoking" the fires of nuclear fusion.

Merged stars and binary systems would be about twice the mass of the cluster's individual stars. They would tend to settle at the core of the cluster, which is the "bottom" of the cluster's gravity well. Ground based observations support the idea that blue stragglers are massive merged stars. The central regions of a few globular clusters have recently been shown to host blue stragglers which are more centrally concentrated than the cluster's subgiants at the same magnitude.

However, high density clusters are generally difficult to resolve from the ground, and have not yet been searched, systematically, to brightness limits faint enough to detect blue stragglers. Hubble Space Telescope's high spatial resolution and ultraviolet sensitivity make it a powerful tool for probing the centers of globular clusters.

To search for blue stragglers the researchers looked at the center of globular cluster 47 Tucanae which lies about 15 000 light-years away. HST resolved approximately 600 stars in a small field (1.5 light years across) at the cluster's core, where ground-based images only yield a few dozen stars. When the researchers compared HST images with two short exposure visible light images of 47 Tucanae (taken by Georges Meylan on 12 December 1988 with the 2.2 meter telescope of the European Southern Observatory at La Silla, Chile) they discovered 21 stars that are exceptionally bright in ultraviolet light.

The researchers next compared the brightness and temperature of these stars against a computer model developed by the FOC team. This model predicts how main sequence stars, like the blue stragglers will appear at ultraviolet wavelengths. (The main sequence, from the Hertzsprung- Russell diagram, contains more than 90% of the stars observed, which are in this stable portion of their lives.) The excellent match between the model and the FOC observations allows the researchers to conclude that the excessively UV-bright stars detected by HST are blue stragglers.

This high concentration of blue stragglers towards the core of 47 Tucanae suggests they are significantly more massive than most of the cluster's stars. Some of the blue stragglers could be massive because they are really double star systems. Such binary systems would play a crucial role in the cluster's dynamic evolution, and help explain why the cores of globular clusters aren't even more densely packed with stars. The cores may in fact collapse but then rebound due the presence of blue straggler stars. Though the blue stragglers represent only a tiny fraction of the cluster's population they would serve as kinetic energy batteries. Only a few blue stragglers could stir-up the motions of thousands of other stars in the cluster, like pair of "egg beaters."

The researchers caution that the FOC images alone don't conclusively allow astronomers to distinguish between the three possibilities for the nature of the blue stragglers of 47 Tucanae. However the binary star model is supported by other observations of high-velocity stars found in the core of 47 Tucanae which could only, be the result of gravitational close encounters between single stars and binaries or between two binary systems. Astronomers have also recently discovered a significant population of millisecond and binary pulsars in 47 Tucanae, which adds additional weight to the notion that binaries are present in this cluster.