Hubble Catches Up With A Blue Straggler Star In The Core Of A Globular Cluster

       Astronomers have long been mystified by observations of a few hot, bright, apparently young stars residing in well-establishedneighborhoods where most of their neighbors are much older. 

       With the help of NASA's Hubble Space Telescope, astronomers now have evidence that may eventually help solve the 45-year-old mystery of how these enigmatic stars, called blue stragglers, were formed.  For the first time, astronomers have confirmed that a blue straggler in the core of a globular cluster (a very dense community of stars) is a massive, rapidly rotating star that is spinning 75 times faster than the Sun.  This finding provides proof that blue stragglers are created by collisions or other close encounters in an overcrowded cluster core.

       Astronomers studied a blue straggler in the tumultuous heart of the nearby globular cluster 47 Tucanae (47 Tuc), located 15,000 light-years away in the southern constellation Tucana.  The observation was made by astronomers Michael M. Shara of the Space Telescope Science Institute, Baltimore, MD; Rex A. Saffer of Villanova University, Villanova, PA; and Mario Livio, also of the Institute.  Their analysis will appear in the Nov. 1 issue of  the Astrophysical Journal Letters.

       "This is an extremely exciting result," Saffer said, "because it may help distinguish between competing theories of blue straggler star formation and evolution."

       "Since their discovery 45 years ago, blue stragglers have been assumed to be stars much like the Sun, although their bluer color and greater brightness imply that they are more massive and much younger than normal globular cluster stars.  Our analysis confirms that, but without having to make any assumptions about the state of blue straggler star evolution," Saffer said.

       Using Hubble's Faint Object Spectrograph (removed during the Second Servicing Mission in February), the astronomers analyzed the spectrum of one blue straggler, measuring its temperature, diameter, and rotation rate. The team then combined these measurements with the blue straggler's apparent brightness, taken from a Hubble Telescope Wide Field and Planetary Camera archival image, to obtain the star's mass.  The astronomers report the derived temperature and mass are consistent with the characteristics of a normal star with a mass about 1.7 times that of the Sun.  However, the star is spinning at least two to three times faster than stars of its kind.

       Astronomers now believe that blue stragglers are created by the merger of two low-mass stars.  But they have two different views of how these stars interact to create blue stragglers.  One merger theory proposes that a violent collision of two unrelated stars creates a blue straggler.  Another hypothesizes that a slow coalescence of a gravitationally bound pair creates the straggler star.

       Based on their analysis of the blue straggler in 47 Tuc, the team favors the slower, gentler merger scenario between binary stars.  In double-star systems where the stars are close enough to touch each other, the more massive star can cannibalize its partner, producing a single, even more massive star.  This process, astronomers believe, more likely results in a rapidly spinning merger product where the fast orbital motions of the binary star produces the rapid spin of the consolidated pair.

       The second merger scenario involves a collision between two unrelated stars, which run into each other by chance in the dense star cluster core.

       "It's a bit like a head-on wreck between two tractor trailers," Saffer explained, "where the enormous energy carried by the fast-moving stars is deposited in the debris from the collision."

       Saffer credits the Hubble telescope's superior spatial resolution with being able to peer into a swirling mix of stars to capture a blue straggler in the cluster core.

       "While some blue stragglers are found in globular cluster outskirts, in 47 Tuc the blue stragglers are only found in the cluster core," Saffer said.  "The crowding of the stars there is too severe for the current generation of ground-based telescopes to resolve them."

       Globular clusters contain up to 1 million stars packed into a swarm only about 20 light-years in diameter.  They also areamong the oldest stellar systems in the Milky Way Galaxy.  Stars speeding through the extremely crowded cluster core are far more likely to collide or pass near each other than stars in the sparse neighborhood of the Sun.  These processes can produce a zoo of stellar animals, such as X-ray binaries, pulsars, blue stragglers, and other exotic species, all of which have actually been observed in globular cluster cores.

       The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD.  The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

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