Unmasked a mechanism for the formation of
Blue Straggler Stars

An international team of researchers leaded by prof. Francesco Ferraro of the Astronomy Department of the Bologna University discovered a significant depletion of carbon and oxygen at the surface of some Blue Straggler Stars in the globular cluster 47 Tucanae. This is the first detection of a chemical signature clearly pointing to a specific scenario for the formation of these peculiar objects: the observed stars likely formed by taking their material off a companion in a binary system.

The research has been published in a paper appeared in the 10 August 2006 issue of the Astrophysical Journal Letters.

By exploiting the multiplexing capability of the spectrograph FLAMES/Giraffe at 8-m telescope ESO-VLT, the team measured the abundance of chemical elements at the surface of 43 Blue Straggler stars in 47 Tucanae, and discovered that six of them contain less carbon and oxygen than the others. Such anomaly indicates that this material comes from the deep interiors of a companion star and has reached the surface of the Blue Stragglers during the mass transfer process between two stars in a binary system.


Oxygen abundance as a function of Carbon abundance (both with respect to Iron and in a logarithmic scale) for the Blue straggler stars observed in the globular cluster 47 Tuc. Normal Blue straggler stars are marked with empty circles, while the 6 Carbon and Oxygen-deplete Blue Straggler stars are marked with red squares. Their names are also indicated. The yellow region correspond to the location of other stars in 47 Tuc previously studied.


47 Tucanae (or 47 Tuc) is an impressive globular cluster that is visible with the unaided eye from the southern hemisphere. It was discovered in 1751 by the French astronomer Nicolas Louis de Lacaille who catalogued it in his list of southern nebulous objects. Located about 16 000 light years away, it has a total mass of about 1 million times the mass of the Sun and is 120 light years across, making it appear on the sky as big as the full moon.


The ESO Very Large Telescope (VLT) at the Paranal Observatory (Atacama desert, Chile) is the world's largest and most advanced optical telescope. It comprises four 8.2m reflecting Unit Telescopes (UT) and several moving 1.8m Auxiliary Telescopes, which can work independently or in combined mode. The photo shows a beautiful view of the Paranal platform with the four 8.2-m UT (ESO Press Release 14/00). The team of astronomers used FLAMES (a multi-object high resolution spectrograph) mounted on UT 2 (the second telescope from the left) to discover the chemical signature of the mass-transfer process in blue Stragglers. This telescope is also named "Kueyen" - "The Moon" in the language of the Mapuche people, an indigeneous population which lives mostly in the South of Chile (some 500 Km south of Santiago de Chile).

The formation mechanism of Blue Stragglers has been a mystery for decades. These objects are unexpectedly young-looking stars found in stellar aggregates, such as globular clusters, which are known to be made up of old stars. They are thought to be created in either direct stellar collisions or through mass transfer processes in binary systems, and as such, they provide interesting constraints on both binary stellar evolution and star cluster dynamics. However, the unambiguous signature of a formation scenario had not been observed up to now. The discovered carbon-oxygen depletion clearly recognize the mass transfer process in binary systems as the formation mechanism of (at least part of) those stars.

The discovery is therefore a fundamental step toward the solution of the long-standing mystery of Blue Straggler formation in globular clusters, and demonstrates that the binary mass-transfer scenario is fully active even in a high-density cluster like 47 Tucanae, where stellar collisions are though to be very frequent.

The team comprises Francesco R. Ferraro and Nicola Compagni (Astronomy Dept., Bologna University, Italy), Barbara Lanzoni, Eugenio Carretta, and Flavio Fusi Pecci (INAF--Osservatorio Astronomico di Bologna), Giacomo Beccari (INAF--Osservatorio Astronomico di Bologna and Teramo), Raffaele Gratton and Sara Lucatello (INAF--Osservatorio Astronomico di Padova), Giampaolo Piotto (Astronomy Dept., Padua University, Italy), Elena Sabbi (Space Telescope Science Institute, USA), Robert T. Rood (University of Virginia, USA), Alison Sills (McMaster University, CA), and Sabine Moehler (Institute fur Theoretische Physik und Astrophysik Kiel & ESO Germany).

The new in the press:

  • Il resto del Carlino 20 Oct 2006
  • La Repubblica 25 Oct 2006
  • Sky & Telescope January 2007
    • For additional information please contact:
    Francesco Ferraro tel +39 051 2095774
    Last Update : 25 Oct 2006