Globular clusters are massive assemblies of stars. More than 100 such clusters are known in the Milky Way galaxy and most of them harbor several million stars. They are very dense -- at their centers, the typical distance between individual stars is comparable to the size of the Solar System, or 100 to 1,000 times closer than the corresponding distances between stars in the solar neighborhood.
Globular clusters are among the oldest objects known, with estimated ages of 11 to 15 billion years. All stars in a globular cluster were formed at nearly the same moment, and from the same parent cloud of gas and dust.
The original chemical composition of all stars is therefore the same. But now, an international group of European Southern Observatory astronomers, working with the UVES Spectrograph at the ESO Very Large Telescope (VLT), have obtained some unexpected results during a detailed analysis of dwarf stars in some globular clusters.
Such stars have about the same mass as our Sun and, like the Sun, they evolve very slowly. This means they still ought to have about the same abundances of most chemical elements.
Nevertheless, the astronomers found large abundance variations from star to star, especially for the common elements Oxygen, Sodium, Magnesium and Aluminum. This phenomenon has never been seen in such stars before.
It appears that those stars must somehow have received "burnt" stellar material from more massive stars that died many billion years ago. In their final phase -- as "planetary nebulae" -- they eject stellar material that has been enriched with certain chemical elements which were produced by the nuclear processes in their interiors during their active life.
Such an acquisition of material from other stars has been proposed but has never before been seen in globular clusters. This new discovery obviously sets stars in globular clusters apart from those in less dense environments, such as the solar neighborhood.
(Reference: The research paper ("The O-Na and Mg-Al Anticorrelations in Turn-Off and early Subgiants in Globular Clusters") is now in press in the European journal Astronomy & Astrophysics.)
Related websites:
Image of Globular Cluster
Spectral Analysis of Dwarf Stars
[Contact: Raffaele Gratton, Luca Pasquini]
06-Mar-2001
