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Size of star nuclei determines amount of combustible available, and hence evolution and life span of a star
Stars draw their energy from nuclear fusion occurring inside their nuclei, where densities and temperatures reach extreme levels. In the nuclei of massive stars, a phenomenon known as core overshooting can occur, which drastically impacts the evolution of the star, especially its life span. Recently, a study lead by the Institute of Astrophysics of Andalusia (IAA-CSIC) measured the magnitude of this phenomenon and was able to establish an unequivocal dependency between it and the mass of the star.
To determine the life span of a star it is necessary to know its type of nuclear cauldron and the types of thermonuclear reactions which it hosts. Stars produce energy fusing together hydrogen atoms and turning them into helium, but the conditions necessary for that reaction are present only in the nucleus, so the size of the nucleus will determine how much combustible is available and, by extension, the life span of the star. The size of star nuclei depends on the way energy is conveyed to the outer regions.
In stars with a mass of at least 1.3 times that of the Sun, the energy produced in the nucleus is conveyed to the outside by convection, similar to the bubbling process in boiling water. The convection cells convey the energy, and the size of the nucleus has traditionally been inferred from criteria based on the acceleration of those cells: when it is absent, the movement ends: that will determine the boundaries of the nucleus.
“However, by inertia, the convection cells may move longer distances than those foreseen by this criterion, resulting in a bigger nucleus. Consequently, more combustible will be available, prolonging the life of the stars, among other details,” says Antonio Claret, IAA-CSIC researcher in charge of the study.
We may use as an analogy for this phenomenon, called core overshooting, a fire in a camp: in principle the fire will extend only as far as the firewood, but because of thermal movements or the wind, it may reach all the way to the leaves in the trees around the hearth.
"Because core overshooting alters the evolution as well as the life span of massive stars, the calculation of its intensity and its probable dependency vis-à-vis the mass of the star is an important challenge for modern astrophysics -says Claret (IAA-CSIC)-. In order to accomplish it, we must be able to rely on very precise data that we can compare with theoretical calculations. A few trials had been carried out in the past, but no conclusive results had been obtained, mainly due to the scarcity of reliable empirical data.”
The present study selected thirty three binary and eclipsing stars situated in our vicinity, as well as the Magellanic Clouds, two satellite galaxies of the Milky Way. The binary star systems are of a type which, given their orientation with respect to us, eclipse each other periodically, and they are the most reliable source of stellar data such as mass, radii or temperatures (with a median error of between one and five percent). The authors compared the data with theoretical calculations of evolutionary models to determine the magnitude of core overshooting and a clear conclusion was finally reached.
“A much clearer relationship was found between core overshooting and stellar mass than in previous studies. We measured a very significant increase in core overshooting in stars whose mass ranges between 1.3 and 2 solar masses”, says Claret (IAA-CSIC). Moreover, researchers have discarded the influence of other factors in this phenomenon, such as the star’s evolutionary stage.
The conclusions of this study have direct implications for subjects ranging from the synthetic study of stellar populations to the formation of compact objects such as white dwarfs, neutron stars or black holes, which are the product of fuel depletion inside stellar nuclei.
A. Claret and G. Torres. "The dependence of stellar core overshooting on stellar mass". Astronomy & Astrophysics, vol 592, August 2016 DOI: http://dx.doi.org/10.1051/0004-6361/201628779