X CANARY ISLANDS WINTER SCHOOL OF ASTROPHYSICS "Globular Clusters"
Course: STAR CLUSTERS AS A TEST FOR STELLAR EVOLUTION
THE AGES OF CLUSTERS
After much hard work over the last few decades, astronomers today know that globular clusters are among the oldest objects in the Universe, containing as they do some of the oldest stars. Their pronounced spherical shape indicates that they hardly rotate. What is more, their masses, luminosities and radii are more or less the same, wherever they are located. This means that they were born more or less in the same way, and at approximately the same stage of the expansion of the Universe. Since they are not rotating, they must have formed from gas clouds having little or no angular momentum, and that they could be the result of the gravitational fragmentation of still un-ionized primordial gas in expansion. By comparing their colour-magnitude diagrams with calculations based on stellar evolutionary theory it is possible to determine the ages of these objects, which are key elements in the study of the origin and evolution of the galaxies.
What do you think are the most important contributions of the study of globular clusters to our knowledge of the Universe? What type of key information may be found when studying globular clusters exclusively?
"One has to bear in mind that each globular cluster represents a snapshot which memorize for us an instant of the evolution of matter in galaxies and, more in general, in the Universe. When interpreting observations in terms of globular cluster age and chemical compositions we are mapping the history of matter in our Galaxies as well as in other galaxies. Each cluster tell us that so many years ago in that galactic location there was a cloud of gas with such a distribution of nuclear species. It follows that globulars represent the most desirable milestones for that 'archaeology' of the Universe which is among the most fascinating objectives of astrophysics. The study of globular clusters will give us key information on the early evolution of spyral galaxies, as our own Galaxy, and on the evolution of galactic halos. They will probably give even more information on the history of both elliptical and dwarf irregular galaxies, mapping over a larger range the evolution of matter in these objects. As a byproduct, the age and composition of globular is already giving severe constraints on cosmological models."
How old are the globular clusters? By how much do you expect this age to vary?
"For galactic globulars, the best guess is now around 11 Gyr and, in my feeling, I will be surprised if this evaluation will be moved by more than ±2 Gyr. It appears to me that this is probably the age when globulars begun to appear everywhere in the Universe."
What are the most relevant uncertainties in the branch of physics related to the theories of stellar evolution? What effects might these uncertainties have on our understanding of stellar evolution?
"There are two different kinds of uncertainties, the ones connected with macroscopic mechanisms, the others with our knowledge of fundamental physics. Among the first ones one has convection, element sedimentation, mss loss and model atmospheres. As for the fundamental physics there are uncertainties on the three basic ingredients affecting every stellar model, namely the Equation of State, the radiactive opacities and the energy generation. Moreover we have further uncertainties on the additional physics affecting post Main Sequence stars, i.e., electron conduction and neutrino cooling. As a result, I guess that we are hardly predict the magnitude of a star better than ±0.1 mag. From my point of view we are facing an extraordinary qualitative success of theories but, at the same time, strong difficulties in giving firm quantitative predictions."
In relation to the subject of this Winter School, what is the most interesting problem in your present research?
"With my working group we are approaching several interesting problems, like the use of the cooling sequence of cluster White Dwarfs as a clock marking the cluster age, or the use of He burning clumps in not-too-old clusters as standard candles. However, the most relevant investigation in progress is probably the match between evolutionary models and pulsational theories, to predict the observational features of variables stars, adding further invaluable constraints to the information that one derives from the CM diagram."
Vittorio Castellani was born on
13 March 1937. He graduated in Physics at the University "La Sapienza" in Rome,
with a thesis on "annichilation e+e- in the Frascati storage ring AdA". Shortly
after the thesis he joined in Frascati the astrophysics group of proof. Livio Gratton,
where four young people (he, V. Caloi, C. Firmani and A. Renzini) decided to start from
the ground an activity in theoretical stellar models. Just at that time he fell in love
with globular clusters, looking at the impressive structure of these wonderful stellar
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