CONTENTS

Globular cluster M13

From w Centauri to 47 Tucanae,

Something more than just beauty in the sky

The constellation Centaurus contains what is for many the most beautiful globular cluster in the sky. The cluster Omega Centauri (w Cen), with close to a million stars, has a stellar designation because to the German astronomer Johannes Bayer (1572-1625) it appeared as a fuzzy star of fourth magnitude.

Also in the southern hemisphere, in the constellation Toucan and competing in beauty, brightness and antiquity, is the splendid globular cluster 47 Tucanae (47 Tuc); once again, and for the reason just given, it is a cluster with a stellar name.

Both objects are observable with small telescopes, and even with the naked eye, which is why they are so well known among amateur astronomers. But their appeal goes far beyond their evident beauty: w Cen and 47 Tuc are representative of a class of objects in the Universe of particular interest to professional astronomy.

Globular clusters hold many astronomical secrets. The technology of past centuries - unlike the giant telescopes of today with their advanced instrumentation - did not permit the contents of globular clusters to be clearly resolved. Today, we know that these attractive objects are very dense agglomerations of from tens of thousands to millions of stars distributed with the particularly spherical symmetry that their name indicates, and that they belong to what the celebrated astronomer Walter Baade in 1944 christened "Population II".

The members of this stellar population are old stars located in the halo of our Galaxy (and, by extension, in the halos of other galaxies), and with a very low metal content. This low metallicity means that globular clusters must have formed at an early stage in the life of their host galaxies, not long after the Big Bang and before the primordial hydrogen and helium were processed in the interiors of stars to form the heavier chemical elements.

All the stars in a globular cluster must have been formed at the same time, after the collapse of a single giant gas cloud. Their distribution in the "Hertzsprung-Russell diagram" (a graphical representation of the temperature or colour of each star plotted against its brightness or absolute magnitude) provides information concerning the common age and distance of all the stars in the cluster. The estimate thus obtained of cluster ages was, until recently, a matter of some controversy: the age of the Universe (some 15,000 million years) was incompatible with globular clusters having an age of 18,000 years. How could clusters that were older than the Universe itself possibly exist? Fortunately, as explained in this special issue of IAC Noticias, the paradox of the ages of the globular clusters has now been resolved after improved calculations of the distances to certain stars.

In our Galaxy there are 150 known globular clusters, scattered throughout the halo and with elliptical orbits about the Galactic centre; in contrast, open clusters, with younger and more metal-rich stars ("Population I") are concentrated in the disc of the Milky Way.

Globular clusters, so it would seem, are key topics for understanding such questions as the age of the Universe, galaxy formation, stellar evolution, etc. For this reason, as noted in their introduction by organizers of this Winter School, it is astonishing that globular clusters have warranted so few congresses and other scientific meetings. Hence the interest in, justification and theme of this X Canary Islands Winter School of Astrophysics, which marks the tenth anniversary of this annual event which provides an opportunity for young astrophysicists to meet and interact with lecturers of high scientific standing.

Just one recommendation before I sign off. Do have a look at what we have entitled ‘Planets in other clusters’, in which the Lecturers of the Winter School invite us to imagine how it would be to live inside a globular cluster.