Age-Dating Stars

From the Sun to Distant Galaxies

Maurizio Salaris

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000392616
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

2022 Winner - CHOICE Outstanding Academic Title

The determination of stellar ages has been - and still is - crucial for the development of our understanding of the universe, and to constrain theoretical models for the formation of galaxies and the evolution of planetary systems. Stellar ages provide scientists with timescales, and these timescales allow us to identify the relevant physical processes responsible for the development of cosmic structures.

This book describes in a simple, yet rigorous, manner the vast array of techniques that have been developed and are currently being used to determine the ages of stars. It also explores how stellar ages inform our knowledge about planets, star clusters, galaxies, even distant galaxies that we cannot resolve into individual stars. Up-to-date with the latest research and technologies in the field, it includes the cutting-edge methods being used based on asteroseismology and discusses open problems that remain to be pondered in future research.

It will be of interest to advanced undergraduates and graduate students studying astronomy, in addition to the general public.

Key Features

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• Presents an entertaining and accessible approach whilst also providing a rigorous and comprehensive presentation of the subject

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• Describes how to unveil the ages of stellar populations in distant galaxies that we cannot resolve into individual stars

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• Contains historical notes about these techniques, outstanding major problems, and a discussion on future developments in the field

CHAPTER 1

You Must Be Crazy

Not long ago, some acquaintances who live in Rome became aware of my job as an astrophysicist in the UK. When they learned the name of the neighbourhood where I grew up until the age of 15, they were somewhat surprised: Science is not one of the expected career paths for my generation raised in the 1970s in the borough of Garbatella.

There are probably few boroughs in the world that know the exact date of their foundation, and Garbatella, in Rome, is one of them. Its birthday is recorded on a plaque stating that on 18 February 1920, in the presence of the King of Italy Vittorio Emanuele III, the first stone was laid of a new neighbourhood: the Garbatella. According to common wisdom, this name comes from the moniker for the owner of a local tavern, not far from the Basilica of Saint Paul. She was a beautiful and graceful lady named Carlotta, so well-liked by the travellers who visited her tavern, that it was referred to as the inn of the lady ‘Garbata and Bella’ (Polite and Pretty) subsequently syncopated in ‘Garbatella’. The reason for the name also goes back to her charitable attitude towards the needy, and today there is a stucco depicting this lady on the facade of a building in the neighbourhood, not far from where my maternal grandparents lived. The name Garbatella also seems to have crossed borders, because when I visited Lebanon years ago to give a seminar at the American University of Beirut, I found a Lebanese restaurant called Garbatella during a trip to the nearby mountains.

The architecture followed initially the English model of the garden cities built for workers that included significant green spaces to provide residents with locally sourced food. After the first small villas came two- and three-floor buildings, then with the advent of fascism a few years later, houses more similar to modern condominiums began to be built. They hosted the displaced people who had been driven away from the city centre to make room for Mussolini's public works¹. Some of the buildings were considered great examples of modern architecture to be shown to important guests visiting the city. On 12 December 1931 Garbatella was even visited by Mahatma Gandhi.

Even today the neighbourhood has preserved its winding streets, steep stairways, and charming little squares. The shade that prevails is red; you can find it in the plaster of the buildings and the exposed bricks, in contrast with the white of the columns, frames, windows, the many decorative elements on the facades, and the black wrought iron of the railings and gates. The other predominant colour is the green of the lush vegetation (see Fig. 1.1). It is the ideal place for a walk among its gardens, and historical places such as the Palladium, a former cinema now used as a theatre².

Figure 1.1 A street of the neighbourhood of Garbatella in Rome, today. Not much has changed since its foundation about a century ago (ValerioMei/​Shutterstock.com).

It looks like an idyllic place, but it certainly wasn’t easy to live in during the 1970s and the 1980s. Economic crisis, political violence and terrorism, common crime, the unstoppable rise of drug addiction, all had a dramatic impact on life in the neighbourhood. I remember well the owner of the fruit and vegetable shop near our flat, murdered in front of the store because of gambling debts: My mother used to shop there almost every day, taking along my brother and myself during the long school summer holidays. Or the kid who attended my school and died crashing with a stolen scooter. I still have vivid memories also of pavements and parks strewn with syringes, some with blood still fresh on the needle. Not touching syringes and accepting nothing from strangers has been a common thread throughout my childhood. The life of too many people my age who grew up in that neighbourhood ended tragically because of drug addiction.

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¹ This is when my maternal grandparents moved to this neighbourhood from their flats in the historical central parts of the city.

²I remember watching at this cinema, for the first time, the masterpiece 2001 A Space Odissey, around 1979–1980.

It was the first months of 1977, and I was in bed with the usual winter flu. The year before, during a monetary crisis, the national currency at the time – the Italian Lira – was devalued by 12%, terrorist violence continued to claim victims, and a disastrous earthquake in the north-east of the Country caused the death of over a thousand people.

At the time I attended the Pablo Picasso middle school (a print of the paint Guernica was hanging right beyond the school entrance), a three-year bridge between primary school and high school, hosted in a building that no longer exists. I was used to walking there with three or four friends from primary school years. The meeting point was near my parents’ flat, and we walked to school as a group, at a brisk pace, going through some narrow streets flanking an old and abandoned warehouse.

At school, I found the lessons generally easy to follow, the teachers were good, and I especially liked maths and science. But bullying and intimidation were standard fare, and it wasn't fashionable to be a good student with top marks. In fact, I will never forget one of my classmates, a girl of whom I still remember the name, telling me one day: Salaris, good students are out of fashion these days!

Close to school, there was a bookshop with always a selection of science books on its shelves. I stopped every day to look at the titles and covers, fascinated especially by astronomy books, which were often displayed alongside publications on UFOs and aliens. I loved science books, an interest probably born during visits to the nearby flat of my uncle. The bookshelves in his living room were filled with not only comics and science fiction books but also several encyclopaedias, and during every visit, I spent hours reading about physics, astronomy, and maths in those volumes. Watching live on television the Apollo astronauts walking or driving on the surface of the Moon must have also played a role. I could not know it at the time, but today it looks like an extraordinary privilege to have grown up at the only time (so far) in history when humans set foot on an alien world. Sometimes I wonder whether it will ever be repeated.

A few days before I was in bed with the flu, I saw on display in that shop a book called Stars and galaxies, and I had enough money with me to buy it (it's still somewhere at my mother's place). It was a hardcover little book, with lots of colour figures and photos, written by a Spanish professor and translated to Italian. I had the pleasure of meeting the author, Ramon Canal of the University of Barcelona, almost 20 years later, when I was a postdoc at the Institute of Space Studies of Catalonia. However, at the time of this meeting, I did not recall he was the author of that ‘fateful’ book.

That winter morning, warm under the covers, I began to read about stars and galaxies. It wasn't the first popular astronomy book I read, and I did not expect any earth-shattering new revelation. After a first half devoted to galaxies, in the following sections about stars I discovered for the first time ‘stellar evolution’. The realization that stars evolve changing size, brightness, temperature, their inner structure and chemical composition was like an epiphany. First of all, the concept that stars evolve and change was fascinating and made them much more interesting in the eyes of a young boy. Secondly, their evolution coherently explained the various stellar classifications found in the popular books I read before.

Two important points escaped me at the time (after all, I was only 11 years old and not a child genius). The first one is that the Big-Bang (the ’birth’ of the universe, we’ll see more about this in chapter 2) produces essentially only hydrogen (H) and helium (He) but if you look at the chemical composition of the human body, you’ll find that about 65% of the mass of a person is made of oxygen (O), about 10% of H, 18% of carbon (C), 3.3% of nitrogen (N), 1.5% of calcium (Ca), 1% of phosphorous (P), plus small percentages of other elements like magnesium (Mg), potassium (K), sulphur (S), sodium (Na). Apart from the 10% of hydrogen, the other elements must have been produced after the Big-Bang, and indeed the theory of stellar evolution shows that they are all synthesized at high temperature in the interior of stars, and eventually ejected in the interstellar space. Out of interstellar gas enriched by stellar ejecta new generations of stars (and planets) form and evolve, and the cycle is repeated. We owe our existence not only to the light coming from the Sun but also to unknown stars that, before the Sun was born, have produced the elements that make up our bodies. It sounds melodramatic and not very original to state that we are all ‘stars’ children’, but indeed we are. And this is a very sobering fact.

The second implication that I failed to grasp, was that if stars evolve, and if we can mathematically predict their evolution, then it must be possible – at least to some degree – to determine their age. And measuring the age of stars ended up – mostly by chance – to be one of the main lines of my research, after I was lucky enough to satisfy what became my crazy childhood dream of becoming a scientist.

1.1 Why do ages of stars matter?

Stars cannot talk to us, they cannot show us their identity cards, and evolve on timescales much longer than human (and whole civilizations) lifetimes. Historically, their unchanging appearance in contrast to the terrestrial realm was taken as a sign that the heavens had a fundamentally different nature. Determining the age of stars is a bit like the work of a forensic anthropologist, although not remotely as macabre. Forensic specialists apply anthropology to criminal investigations and help identify human remains, estimating the age of the victims, as we have seen hundreds of times in television series. Their work rests on the knowledge of how human bodies develop over time and is based, among others, on the degree and location of bone growth. Our bones are mostly soft cartilage at birth, which is then slowly replaced by hard bone at over 300 different centres of growth. These centres of growth eventually fuse to form the bones we find in adult bodies. And after the body stops growing, some bones begin to fuse together. Given that we know the speed at which these processes advance, anthropologists can use the observed patterns of bone development to estimate ages.

Along the same lines, if we can calculate reliable models of stars and their evolution, we should be able to find time-varying features to be observed and exploited for age determinations. But why should we be so keen to determine the ages of stars, apart from our innate compulsion to learn about the world around us?

Let's start with an example closely connected to our existence as a species, If we are completely ignorant about the structure and evolution in time of our star, we can't predict if/when its luminosity and its size will change,...