We all share the same sky
As told by Amina Helmi

My childhood
Mine was an atypical family that lived, of all places, in Bahia Blanca, Argentina. A Dutch mother, an Egyptian father and me. Born on a Tuesday morning in October 1970, I was the first Argentine in our line. My parents had come from Egypt, where they'd lived for a decade after meeting in the Netherlands, and Argentina was meant to be temporary. Two years, maybe three. But something about the place agreed with them: the people, the cultural middle ground it offered, the weather, and two years became a lifetime.

Astronomers recognize daily that we all share the same sky, making national borders meaningless

Argentina is a country of immigrants so I was only somewhat unusual. Yet, few of my peers have experienced such a multicultural upbringing, and growing up in that world has shaped me in many ways.

My childhood was a mix of simple joys. At home, I cooked imaginary meals with my mom and played with dolls, while my dad was at work. On weekends, he’d take me to a park, teach me to ride my bicycle or we’d watch football or formula 1 races together on TV. My dad was professor of soil chemistry and his passion for science was contagious. I loved going to his Institute! I'd find myself daydreaming over his desk, pretending to be his secretary, shuffling papers and typing on an old Remington. The real science, aromatic compounds bubbling just across the room, remained mysterious and appealing, but out of reach.

As an only child, I learned early to be independent and creative to entertain myself. With no siblings or cousins around, school was especially fun both for the friendships and the chance to learn. I was an avid reader, and enjoyed feeling transported to another world. I was drawn to novels and adventure stories, especially those with a young female protagonist who solved mysteries. But books were expensive, and my parents’ moves across the world had kept our home collection small. So, I read my way through every book in my primary school library instead.

By chance, the mother of my best friend turned out to be a secondary school teacher with an extraordinary book collection. Martha became not just my favourite book-lender but almost inadvertently, also a role model for me: an intelligent, beautiful, and fun single working mother raising two wonderful girls my age.

In fact, I might owe Martha my first exposure to Astronomy. In the winter holidays of 1982, my mom and I, Martha and her girls, all went on a memorable trip to Buenos Aires. The planetarium show we saw then left me breathless. Yet, the first time I considered becoming an astronomer was a year later. “We always see the same side of the Moon”, the school teacher explained. I vividly recall the associated little experiment and how, suddenly, I saw it, I experienced the eureka moment of understanding. As a 12 years-old, I had managed to grasp what was happening thousands of kilometers away (in space!) just with my brain. That made me feel so small yet so big…

At 15, I read Contact by Carl Sagan about a female astronomer using maths and physics to discover something deep about the universe! I firmly decided to study astronomy

Even though a “bookworm”, I was not a typical “nerd”. I have never watched Star Trek or Star Wars. Even when Cosmos (Carl Sagan’s stunning astronomy series) aired, I preferred to go on cycling adventures with my friends. But I did love maths. To me, equations were like small puzzles, and I was good at solving them. And then, as a 15 years-old, I read Contact by Carl Sagan and was seized by the story: A female astronomer using maths and physics to discover something deep about the Universe! It was then that I firmly decided to study astronomy.

Studying Astronomy in La Plata: a dream come true
What has always captivated me, even more than the inspiring starry panoramas, are the patterns and understanding behind them. To this date, I am drawn to make sense of the world around us and to solve its mysteries. Like in the children’s books, but now to explain the Universe.

Studying Astronomy at the University of La Plata felt natural. It wasn’t easy though. I was often homesick, hence like so many students, I travelled regularly back to Bahia Blanca. Those 8-hour night bus rides across the pampas always stopped once, allowing me to gaze at innumerable stars and feel privileged to be gradually grasping their workings.

To achieve that sense of fulfilment, however, I made really long study hours. Besides the challenges inherent to studying a difficult subject, I was faced with my own insecurities (do I really understand all the material inside out?). My parents’ sacrifices and unconditional support further fed my sense of responsibility and pushed me to do my best. But I loved it: it was my destiny to become an astronomer.

I am proud of the university education I received in Argentina. Its strong foundation prepared me well for research

Throughout my student years I also made lifelong friends. During mate sessions filled with laughter and games, there was always room for deep conversations. And in 1991, I met Mariano, my future (ex) husband—also an astronomer.

I am proud of the university education I received in Argentina. Its strong foundation prepared me well for research, particularly in theory. Yet, I was painfully aware (both from my own experience and my father’s) that being a scientist in my home-country would be hard. With limited funding and persistent economic instability, focusing on science is a constant challenge. I also learned as a young girl, that hard work and talent are undervalued in my beloved Argentina. Too often, “connections” outweigh merit.

A defining moment came when I participated in the Vatican Observatory Summer School (VOSS) in 1995. Together with 25 bright young students from all over the world, I was exposed to cutting-edge science. This school, still held every few years, provides a unique opportunity, especially to students from developing countries. By working with world-famous astronomers, the VOSS acts like a springboard. This also happened to me, and I am particularly indebted to the late George Coyne, SJ for his support.

After the VOSS, Mariano and I turned our desire to pursue careers abroad into reality. Mariano’s postdoctoral fellowship from CONICET (Argentina’s research council) and my Amelia Earhart fellowship (Zonta Foundation) made it possible. We were ready to, temporarily, emigrate to the Netherlands.

A graduate student at Leiden: challenging at first, rewarding at last
Choosing the Netherlands, a leading country in Astronomy, felt natural given my personal background. A Leiden professor I’d met at a conference in Buenos Aires recommended the Observatory and provided an initial contact.

However, my first year at Leiden was particularly tough. The Amelia Earhart fellowship got me started, but I still needed to secure full PhD funding. This proved more difficult than I had anticipated. Was it naïve to believe that several articles published in well-known refereed journals and good references from the VOSS lecturers would be enough? I knew I would have to prove myself, but it took nearly too long, and I almost gave up on Leiden. It was thanks to the efforts of Tim de Zeeuw that, after 1 year, I was granted a PhD scholarship.

Tim was very generous in letting me pursue the subject of my interest and he put me in touch with Simon White, a leading cosmologist in Germany. The three of us together defined the topic of my PhD at the interface of two fields, cosmology and the Milky Way. The goal was to work out what the hierarchical cosmological paradigm predicted in detail for the formation of a galaxy like ours, for example in terms of mergers and their signatures. That I would walk the path of renowned Dutch astronomers to understand the Milky Way, was extremely exciting and felt like an honour to me. And I’ll always be deeply grateful to Tim and Simon for their support.

I became a theorist who likes making testable predictions and, if possible, peeking at the data. It is perhaps no coincidence that I was always captivated by the story of French astronomer Leverrier. He had predicted the existence of Neptune purely from the motion of Uranus, and was later proven right. How cool was that?! As it turned out, and without realizing it at the time, I found myself following a similar path. During my PhD, I developed models to understand the evolution and characteristics of merger debris, which led me to predict the existence of hundreds of streams from past mergers in the Milky Way halo. Then came the moment I had not dared to hope for: the actual discovery of the first streams crossing the Solar vicinity, the remains of a small galaxy accreted long ago.

I have been blessed because not only did I make an important discovery at the beginning of my scientific career, but I also got involved in the amazing Gaia satellite. At the time, Gaia was being considered as a cornerstone mission by the European Space Agency. With my models and newly developed methods, I contributed to defining the specs required to uncover the Galaxy’s merger history. Little did I know it would take another 18 years to achieve that goal...

A school in Galactic Archaeology: Learning while preparing
After less than three years as a postdoctoral fellow, I became assistant professor at the Kapteyn Institute in Groningen in 2003. Since then, I’ve built my own group and research line, largely thanks to significant personal grants. During my ERC Starting Grant interview, when asked about my ambitions, I remember saying I wanted to make Groningen renowned for Galactic Archaeology: a hub for young researchers eager to reconstruct the Milky Way’s history. Inspired by my father and the mentors I admired, I’ve spent the years since turning that dream into reality by building a “school”.

I have had the privilege of working with amazing students, postdocs and colleagues. Together we developed new understanding on dwarf galaxies with the DART team (PI: Tolstoy), and models for the dark matter distribution around the Milky Way and its satellites. We also (sometimes implicitly) prepared for the Gaia revolution. We did this by developing and exploiting our own simulations and those by the Aquarius project (PI: Springel), as well as by using data from the Spaghetti (PI: Morrison) and RAVE (PI: Steinmetz) surveys. I am deeply indebted to all my team members and collaborators; without them I would not be writing this autobiography.

Goosebumps and a milestone
It was 25 April 2018, and I took my son (who had been ill for a while and could not follow school), to the event planned around the 2nd data release from Gaia.

It was clear from the first plots that day that something extraordinary was hidden in the Gaia DR2 data

Together with hundreds of scientists across Europe, my team had been working for years to ensure that the data released that day would be of sufficient quality for science exploitation. I had also led Gaia’s performance verification paper on the motions of globular clusters and dwarf galaxies around the Milky Way. Yet, on that April morning, I had no idea what discoveries lay ahead—we were restricted from exploring the data beyond our projects’ boundaries to ensure fairness for all. Yet, I already felt immense pride and satisfaction in being part of the Gaia collaboration.

It was clear from the very first plots we made that day (thanks to team member Maarten Breddels) that something extraordinary was hidden in the Gaia DR2 data. Together with Carine Babusiaux, Helmer Koppelman, Davide Massari, Jovan Veljanoski, and Anthony Brown, we unveiled hitherto mysterious clues. We noticed curious stellar motions, unexpected sequences exposing distinct stellar populations and traces of varied chemical histories. Step by step, the Milky Way’s last major merger revealed itself to us in all its glory.

Using data from Gaia DR2 and the APOGEE survey, and in just four intense weeks, we not only uncovered the merger but also determined when, how, and with what kind of object it had occurred. We could even see the immense scar it had left: a puffed-up Galactic disk. This was a true milestone in Galactic history.

The merger with Gaia-Enceladus as we named it, happened long before the Sun and most of the stars that make up the “milky way” on the sky were born. And I was there, 10 billion years later, to put the pieces together. Feeling again small and big at the same time. Goosebumps. And profoundly grateful.

This discovery and the ones that followed, proved that mergers have been important in the history of our Galaxy, and that the feeble streams I discovered nearly 20 years earlier and which currently carry my name, were not something anecdotal. Our research community is still figuring out the impact of mergers, but what is clear, especially in the case of Gaia-Enceladus, is that it is bigger than anticipated.

A long-term pursuit of the Galactic community is to use the vast amounts of detailed, high-quality data, i.e. the fossil record of stars in our Galaxy, to constrain galaxy formation and cosmological models. For example, streams may be key for testing the nature of dark matter and alternative gravity hypothesis. And although until recently it was impossible to simulate sufficiently realistic analogues of the Milky Way, this is now within reach. Thanks to a new methodology developed in my team jointly with scientists from Germany, France and Sweden, we are able to generate initial conditions that when evolved match a set of key properties of the Milky Way and its big sister, the Andromeda galaxy. With these cosmological initial conditions, we can run sophisticated hydrodynamical simulations to compare with transformative datasets from Galactic Archaeology. This very exciting enterprise will, iteratively, allow us to better understand galaxy formation physics, and ultimately perhaps, our place in the Universe.

Personal thoughts
My place is being a mom and an astronomer. My son, now 20 years old, has given me the greatest possible gift: motherhood. Manuel means the world to me. Balancing motherhood and science can be challenging (even from my son’s perspective!). But I couldn’t be just one or the other. I’d be a boring mom if I weren’t a researcher, and an unhappy researcher without my son. Our dog, Samira (and before her, our beloved Sandy), completes our little family. We enjoy long-walks, watching movies and traveling. Our destinations: often the sea, and lately, my Egyptian relatives.

In 2019, my parents moved to Groningen, fulfilling my long-cherished dream. I’m grateful that I could care for them, especially during the pandemic, and even if only for a few years. I treasure the many wonderful moments we shared. After they passed away, my son fell seriously ill. The past few years have been particularly challenging.

Given my personal background, I feel at home in this global community.

Yet, through it all, my love for science remained constant. Science has this beautiful aspect of being fundamentally curiosity-driven: fuelled by our innate quest for understanding, (almost) selfless by nature. This spirit is reflected in the astronomy community. Astronomers recognize daily that we all share the same sky, making national borders meaningless. The biggest challenges we face, as astronomers and as humans, demand collaboration. As an example, no single country can build missions like Gaia or extremely large telescopes alone. This very international character exposes astronomers to different cultures, teaching us to value our differences and, above all, to find what unites us.

Given my personal background, I feel at home in this global community. I’m grateful for the friendships and opportunities it has brought me, and I’m deeply honoured to share my story with you.