Introduction

> Laying under the southern Milky Way, I felt an unmistakable connection to the universe. Even though I'm technically small, I recognize that many small things come together to form something magnificently whole.

> Studying the oldest stars gives us profound insight into the early universe. By understanding the chemical and physical conditions of these ancient celestial bodies, we uncover the origins and evolution of the Milky Way, piecing together the story of our cosmic heritage.

First elements

> The universe started with the Big Bang, creating a hot, mostly hydrogen and helium gas. Massive stars, a hundred times the mass of our Sun, formed and exploded in supernovae, seeding the cosmos with heavier elements like carbon and oxygen. This transition from a chemically pristine universe to one peppered with heavier elements allowed for the formation of smaller, long-lived stars like our Sun.

> The early universe's first stars lived incredibly short lives, only a few million years, compared to cosmic timescales. Their supernovas played a crucial role in cooling gas clouds, enabling the formation of subsequent generations of stars and eventually guiding the creation of galaxies. I find and study ancient low-mass stars, which preserve the chemical composition of those early gas clouds, effectively allowing us to "unpack everything that happened back then."

> The Milky Way formed through hierarchical growth, absorbing smaller protogalaxies. The oldest stars, remnants from these early structures, tend to reside in the galaxy's outer parts. By studying these ancient stars and their compositions, we can trace back and understand the early universe's conditions and the formative stages that led to our own galaxy's development.

Milky Way

> When I think about the Milky Way, I feel like I'm part of something larger. It's not about feeling small, but about being in awe and recognizing our place in this vast galaxy.

> Even though we may never physically travel to distant parts of the universe, our ability to explore and understand it with telescopes and sensors is a form of traveling and connecting with the cosmos.

Alien worlds

> The immense variety and mystery of planets and life forms in the universe are truly compelling. Imagining the myriad of worlds and potentially primitive organisms that could exist out there is fascinating. Every other star in our galaxy seems to have a planet, and the conditions on these planets could be wildly diverse, creating a canvas of possibilities that we are only beginning to understand.

> The process of star and planet formation remains one of the most intriguing puzzles. While we grasp the broad strokes—like needing a gas cloud that cools and clumps to form stars and possibly planets—the specifics are highly varied and complex. It's analogous to human diversity: broadly similar structures but immense individual differences. Our technological limitations keep us from directly observing these processes in detail, but with advancements, we might move closer to unraveling these cosmic mysteries.

Protogalaxies

> Understanding the formation of galaxies is intertwined with understanding black holes. "Most large galaxies have a supermassive black hole in the center," but we still grapple with whether the black hole formed before the galaxy or came into existence alongside it. This fascinating "chicken or egg" question drives many of us in astrophysics to investigate deeper.

> The James Webb Space Telescope is revolutionizing our observations, collecting infrared light from some of the earliest galaxies. It's exciting to think that these faint blobs we see have been traveling for 13 billion years! "We're seeing that they were there, surrounded by already bigger galaxies," and I can't wait for my colleagues to push even further into uncharted territory.

Black holes

> When I work on observing stars, the physics is different from theoretical discussions about black holes. Despite the differences, collaborating with theory colleagues can lead to fascinating intersections, even if we speak different scientific languages.

> It's intriguing how different fields like artificial intelligence and neuroscience, or observational cosmology and simulation, approach understanding similar mysteries from unique angles. Playing with simulations is like a cosmic sandbox where diverse perspectives can lead to valuable breakthroughs.

Stellar archeology

> Stellar archeology allows us to study old stars that have preserved information from the early universe, giving us a unique glimpse into the past. These stars burn hydrogen to helium, maintaining their composition unchanged for billions of years, making them valuable for understanding cosmic evolution.

> By analyzing the chemical composition of stars, we can trace back the history of element formation in the universe from the Big Bang to the creation of heavier elements like gold and platinum. Each supernova explosion adds more elements up to iron, while the heavier elements are produced through different processes like neutron-capture, revealing insights into past cosmic events.

> Old stars carry the signatures of multiple generations of stellar processes, with some potentially forming from gas enriched by various events like supernovae and neutron star mergers. Unraveling these signatures in ancient stars helps us piece together the complex events that shaped the cosmos billions of years ago.

Oldest stars

> "The beauty of old stars lies in the story they tell about the early universe. By analyzing their chemical signatures and kinematics, we can unravel their origins and the galactic archeology they represent. I love stars, but it's really about the narrative they help us construct."

> "Our discovery of stars with low strontium and barium abundances, moving retrograde at high velocities in the galaxy, indicates they are some of the oldest stars, predating the formation of the Milky Way. It’s fascinating how these 'chemically most pristine' stars reveal so much about cosmic history."

> "Creating a new class at MIT where students directly engage in research has been incredibly rewarding. By providing a structured framework and working with enthusiastic undergrads, we not only advanced the field but also made the research process more accessible and inclusive, showing that anyone can contribute to unraveling the universe’s mysteries."

Metal-poor stars

> The term "metal-poor stars" simplifies complex chemical compositions: "Let's call hydrogen X, helium Y, and all the other elements combined metals, Z."

> Discovering stars like HE 1327-2326 and HE 1523-0901 is pivotal for understanding early cosmic evolution: "They provide an important part to the story... Does that happen regularly? Is there something that we can learn?"

> Unraveling the mystery of iron-deficient stars led to the discovery of a new supernova mechanism: "We needed to concoct a theoretical supernova... because you can't take stuff away."

> Reflecting on carbon's role in cosmic evolution and human existence: "We need long lifetimes if you want to have a stable planet and develop humans... We're made of star stuff... It's wonderful to help contribute unraveling our cosmic history."

Neutron capture

> Two main insights resonate with me from this discussion. First, the fascinating old star HE 1523, with its 13.2 billion years of history, teaches us about the universe's early chemical makeup. The significant presence of heavy elements like thorium and uranium in this star is “really good when you want to explore the early universe.” These elements not only give us clues about stellar evolution but also lead to pivotal questions about their origins and the processes that create them.

> Second, the rapid neutron-capture process is a marvel of astrophysics. The idea that “in literally the snapping of my hand, it’s all there” emphasizes just how quickly these heavy, unstable nuclei can form. It’s amazing to think about the minute interactions that birth the very elements that make up our world today, and understanding this chaos provides perspective on the broader story of the cosmos.

Neutron stars

> When two neutron stars merge, they create a super neutron star with violent collisions, leading to a rich source of heavy elements. This process generates gravitational waves and an electromagnetic counterpart, demonstrating nuclear synthesis and the r-process in action.

> The discovery of ancient iron-deficient stars in the dwarf galaxy Reticulum II, showcasing a strong signature of heavy elements, challenged previous assumptions about low levels of heavy elements in early star formation. This finding suggests a neutron star merger in Reticulum II early on, influencing the composition of the galaxy's stars.

Dwarf galaxies

> Studying ultra-faint dwarf galaxies reveals ancient stars untouched since the early universe, providing valuable insights into stellar archeology. These galaxies lost their gas early on, halting star formation and preserving a unique snapshot of the past.

> While data on stars in the Milky Way is richer due to brightness and proximity, the environmental context from dwarf galaxies is crucial. It helps constrain processes like neutron star mergers needed for heavy element formation, highlighting the significance of studying both types of systems for a comprehensive understanding of stellar evolution.

Star observation

> The magic of observational astronomy: Spending nights on mountaintops with the Magellan Telescopes in Chile, using efficient spectrographs, is a significant part of my work and passion. The serenity of these remote locations, away from distractions, allows for deep contemplation and connection with the universe. Lying under the southern Milky Way, I often feel a profound sense of unity with the cosmos, understanding that we are all small parts of a greater whole.

> Spectrum analysis and discovering ancient stars: The process involves using spectrographs to split starlight into its wavelength components, recording absorption lines that indicate the presence of different elements. The strength and position of these lines help us determine the chemical composition of stars. This method, though complex, is like reading the tea leaves, allowing me to intuitively identify promising stars worth further investigation.

> Search strategies and breakthroughs: Discovering the oldest stars is akin to finding needles in a haystack, involving meticulous filtering techniques and patient data collection. The saying "One star is a discovery, two is a sample, and three is a population" encapsulates the challenge of proving findings. Developing new methods and technologies, such as using narrow-band imaging, has improved our chances of finding these elusive stars.

> Highs and lows of data collection: The excitement of discovering rare stars, like the extremely iron-poor star HE 1327, is often juxtaposed with the frustrations of false positives and poor weather conditions. These experiences, while challenging, are an integral part of the scientific journey, requiring resilience and patience to navigate the inevitable setbacks.

> Serendipity in science: The discovery of the first r-process galaxy was a testament to the role of luck in scientific breakthroughs. Despite poor weather and degraded data quality, the extreme enhancement of heavy elements in the stars allowed us to identify the galaxy. This serendipitous discovery highlighted the unpredictable nature of astronomy and the importance of persistence and attentive observation in making significant findings.

James Webb Space Telescope

> Observing metal-poor stars individually has provided unparalleled insights into the early universe's evolution by allowing for meticulous, question-driven data collection. Reflecting on this hands-on approach, I’ve found immense satisfaction in understanding each unique star and uncovering new patterns that shape our cosmic narrative.

> The future of stellar archaeology is promising, especially with upcoming large-scale spectroscopic surveys that will deliver unprecedented data. While I cherish the detailed knowledge gained from personal observations, embracing big data will revolutionize our understanding and enable new discoveries, albeit with cautious skepticism about the limitations of aggregated data.

Future of space observation

> The field of study around metal-poor stars is now about meticulously filling in the last intricate details rather than individual monumental discoveries. It's like assembling a thousand-piece puzzle where we have 900 pieces; our task is to methodically find those remaining pieces to complete the picture and truly understand the big picture and the fine details.

> While the individual groundbreaking discoveries are becoming less frequent, our focus has shifted to creating a comprehensive, airtight understanding of the universe. This entails probing deeper into the specificities of ancient stars, with an obsessive commitment to detail, enabling us to say, "we really know now." Such detailed exploration continues to unfold new doors and avenues for future investigation, even if they don't fit the traditional mold of a 'discovery.'

Age of the universe

> Reflecting on the philosophical edges of the Big Bang and the limits of physics and math, it's fascinating how our mathematical models can suggest things that don't quite make sense to us physically. For instance, our star age calculations can produce results like "15 billion years," which contradict our cosmological understanding. This interplay showcases both the power and the potential misinterpretations inherent in our models.

> Exploring the early universe through the study of ancient stars is incredibly rewarding. It's like navigating a cosmic time machine, even though the light I've been studying is just a few thousand years old as opposed to JWST's 13-billion-year-old light. These stars reveal crucial insights into the early chemical evolution of the universe, although they wouldn't have planets due to the lack of heavy elements required for their formation.

> Highlighting the contributions of women in astronomy has been a passion. The "Harvard Computers" significantly advanced our understanding despite many lacking formal education. Their dedication and intelligence led to groundbreaking discoveries, like Cecilia Payne-Gaposchkin asserting the hydrogen and helium composition of the sun, which revolutionized our understanding of stellar physics. This history underscores the importance of recognizing and crediting diverse contributions to science.

Most beautiful idea in astronomy

> One of the most beautiful ideas in stellar astronomy for me is the connection between old stars and the study of elements. It was a magical moment when I realized that studying old stars could help us understand element formation and fusion processes.

> Finding my passion in old stars and element creation felt like winning the lottery. It's an ongoing love story with the stars, where following my instincts and contributing to our cosmic knowledge brings me immense joy and fulfillment.

Advice for young people

> It's essential for young people to realize that "you can only have sort of one job or one type of profession" at a time, and diving deep into a single pursuit can lead to fulfilling experiences and expertise, rather than feeling overwhelmed by endless choices.

> I genuinely believe that "a fulfilling life is in part likely to be discovered in a singular pursuit of a thing," and it's important for the young generation to find something they can genuinely commit to, even if it feels like a risk – that commitment allows you to witness the magic of what you can create.

> The intersection of art and science is a powerful avenue to explore human experiences; creating a one-woman play about Lise Meitner has allowed me to express the beauty of discovery by illustrating "the moment of discovery," which resonates with everyone and transforms the often-obscured emotional journey of scientists into something relatable and meaningful.

Meaning of life

> The intricate interplay between physical, chemical, biological, and psychological evolution fascinates me. It's all interconnected, creating a seamless tapestry where life seems like a necessary consequence, making it a reproducible phenomenon rather than a product of mere chance. This interconnectedness is profoundly satisfying to a scientist in search of patterns and coherence in the cosmos.

> The notion of "turtles all the way down" resonates deeply with me as I study the universe's oldest stars, the foundational "turtles" on which the cosmos is built. These ancient stars offer invaluable insights into the origins and evolution of everything and remind me that understanding the past is crucial for making sense of our place in the universe.