Anna Frebel studies stars residing of the Milky Way, visible here above Chile’s Las Campanas Observatory.
PHOTO: YURI BELETSKY
Equipped with two 6.5-meter, single-mirror Magellan telescopes, the remote facility offers astronomers splendid and unobstructed views of the star-rich southern sky. Which is what draws Anna Frebel and her team of graduate students and postdocs to travel there several times a year from MIT; the Institute is a partner in the observatory.
“In the evening, after dinner, our whole group meets on the catwalk at the telescope to watch the sunset,” says Frebel, the Silverman Family Career Development Assistant Professor in MIT’s Department of Physics, and a member of the Kavli Institute for Astrophysics and Space Research. “It’s a beautiful ritual. Then, once the sun goes down, we get to work observing the stars until morning.”
Anna Frebel is a leading light—one is tempted to say a rising star—in a new generation of astronomers who call themselves “stellar archeologists.” Frebel studies metal-poor stars—stars whose percentage of heavy elements, known in astronomy as “metals,” is significantly lower than that of our sun. Metal-poor stars are some of the oldest stars in existence, and contain precious information about the early universe and the origins of the matter that composes the universe. Like scientists sifting through ruins of past civilizations to understand our human past, stellar archeologists analyze these ancient stars to sketch the history of the universe.
“Every star in the universe contains a record of the elements in the gas cloud out of which it was formed,” says Frebel, who joined the MIT faculty in 2012. “When we analyze the contents of these metal-poor stars, which were formed in the first billion years after the Big Bang, we’re unearthing a record of a significant period in the early universe. And from this record, we can begin to reconstruct how our universe took shape, how galaxies formed, and how nature, in this amazing way, creates the elements that compose the stars, the planets, and even our bodies.”
As Frebel explains in her recently published book, Searching for the Oldest Stars: Ancient Relics from the Early Universe, the first stars emerged out of giant clouds of gas approximately 13.5 billion years ago, a few hundred million years after the Big Bang. Composed only of hydrogen and helium—with trace quantities of lithium—these first stars were massive and not particularly dense. They burned for a scant few million years, generating energy by fusing hydrogen and helium into heavier elements such as carbon and oxygen in their cores. Then they exploded as supernovae, spewing these heavier elements into the interstellar medium. With their superior cooling qualities, these newer elements enabled the formation of smaller, denser stars—of the kind Frebel and colleagues are studying—with lifespans measuring into billions of years. The ongoing nuclear reactions in the cores of these stars produced increasing amounts of elements heavier than hydrogen and helium.
With the exception of hydrogen and helium, which were formed by the Big Bang, all of the elements in our universe were synthesized by stars—including iron, one of the principal components of our planet, and carbon, the basis of all life on earth. (The formation of elements heavier than iron requires either an extraordinary stellar event, such as a supernova explosion or neutron star merger, or a particular late stage in the life of lower-mass stars.) The production of elements continues. When our sun was born some 4.6 billion years ago, heavy elements constituted about 1.5% of the matter in the universe. Today they account for close to 2%.
The goal of stellar archeologists is to answer fundamental questions about the origins and evolution of the universe. What elements were present 300 million years after the Big Bang? At what point were there sufficient quantities of heavy elements to form galaxies and planets? Where does the carbon that forms the basis for life on Earth originate? “By analyzing the different chemical and physical processes involved in this evolution,” writes Frebel in her book, “astronomers can inch their way closer to understanding the nature of the whole universe.”
Frebel’s precise, painstaking quest for knowledge begins with a thorough examination of large sky surveys—photographic and photometric surveys of large swaths of the night sky, like the Sloan Digital Sky Survey or the Hamburg/ESO Survey—to identify potential objects of interest. Once identified, these objects are reexamined with medium-resolution spectroscopy, and, if still of interest, through high-resolution spectroscopy. Spectroscopy analyzes the light that emanates from a star’s core and passes through its outer shell, where most of the star’s metals reside. Each heavy element on the outer shell absorbs a specific wavelength of light. The light that emerges produces a spectrum much like a chemical fingerprint that identifies each element present in the star. The spectra appear on screens during observation times at Las Campanas.
Each new metal-poor star constitutes a piece of the puzzle scientists like Frebel are trying to put together. “It’s amazing that we can even begin to gather evidence to try and answer these fundamental questions about the universe,” says Ani Chiti, a second-year graduate student who came to MIT specifically to work with Frebel. “And it’s mind-boggling that we try to answer them by staring at tiny points of light in the sky.”
Educated both in Germany—where she was born in 1980—and in Australia, Frebel first drew international attention with her PhD thesis, which she completed in 2006 at the Mount Stromlo Observatory at the Australian National University (ANU). Sifting through data from close to 2,000 stars, Frebel identified two immensely significant objects: HE1523-0901, which is one of the earliest stars ever observed and contains significant quantities of thorium and uranium; and HE1327-2326, the most chemically primitive object observed to date, with an iron content more than 100,000 times lower than the sun’s. The thesis was awarded the Charlene Heisler Prize by the Astronomical Society of Australia.
“It’s true that Anna was perhaps more lucky than we might have dared to hope,” says John Norris, her PhD supervisor at ANU, and now a close colleague. “But what was truly impressive was the manner in which she took the opportunity these objects presented to obtain high-resolution spectra and unlock their secrets.”
“She has a clear idea of what questions are important, and an uncanny ability to synthesize the answers,” says Alex Ji, a fourth-year graduate student in Frebel’s group, who cites his mentor’s contagious enthusiasm as well as her “superhuman” focus. “I don’t think I would have been interested in stars were it not for her.”
In addition to her research in metal-poor stars, Frebel and her group are investigating ultrafaint dwarf galaxies, which can yield information about how the Milky Way galaxy and its extended halo were formed. “Stars tend to be formed in these ultrafaint dwarf galaxies,” Frebel explains. “They then ride these small galaxies like a gondola into larger galaxies, where they are disbursed and absorbed.” Frebel and her group have also created a large supercomputing project, Caterpillar, in collaboration with MIT physics professor Mark Vogelsberger, to simulate this galactic assembly process.
Frebel says she always knew astronomy was what she was born to do. “As humans, we have a strong urge to bridge heaven and earth, to connect the dots around us. I have a two-year-old son, and I’m waiting for the day very soon when he starts to ask ‘why’ about the things he sees around him. My work is just the grown-up version of his ‘why’.”