The Stellar Archaeologist
This past February, Anna Frebel and her colleagues announced the discovery of what may be the oldest known star. On a clear night, even an amateur astronomer might be able to train a strong telescope on SMSS J031300.36-670839.3, located in the halo of the Milky Way.
SM0313—the shorthand by which it’s known—has an unusual chemical signature. While it likely originated in an early dwarf galaxy that formed soon after the Big Bang, it does not share any characteristics with stars present in the dwarf galaxies that have been discovered so far. This fact has astronomers puzzled. “The Milky Way, in all likelihood, is gobbling up dwarf galaxies, incorporating their stars into its own halo,” says Frebel, a member of the MIT Kavli Institute for Astrophysics and Space Research and an assistant professor of physics in the Astrophysics Division.
Frebel, 34, belongs to a new breed of astronomers who can be thought of as stellar archaeologists. Instead of sifting through layers of dirt for artifacts of past civilizations, they mine the sky for light from ancient stars. By studying that light, they determine the chemical composition of those stars and piece together how our present galaxy and the elements in the periodic table came to be. Finding a star as primitive as SM0313 adds an important chapter to the story of how the galaxy, the universe, and life itself unfolded.
A few hundred million years after the Big Bang, the first stars formed from gaseous clouds made of the two lightest elements, hydrogen and helium, with traces of lithium. Over time, the nuclei of these lightweight elements fused together in the fiery cores of those primordial stars, creating heavier elements like carbon, oxygen, and iron. When the stars died, exploding as supernovae, they expelled these new elements into space, enriching gas clouds from which new, more chemically diverse stars were born. As subsequent generations of stars were created, they became increasingly rich in metals, as astronomers call all elements besides hydrogen and helium.
Frebel says that ancient metal-poor stars strewn among the few hundred billion younger stars in our galaxy are simply waiting to be found, like “old cans of beans in the back of the cupboard.” She analyzes them with a telescopic attachment known as a spectrograph, which splits the star’s light into a rainbow. Because every element absorbs light at a particular wavelength, she can deduce which elements are present in the star’s atmosphere by observing the precise shades that are missing from the spectrum.
Determining the age of a star, however, is tricky. Very rarely, an ancient star contains a radioactive element such as uranium, which allows astronomers to use a technique similar to carbon dating. In 2007, Frebel used that method to calculate that a star known as HE 1523-0901 was roughly 13.2 billion years old. She and her colleagues are quite sure it’s from one of the earliest generations of stars, because the universe itself, at about 13.8 billion years, is only slightly older. But for stars without radioactive elements, the rule of thumb is that the fewer the elements, the older it is. Because the researchers could derive the abundance of only four elements in SM0313, Frebel says it “most probably” belongs to the second generation. (They were also surprised by the relative abundance of carbon in such an ancient star, which suggests that the element was critical not just for the emergence of life but in the early universe as well.)
What has Frebel convinced that it may be the oldest known star is that it seems to be devoid of iron. In the metallicity scale astronomers use, −1.0 means a star has one-tenth the iron content of our sun, −2.0 is one-hundredth, and so on. In 2002, astronomer Norbert Christlieb reported the discovery of a star whose metallicity was −5.2; before she defended her thesis in 2006, Frebel found a star that came in at −5.4. With the discovery of SM0313, which has no detectable iron at all, Frebel and her colleagues set a new record at −7.1—or possibly even less because only an upper limit could be determined.
Frebel began her quest as an undergraduate physics major. She took a year off from studies in her native Germany to do astronomy research in Australia, where she got views of the sky that only the Southern Hemisphere can provide. While there, she met Christlieb, then of the University of Hamburg, shortly after he’d discovered the −5.2 star. “I quickly realized that Anna is not afraid of exploring unknown territory and that she is not a person who easily gives up,” he recalls.
After earning a PhD from the Australian National University and completing postdocs at the University of Texas and the Harvard-Smithsonian Center for Astrophysics, Frebel joined the MIT faculty in 2012. A major draw was the Institute’s access to some of the world’s most powerful telescopes. Last year, she spent 10 nights gathering data with the twin Magellan telescopes at Las Campanas observatory in Chile. (She’s among those offering input into the spectrograph of the new Giant Magellan Telescope, which will have six times the light-gathering capacity of today’s largest telescopes.)
Despite the long journey to this remote observatory in the Atacama Desert, she says she always feels rejuvenated by the diamond-flecked skies. All night her eyes are glued to the computer that is recording the spectral data. With instrument time so precious, Frebel has to balance her observations so she can get enough data on all her target stars. Dwarf galaxies are so dim that it takes close to 10 hours to collect enough photons, she explains; bright halo stars don’t require as much exposure. When the weather doesn’t coöperate, she checks e-mail or indulges in her hobby of astrophotography, since cloudy skies make for good pictures. She analyzes the data only after she gets home.
Growing up in the small town of Göttingen, Frebel could always see the stars on clear nights—not so easy in Boston, with its “orange skies.” Her romance with stars began early, but as a teenager, she seriously considered being a fashion designer. Though she had no formal training, she designed and sewed many of her clothes, often duplicating fancy outfits she liked or dreaming up her own. Ultimately, astronomy won because, she says, “I loved it more. I could keep fashion as a hobby.” Now, instead of working with her mind and hands to re-create the pattern of a dress, she imagines how the early universe might have looked and gathers data to test her idea. “I think my dressmaking provided me with a training tool to develop ideas and then rigorously pursue them,” she says.
Frebel is eager to share those ideas with the public; Princeton University Press plans to publish her German popular science book, Searching for the Oldest Stars, in translation. And she happily works with astronomers of all stripes. Until recently, theorists and observers in the “first stars” field have “largely been working without much contact,” says Volker Bromm, a University of Texas astronomy professor and a collaborator since Frebel’s postdoc days. “Anna is unique among observers in her desire to interact with theorists, embed her observations in the big-picture theoretical context, and guide them to relevant empirical predictions.”
Stellar archaeologists’ observations are certainly shaking up theoretical assumptions, such as the notion that all early stars blew up in massive explosions that spewed heavy elements like iron. Frebel and her colleagues believe that the progenitor of SM0313 must have gone out as a subdued supernova. Only a low-energy explosion would yield lighter elements like carbon but fail to expel heavier elements like iron made in the original star’s core. Astronomers seek a dozen other second-generation stars to bolster this new idea.
In May, Frebel and her team published their discovery of the least chemically evolved galaxy found so far—Segue 1, located 75,000 light-years from Earth. With only about a thousand stars, all metal-poor, this “wimpy” galaxy may give us a clearer glimpse into the conditions of the early universe. But further study of its dim and distant stars will take some doing.
Still, our telescopes are only getting better. The odds of finding the ancients both near and far—and unlocking their secrets—seem to be in Frebel’s favor.
Editors note: Originally it was reported that a star known as SM0313, which Anna Frebel and colleagues discovered in the halo of the Milky Way, supports the theory that our galaxy has been growing by feasting on its neighbors. While other evidence suggests that the Milky Way is doing just that, SM0313’s unusual chemical signature is unlike that of stars in nearby dwarf galaxies discovered so far, making this star unsuitable for investigating the Milky Way’s evolution.
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