Stellar archaeologist has turned up stars from the second generation ever to form
Anna Frebel, 36
Massachusetts Institute of Technology
Anna Frebel can’t explain her fascination with the stars. It’d be like explaining why “berry purple-pink” is one of her favorite colors. “They are just a part of me,” says Frebel, an astronomer at MIT. “What’s going on with them and what they can tell us — there is something magical.”
Frebel’s fascination has led to the discovery of at least three record-breaking stars. Dating back roughly 13 billion years, the stars — all within the Milky Way galaxy — might be elders from the second generation of stars ever formed in the universe. She has also found that one of the tiny galaxies flitting around outside the Milky Way might be a fossil that has survived from not long after the Big Bang. The light from these ancient relics encodes stories about the birth of the first stars, the assembling of galaxies and the origin of elements essential to creating planets and life as we know it.
“Anna has a really good track record of finding these amazing things,” says Alexander Ji, one of the three graduate students Frebel mentors at MIT. “She’s always finding things that change our understanding of the universe.”
[Stars] are just a part of me. What’s going on with them and what they can tell us — there is something magical.
— Anna Frebel
As a young girl living in Germany, Frebel wanted to be an astronaut, but she passed on that dream when she learned about the centrifuge that whips trainees around to simulate launch acceleration. Not for her. She instead studied physics and astronomy, first at the University of Freiburg in Germany and then at the Australian National University in Canberra. Since then, Frebel, now 36, has earned a reputation as a “stellar archaeologist,” with the patience and perseverance to search through the universe’s most ancient debris.
Only someone with a galaxy’s worth of patience could sift through the tiny rainbows of light, the spectra, produced by thousands of stars, handpicking the specimens that might preserve clues to the conditions shortly after the first stars lit up the universe. And only a persistent person would spend more than two years pointing Australia’s 2.3-meter-wide Advanced Technology Telescope at 1,200 of the most promising candidates (“105 stars per night was my record,” she says) and eventually, with observations from other telescopes too, land on one star that was, for a while, among the oldest known.
She was first drawn to this research after hearing astronomer Norbert Christlieb, then a visiting researcher at the Australian National University, talk about his work on old stars. “It hit me: Oh my God, this project combines all my interests,” Frebel says. There were stars, chemistry, nuclear physics and the periodic table. “There are so many, for me, cool topics that come together.”
In combing through her stars, Frebel was looking for ones that contained hydrogen and helium — but little else. Most heavier elements up to iron are forged in the cores of stars, where atomic nuclei smash together. As the universe aged, its inventory of atoms such as carbon, silicon and iron steadily increased. The earliest stars, however, came on the scene when there were far fewer of these pollutants floating around.
Her efforts paid off in 2005 with a star branded HE 1327-2326, reported in Nature as the most pristine star known at the time. “She found one that took us closer back to the beginning of time as we know it,” says Frebel’s Ph.D. adviser, astronomer John Norris of Australian National. “It became clear to us early on that she was quite gifted.”
Her gifts netted her the Charlene Heisler Prize in 2007, given by the Astronomical Society of Australia for outstanding Ph.D. thesis. She has since won several recognitions, including the Annie Jump Cannon Award in 2010, given to notable young female researchers by the American Astronomical Society, for her “pioneering work in advancing our understanding of the earliest epochs of the Milky Way galaxy through the study of its oldest stars.”
The geriatric stars that Frebel finds are not perfectly pristine; they preserve in their atmospheres the chemical makeup of interstellar gas that had been seeded with a smidgen of heavy elements from the explosions of stars that came before. Chemical abundances in many of these stellar fossils are out of balance compared with modern stars. The fossil stars have much more carbon relative to iron, for example — carbon that had to have come from the debris of that very first crop of stars.
Frebel worked with theorists to show that excess carbon could have allowed successive generations of stars (and planets) to form, reporting the work in 2007 in Monthly Notices of the Royal Astronomical Society Letters. “I’ve always been interested in understanding the main message of the data,” she says, which leads her away from the telescope to computer simulations and theory. In this case, the message is that carbon “might have been the most important element in the universe.”
Gas needs to be cold, around –270° Celsius, just a few degrees above absolute zero, to clump and form stars. And carbon is an excellent coolant; its electrons are arranged in such a way to let it efficiently radiate energy. The first generation of stars didn’t have carbon’s help. They were probably slow to form and ended up as gargantuan fluffy orbs hundreds of times as massive as the sun. But once those stars exploded and seeded the cosmos with carbon, Frebel’s data suggest, subsequent generations of stars formed that would have looked more like the stars we see today.
Frebel likens her studies to watching her young son learn to walk and talk. “My overall interpretation is that the universe was still trialing things.”
Before she became a parent, she regularly went to one of the twin Magellan telescopes, 2,380 meters above sea level in the Chilean Atacama Desert. On long nights, while waiting for the telescope to soak up light from a star tens of thousands of light-years away, Frebel would feel the pull of the night sky. “I just lie on the ground and stare into the sky and get lost in the universe,” she says.
In recent years, Frebel has expanded her repertoire to include a horde of teeny galaxies that orbit the Milky Way and also serve as archaeological sites. “Now we can use not just one star,” she says. “We can use the entire galaxy as a fossil record.” One of these runts, called Segue 1, appears to be a remnant from the cosmic dawn and might be typical of the pieces that assembled into large galaxies like the Milky Way.
Frebel and her student Ji discovered that another dwarf galaxy, dubbed Reticulum II, contains clues about one of the mechanisms responsible for creating most of the elements heavier than iron. A long-ago smashup between two neutron stars once bombarded the gas in Reticulum II with neutrons, producing atoms, such as uranium, that can’t be formed in stellar cores. Similar run-ins in other galaxies might have helped build up the universe’s stockpile of heavy elements.
Frebel plans to continue her quest to understand the origin of atoms, stars and galaxies. Though the celestial bodies she studies are ancient, “my days never get old,” she says.
A.P. Ji et al.R-process enrichment from a single event in an ancient dwarf galaxy. Nature. Vol. 531, March 31, 2016, p. 610. doi: 10.1038/nature17425.
A. Frebel and J.E. Norris. Near-field cosmology with extremely metal-poor stars. Annual Review of Astronomy and Astrophysics. Vol. 53, August 2015, p. 631. doi: 10.1146/annurev-astro-082214-122423.
A. Frebel, J.D. Simon and E.N. Kirby. Segue 1: an unevolved fossil galaxy from the early universe. Astrophysical Journal. Vol. 786, April 26, 2014, p. 74. doi: 10.1088/0004-637X/786/1/74.
S.C. Keller et al. A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36-670839.3. Nature. Vol. 506, February 27, 2014, p. 463. doi: 10.1038/nature12990.
A. Frebel and V. Bromm. Chemical signatures of the first galaxies: criteria for one-shot enrichment. Astrophysical Journal. Vol. 759, October 26, 2012, p. 115. doi: 10.1088/0004-637X/759/2/115
A. Frebel, J.L. Johnson and V. Bromm. Probing the formation of the first low-mass stars with stellar archaeology. Monthly Notices of the Royal Astronomical Society Letters. Vol. 380, September 1, 2007, p. L40. doi: 10.1111/j.1745-3933.2007.00344.x.
A. Frebel et al. Discovery of HE 1523-0901, a strongly r-process-enhanced metal-poor star with detected uranium. Astrophysical Journal Letters. Vol. 660, May 10, 2007, p. L117. doi: 10.1086/518122.
A. Frebel et al. Nucleosynthetic signatures of the first stars. Nature. Vol. 434, April 14, 2005, p. 871. doi:10.1038/nature03455.
E. Conover. Ancient dwarf galaxy was heavy-element factory. Science News. Vol. 189, May 14, 2016, p. 9.
C. Samoray. Early stars found swirling in the Milky Way center. Science News Online, November 11, 2015.
C. Crockett. First stars may lurk in our galactic neighborhood. Science News. Vol. 188, November 14, 2015, p. 12.
R. Cowen. Cosmic primitive: Old star sheds light on early stellar formation. Science News. Vol. 167, April 16, 2005, p. 244.