Investigating the early universe through “stellar archaeology”
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The Nature paper “A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36-670839.3” has inspired international media coverage. In the New York Times, science writer Curtis Brainard wrote a Science Times front-page-dominating commentary calling a relatively new astrophysical method “archaeology of the stars.”
The opening paragraphs from Space.com’s report introduce the news from Nature:
Astronomers have found what appears to be one of the oldest known stars in the universe.
The ancient star formed not long after the Big Bang 13.8 billion years ago, according to Australian National University scientists. The star . . . is located 6,000 light-years from Earth and formed from the remains of a primordial star that was 60 times more massive than the sun.
“This is the first time that we’ve been able to unambiguously say that we’ve found the chemical fingerprint of a first star,” lead scientist Stefan Keller, of the ANU Research School of Astronomy and Astrophysics, said in a statement. “This is one of the first steps in understanding what those first stars were like. What this star has enabled us to do is record the fingerprint of those first stars.”
A news report from The Australian—“Aussies turn universal thinking on its head”—narrates how Mike Bessell, who first spotted the star, and Keller proceeded just after the initial discovery. With colleagues, they used ANU’s SkyMapper telescope and then the Magellan telescopes at Las Campanas Observatory above Chile’s Atacama Desert.
Space.com quotes Keller on the discovery’s importance:
To make a star like our sun, you take the basic ingredients of hydrogen and helium from the Big Bang and add an enormous amount of iron—the equivalent of about 1,000 times the Earth’s mass. To make this ancient star, you need no more than an Australia-sized asteroid of iron and lots of carbon. It’s a very different recipe that tells us a lot about the nature of the first stars and how they died.
At the New York Times, the teaser blurb at the foot of the 11 February front page advertised Brainard’s science-section commentary in two ways:
* Anachronically and maybe misleadingly, but certainly enthusiastically, the blurb included a photograph not of either Magellan telescope at Las Campanas, but of the Giant Magellan Telescope envisioned there. (It’s shown as it’s planned to look in full operation in 2024, when—with all seven mirrors installed, and using adaptive optics—it’s expected to excel the Hubble Space Telescope in performance by a factor of 10, even though it is earthbound.)
* The blurb’s text emphasized the science-method analogy: “Archaeology of the stars: The evolution of the universe is being traced through its elements.”
In January, when Nature Publishing Group named Brainard blogs editor for Scientific American, he took the latest step in a career that has involved writing for internationally prominent publications and creating Columbia Journalism Review’s department for science and the media, The Observatory. So before using the reported research to promote the archaeology analogy, Brainard might well have known that the leading US contributor to the reported research, Anna Frebel of MIT, has been publicizing the analogy for at least two years.
In 2013, Harvard Magazine carried an article that began, “Some explore the origins of the universe by seeking to observe the most distant galaxies. . . . Anna Frebel instead practices ‘stellar archaeology.’”
Earlier, in April 2012 in Physics Today, Frebel and her fellow astronomer Volker Bromm, of the University of Texas, published “Precious fossils of the infant universe,” in which they pointed out that the “ancient, metal-poor stars at the outskirts of the Milky Way provide a window on the conditions that governed the universe shortly after the Big Bang.” Whether or not their figurative references to fossils mixed their metaphors by bringing in paleontology, they offered a brief explanation of the analogy of retrospective Earth science and astrophysics:
The traditional [astrophysical] approach is to probe the early cosmos directly, with in situ observations at high redshifts; today’s telescopes can see back in time to about 1 billion years after the Big Bang. . . . A second approach, stellar archaeology, explores the ancient past by scrutinizing fossil stars located at the outskirts of the Milky Way. The chemical surface composition of those roughly 13-billion-year-old stars reveals the conditions in the early universe just a few hundred million years after the Big Bang and before the emergence of our galaxy. The fossils contain only traces of what astronomers call metals—elements heavier than hydrogen, helium, and lithium—produced in stars and supernova explosions. The chemical fingerprints of those old, metal-poor stars provide unique constraints on the nucleosynthesis inside the first stars and supernovae and on the chemical evolution of the Milky Way. In contrast to the traditional far-field approach that relies on high-redshift studies, stellar archaeology is a near-field cosmology, enabled through the study of local stars.
Brainard’s Times commentary carries the subhead “A new breed of astronomers traces the universe’s evolution through its elements.” It explains that for a decade, Frebel and others have been using powerful telescopes and high-resolution spectroscopy to investigate the chemical compositions of older, relatively close stars, yielding information about the creation of elements and about how the first stars and galaxies formed.
Then comes the archaeology analogy:
These astronomers are like Egyptologists combing the desert for relics of bygone civilizations, and call themselves stellar archaeologists. Their work relies on the fact that the rare, primordial stars they are looking for have very few atoms heavier than hydrogen and helium, the gases from which they came together. By contrast, our sun and other relatively young stars are rich in other elements, which astronomers collectively refer to as metals.
Now Frebel and others have investigated this particularly iron-poor star. Brainard quotes her concerning looking at stars’ chemical compositions “to answer questions like ‘How massive were the first stars? How many were there? How and where were the elements produced? How did they explode? And how did the first low-mass stars form?’”
Metal-poor “remnants of the early universe,” Brainard observes at the end, “are uncommon and hard to find, but stellar archaeology is a young field, and new artifacts will undoubtedly come to light.”
Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA's history program, and is a science writer at a particle-accelerator laboratory.