By her own account, MIT Professor Anna Frebel has a very challenging job. She calls herself a “stellar archaeologist,” an astrophysicist who studies the earliest generations of stars in the universe. But how do you recognize those venerable oldsters among the billions and billions of stars that speckle the night sky? To the untrained eye, they all look like featureless points of light, and even through the largest telescopes they appear as unresolved dots—no gray beards or blue hairdos to signal their advanced years.
There is one characteristic the oldest stars share, however: they are all deﬁcient in the chemical elements heavier than helium, which astronomers lump together with the not-quite-descriptive term “metals”. The very earliest generations of stars, according to current understanding, condensed from the expanding gas of the Big Bang, a few hundred mil-lion years after the initial explosion, at which time the universe consisted almost entirely of hydrogen (about 75% by mass) and helium (25%). These ﬁrst pure H-He stars created heavier elements through nuclear reactions in their interiors, and, near the end of their lives, ejected these elements back into the interstellar gas from which new generations of stars could form. Over the 13.7 billion year history of the universe, the heavy element abundance of the later stellar generations gradually in-creased. This development has been is a good thing for astronomers and humans in general: virtually all the carbon, nitrogen, oxygen, and other elements essential for life were forged in the nuclear ovens of earlier stars.
The key to ﬁnding and studying ancient, metal-poor stars, then, is spectroscopy, the analysis of the wavelengths of starlight, to look for speciﬁc dips in intensity, called spectral lines that are the signatures of the chemical elements. Frebel looks for stars that are deﬁcient in key elements, notably iron, which means that she’s looking for the near absence of lines that are prominent in the spectra of younger stars, like the sun. Recognizing such faint spectral features among the myriad of stars, she laments, is “like searching for the needle in a haystack,” with “the haystack the entire halo of the Milky Way, and the needles…the rare metal-poor stars.” This process requires the world’s largest telescopes—in Australia, Chile, and Hawaii—and it takes enormous amounts of observing time. But she’s been notably successful: one of her discoveries, the star HE 1327-2326, had an iron abundance of only 1/250,000 the iron abundance of the sun.
Frebel’s narrative provides a rich picture of the understandings astronomers have gleaned from studies of the elements in stars. We are gradually developing a picture of how the elements evolved from the earliest moments of the Big Bang and how these elements were distributed through our Milky Way galaxy, eventually forming the solar system we inhabit today.
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