THE STUDY “Discovery of HE 1523-0901, a Strongly r-Process-Enhanced Metal-Poor Star With Detected Uranium,” by Anna Frebel et al., The Astrophysical Journal, May 10, 2007.
THE MOTIVE How do you determine the age of an ancient star? For distant galaxies, astronomers measure redshift—the stretching of light that indicates how fast the stars are receding and therefore how old they are relative to the age of the universe. But this doesn’t work for an ancient star nearby. So Anna Frebel of the McDonald Observatory at the University of Texas at Austin looked for chemical clues. She reckoned that a rare form of old “metal poor” star, one with one-thousandth the iron content of our own young sun, carries an internal clock, one composed of the radioactive elements uranium and thorium. Because these metals decay at a steady rate, an astute observer can extrapolate backward and pinpoint the star’s moment of birth. Frebel has found one such star in our own Milky Way and dated its birth to 13.2 billion years ago—barely 500 million years after the universe itself was born.
THE METHODS The older a star is, the fewer metals it contains. The first stars, which formed a few hundred million years after the Big Bang, were composed of only hydrogen, helium, and traces of lithium. Some tens of millions of years after their birth, these massive, puffy stars exploded as supernovas, and new heavy elements were born in their fiery depths. Frebel first hunted for their offspring, an old star with a chemical fingerprint that could be dated: 74 percent hydrogen, 25 percent helium, smidgens of uranium and thorium inherited from a parent supernova, and very little iron—a relatively light element that accumulated later in history as the universe evolved and that would obscure any signal from the radioactive components. To detect uranium and thorium, Frebel could measure the strength of their absorption lines in a spectrum—in other words, calculate how much light each element absorbs at a particular wavelength. Frebel used the Clay Magellan Telescope in the Chilean Andes to search the halo of the Milky Way—its outer reaches, where old stars lurk—and turned up a bright red giant about eight-tenths the mass of our sun, dubbed HE 1523-0901, that appeared to meet all the requirements.
Frebel then turned to the European Southern Observatory’s Very Large Telescope in northern Chile. The VLT incorporates the Ultraviolet-Visual Echelle Spectrograph, or UVES, which acts like a supersensitive prism, splitting light collected by the telescope into a rainbow. Staff at the VLT collected the star’s spectra, from which Frebel calculated its uranium and thorium content.
Finally, Frebel compared her results with theoretical models that compute the quantities of heavy metals in stars at the moment of their formation. These programs model the process of nuclear synthesis in supernovas and infer how much uranium and thorium would be present at the birth of new stars. By subtracting her own figures on the star’s current heavy-element content from these theoretical estimates, Frebel determined how much radioactive decay had occurred and, therefore, established its age. HE 1523-0901 “is certainly among the oldest stars,” Frebel says, “and was probably formed as the second or third generation of stars.”
THE MEANING “Recently the Wilkinson Microwave Anisotropy Probe measured the age of the universe to be 13.7 billion years from the cosmic microwave background,” Frebel says. “The actual age measurement of a star provides an independent lower limit of the age of the universe.”