The universe created in the big bang consisted mostly of hydrogen, with a significant though much smaller quantity of helium, about 24% by mass, and some traces of lithium. All of the other elements essential to life -- carbon, oxygen, and nitrogen, not to mention the other approximately 100 known elements -- were made much later, in stars. But these other elements did not appear all at once, even within a single galaxy like our Milky Way. The stars that formed them, and the later generations of stars that further reprocessed them, lie in clusters of activity around the galaxy, each with its own life history. As a result, the relative abundances of the elements varies from place to place, and from star to star. The consequences are important: many (although by no means all) astronomers think that life as it exists on Earth, and that relies on a rich mix of all the elements, could not evolve in other places in the Milky Way where those ingredients are deficient. Measuring the relative abundances of the elements around the galaxy and in other galaxies is one of the important tasks that astronomers undertake in order to understand both the history of each location, and also its current, essential character.
A CfA graduate student, Joel Hartman, has joined with four colleagues from other institutions to develop a new way to measure the relative amount of elemental abundances. Their technique uses the pulsating behavior of a class of stars called Cepheid variables, stars whose masses and ages put them into an evolutionary phase in which their brightness varies every few days to weeks with great regularity. It turns out that one subclass of Cepheids pulses with a complex period that depends sensitively on the quantity of heavier elements in the star's atmosphere, thereby allowing a determination of the elemental abundances. Using a new survey of variable stars in a nearby galaxy, the team identified (from a set of 3023 periodic stars whose character marked them as being Cepheids) a group of five that were in this subclass. The team then used them to measure the variation in the abundances of elements in five locations across the galaxy. The results, which are independent of traditional techniques that measure abundances from the elements' spectroscopic signatures, offer a powerful new way to probe the enrichment of the cosmos.