Volume XXVII Number 1
Photo © Tyagan Miller
Star cluster NGC 7789
What Makes Stars Tick
Constantine Deliyannis is excited.
Apologizing for his distraction as he hangs up his office phone, he explains that a colleague at another university has discovered what may be one of the universe's oldest stars.
The star could be 11 billion years old or more--very close, astronomically speaking, to the universe's age of about 14 billion years. It's a pristine star, according to Deliyannis, meaning it's very "metal poor."
During the first minutes after the Big Bang, as the universe was cooling down, the light elements hydrogen and helium were synthesized, along with a little bit of lithium, the lightest of all solid elements. Nearly all other heavy elements were created inside stars, then released when those stars exploded or lost their mass by other means. The less metals a star contains, then, the closer its birth was to that of the primordial universe.
Previously, the most metal-poor star known contained only 1/10,000th of the amount of metals found in our Sun (which contains a relatively large 2 percent of metals). The new metal-poor star Deliyannis is talking about could contain far less.
Although another astronomer discovered the star, Indiana University scientists played a role. The star's discoverer needed more data, but didn't have telescope time scheduled. He contacted Deliyannis, who asked his IU colleagues working at the WIYN (Wisconsin-Indiana-Yale-NOAO) 3.5-meter telescope in Arizona to use some of their time to collect data. They did; then a recent student of Deliyannis's, now a postdoc in Florida, worked with the data. After that, Deliyannis transferred the results.
Deliyannis relishes this kind of cooperation--"I like synergies, interactions, talking to each other," he says. He has been an active collaborator since his days as a graduate student at Yale and a postdoc in Hawaii. But his very first collaborator may have been his Greek grandmother. "When I sat on her knee," he recalls, "she read astronomy books to me."
Deliyannis's early fascination with space eventually led him to IU Bloomington in 1997, where he is currently an associate professor of astronomy and a specialist in stellar evolution. In Deliyannis's words, he studies "what makes stars tick."
With broader interests in cosmology and Big Bang theory, Deliyannis co-founded a project at IUB that engages his enthusiasms. It's called the WIYN Open Cluster Study, or WOCS (rhymes with "rocks").
Open clusters are groups of stars that formed at the same time, meaning the stars in the cluster are approximately the same age and initially had similar chemical compositions. Some open clusters are familiar to stargazers, such as Ursa Major (the Big Dipper) and the Pleiades (Seven Sisters). Because of their stars' similarities, open clusters offer well-defined samples that make "fantastic laboratories for studying stars," says Deliyannis.
The problem is, only a handful of clusters have been thoroughly researched. One of WOCS' major goals is to establish a much larger, more comprehensive database of open clusters and their fundamental properties. "We're looking at open clusters so we can provide detailed observations for use in subsequent studies," says Deliyannis. "We do photometric measurements to reveal a cluster's overall composition, distance, and age."
A lot of WOCS photometry (the measurement of light intensity) is done by IU graduate and undergraduate students as well as undergraduates who come to IU as part of the National Science Foundation's Research Experiences for Undergraduates program--another collaboration Deliyannis calls "delightful." They use the .9-meter telescope at WIYN, one of two instruments WIYN operates at the Kitt Peak National Observatory (KPNO) in Arizona. When the KPNO's priorities shifted toward larger telescopes, Deliyannis saw a "golden opportunity," he says, for the smaller .9-meter instrument to complement the larger WIYN telescope. WIYN took on the .9-meter's operations, and today, IU astronomy students use the refurbished telescope to master fundamental techniques.
While WOCS is a joint effort to provide good data for the astronomy community, the project's second major goal is to tackle specific problems in astronomy. For Deliyannis, one of those problems involves stellar interiors, or as he puts it, "the good stuff going on inside stars."
He's particularly interested in the stuff of lithium. We may associate it with today's rechargeable batteries, but the No. 3 element on the periodic table showed up in the earliest moments of the universe. Stars have formed out of matter containing lithium ever since. Deliyannis uses the element's presence in stars as his "probe" to look inside.
Stars differ in how much lithium they contain. For example, the Sun has only about 1 percent of its original lithium. Other stars have preserved more. To understand what causes these variations, Deliyannis has used the world's largest and most expensive telescopes--the Keck I 10-meter and the Hubble Space Telescope--to trace lithium in the spectra of stars. At WIYN, Deliyannis takes advantage of the Hydra spectrograph that can observe up to 90 objects at the same time, making it especially useful for looking at stars in open clusters. "The Hydra makes the WIYN 3.5-meter more powerful than the Keck in some respects," says Deliyannis.
So far, his research has led him to conclude that stars lose their lithium mainly through slow mixing caused by stellar rotation, but also, possibly, through the action of diffusion. Models suggest that rotation causes star interiors to mix in ways that wouldn't be possible if the stars didn't turn, according to Deliyannis. Diffusion means elements get separated--heavier elements sink, for example.
Combining rotation theory ("real stars rotate," Deliyannis says with a grin) and diffusion theory to study lithium abundance can help to fix the age of a star or star cluster and determine overall metallicity--and those things help astronomers determine if Big Bang theory is on the right track. Does the amount of lithium in a metal-poor star we see today match the amount that would have been "locked in" when the star formed soon after the universe's birth? If not, why not? "These are the kinds of mysteries scientists love to solve," says Deliyannis.
Studying the lithium in other stars can also expand researchers' understanding of our Sun and the universe beyond it. Because our solar system's central star has been widely studied, its properties are well known. Do other stars match up? In other words, how normal is the Sun?
By comparing the Sun's properties with open clusters of a similar age--such as the M67 cluster that Deliyannis has studied--scientists can get closer to determining whether stars like the Sun are common or not. If the Sun is "pretty darn typical," as Deliyannis speculates, then it's possible the life our Sun sustains occurs elsewhere too.
Someday, maybe sooner than we imagine, Deliyannis says, "we're going to realize that the universe is far more than we know today. I think it's fascinating to learn about the rest of the universe, which our species is about to break into."
Lauren J. Bryant is editor of Research & Creative Activity magazine.