Volume XXVII Number 1
Haldan Cohn, left, and Phyllis Lugger
Photo © Tyagan Miller
The WIYN 3.5 meter telescope
Photo NOAO/AURA/NSF, © WIYN Consortium, Inc., all rights reserved
Of Globulars and GRAPEs
Seventeen years ago, when this magazine first described the work of Haldan Cohn and Phyllis Lugger,they were studying globular star clusters (which they describe as "beehives" of stars) using the most advanced technology then available, including a low-speed pre-Internet entity known as ARPANET.
Today, Cohn and Lugger, both professors of astronomy at Indiana University Bloomington, are grateful beneficiaries of the worldwide boom in scientific computing. The extraordinary development of the Internet has revolutionized their research, enabling the astronomers to quickly download data to IU's mass storage facility and analyze these data on high-speed graphics workstations in the Astronomy Department. Lugger and Cohn now make use of a powerful suite of new tools, including the Hubble Space Telescope, the Chandra X-ray Observatory, the 3.5-meter WIYN (Wisconsin-Indiana-Yale-NOAO) telescope, and a specialized supercomputer at IUB called the GRAPE 6. Their theoretical simulations, once constrained by "fast" computers that worked at a maximum of one million instructions per second, now flourish on the GRAPE 6, which runs at nearly one trillion floating-point operations per second (one teraflop).
It's a far cry from long nights spent at faraway telescopes such as the Cerro Tololo Inter-American Observatory in Chile, which was just establishing e-mail connections with the rest of the world two decades ago. Today, they routinely use telescopes such as WIYN, near Tucson, Ariz., remotely via the Internet. Thanks to the Internet's present high-speed capability, Lugger, Cohn, and graduate student Allen Rogel now send commands to WIYN from their offices in Swain Hall and view the data being obtained in real-time.
Using the Hubble Telescope, Lugger and Cohn can see most of the individual stars within globular clusters and look into the crowded central regions. They also study spatial distributions and motions resulting from gravitational interactions with other stars. Due to the worldwide demand for use of the renowned Hubble--which was scheduled to be eliminated by NASA in 2007 until an outcry from the astronomy community caused the space agency to reconsider-- scientists must compete for its use. Although only one out of every five proposals to use the Hubble is approved, Lugger, Cohn, and their collaborators at other universities have been granted time nearly every year since 1990. Unlike most projects, which utilize a single block of time, their current project--measuring the motions of stars in a particular cluster--acquires data on a monthly basis.
"We are simply taking a picture of a star field, then taking the same field some time later," Cohn explains. "We're looking for slight shifts in the position of the stars in the sky. With the Hubble, we can see how fast the stars are moving, and whether the direction of motion changes over time. This enables us to see if they are binary stars, composed of normal stars that have collapsed stars orbiting about them."
Cohn and Lugger have been studying the same globular clusters for a number of years, since marrying his penchant for theoretical calculations of star cluster evolution with her observational work with telescopes. (They have long been married in their non-astronomical life and have two daughters who participate in various family astronomical adventures.) Although each contributes to every aspect of their joint projects, Cohn concentrates on theoretical simulations of the properties of globular clusters and their inhabitants, while Lugger orchestrates the actual observations and data-gathering that test the theories.
"Our initial interest was to look for a process known as core collapse, to see if the stars near the center of globular clusters gradually migrate toward the center, creating much higher density over time than at the outer portions of the cluster," Lugger says. "Another group proposed that there is a large black hole at the center of the cluster M15. We argued that the process of core collapse, with the concentration of massive white dwarfs and neutron stars towards the cluster center, would more easily explain the higher density of the central regions, since these collapsed stars are known to be there."
The data are consistent with the Lugger-Cohn hypothesis, though in galactic astronomy few things are certain. Lugger believes astronomers should take the most conservative--hence simplest--theory that fits the observed data, rather than relying on more exotic explanations such as a massive black hole. Still, there is a possibility that black holes exist at the center of globular clusters. "New tests are needed to choose between cluster models with and without central black holes, such as measuring the motions of stars in the central regions," says Lugger.
Indeed, Lugger and Cohn's ability to use new instrumentation has enabled them to understand much more about black holes and related stars, known collectively as compact objects.
Stars, it turns out, go through a relatively predictable aging process. Our Sun is middle-aged, about 5 billion years old, half of its anticipated 10-billion-year life-span. The aging process, Lugger explains, "is driven by the fusion of lighter elements into heavier elements deep within stellar cores. Fusion of hydrogen into helium is the process that powers stars like our Sun. Ultimately, a star spends all of its nuclear fuel, and its central regions will collapse to form a compact object."
A star of ordinary size, like the Sun, will become what's known as a white dwarf, a small star that is a million times more dense than the present-day sun. "A white dwarf is an object originally the size of the Sun that has been squeezed by gravity into an object the size of the Earth," Lugger notes. Many of the X-ray-emitting binary stars that Lugger and Cohn study in globular clusters consist of a normal low-mass star that closely orbits about a white dwarf. The X-rays arise from gas that is transferred from the normal star to the white dwarf and heated to millions of degrees in the process.
In the 1930s, the astronomer Subrahmanyan Chandrasekhar, for whom the Chandra telescope is named, predicted that white dwarfs had a maximum possible mass--about 1.4 times the mass of our Sun--and that a more massive collapsed star must be much smaller and denser than even a white dwarf. In 1966, pulsing sources of radio emission were identified as neutron stars, the second class of compact objects.
"A neutron star is so dense," Cohn says, "that instead of being collapsed into the size of Earth, it would fit within the city of Bloomington. The typical neutron star spins very quickly, which is what causes the radio emissions. The speed of rotation--in one documented case, a stunning 642 times a second--is due to what's called the law of conservation of angular momentum, more popularly called the ‘ice skater effect.'"
"Imagine an ice skater," Lugger continues. "She first spins slowly, with her arms outstretched. As she brings her arms in, condensing her body, she speeds up considerably. As they collapse dramatically in size and gain in density, neutron stars speed up enormously, just like the ice skater."
The third compact object, the infamous black hole, is formed when the core of a very massive, aged star collapses much further than a neutron star, to the point where nothing can escape from its gravitational pull--not even light. A black hole, Lugger notes, "would be no more than four miles wide if it were the size of our Sun prior to its collapse. The centers of galaxies are believed to harbor super-massive black holes, with masses millions to possibly billions that of our Sun.
"Until there was firm evidence for the existence of neutron stars and black holes, this was considered a rather esoteric field," Lugger says. "Even today, these are classed as ‘exotic stars' because these sizes, densities, and rotation rates stretch the limits of imagination."
While there is no earthbound invention from their work on the near horizon, Lugger's and Cohn's studies of materials, forces, and speeds so much greater than anything humanity has yet harnessed may someday result in a tangible benefit. "But that's not why we do it," Cohn points out. "We appreciate all the support we receive from the university, the National Science Foundation, NASA, and others to try to understand the universe and what's in it. And that, we believe, is worthwhile in and of itself."
In their specific research, Lugger and Cohn are studying not only the behavior of stars within a globular cluster, but also which stars seem to be the sources of powerful X-rays being emitted by the cluster. For this work, they utilize NASA's orbiting Chandra telescope, which can measure high-energy radiation precisely.
"Chandra's orbit takes it very far from Earth, extending almost a third of the way to the Moon," Cohn notes. That puts it beyond the radiation belts that are near Earth, allowing it to detect outer-space radiation in some detail. Chandra, Cohn says, "is the latest and greatest satellite-based X-ray telescope. It pinpoints the location of cosmic X-ray sources with exquisite accuracy." Cohn and Lugger are using Chandra and Hubble in coordinated observations to identify X-ray binaries in globular clusters and study their properties.
In X-ray binaries, gas from a normal star is being fed down to the collapsed object, forming a disk that orbits the collapsed star and is heated to a very high temperature, emitting prodigious amounts of radiation. The normal and collapsed star orbit closely together--a few hundred thousand miles apart, like the Earth and the moon, instead of the hundred million miles, roughly, between the Sun and Earth. That closeness creates heat, radiation, and gravitational distortions. Sometimes a third star will encounter the pair and be drawn in. But as on Earth, Cohn notes, "two's company and three's a crowd," so one of the three stars will soon be spun off.
For the most part, Lugger and Cohn keep their eyes trained on globular clusters, where the interactions of stars are frequent and complex. Given that there are thousands of rapidly moving stars within each cluster, it's easy to understand why they need a supercomputer just to track enough stars to create a reasonable simulation.
The GRAPE 6 (stands for gravity pipe) computer is specially designed for calculating forces between particles. Each particle, Cohn explains, feels the force of every other particle, or star, in the cluster. If you map a thousand stars, you would need to consider the effects on half a million pairs of stars; if you mapped a hundred thousand stars, you must consider five billion pairs of stars. A globular cluster can have up to a million stars in it. "To calculate the gravitational forces on all that makes a very computationally intensive problem," Cohn notes. The GRAPE is hardwired to do that kind of problem.
As computing and telescopic power continue to expand, Cohn and Lugger continue to learn remarkable things about our extraordinary universe. For the past four years, they and Allen Rogel have been participating in a Milky Way-wide census of X-ray binaries, which will reveal much about binaries' origins and patterns of distribution, leading to other questions.
"The answers to one problem always suggest the next questions," Lugger says. "We are never low on things to explore."
Michael Wilkerson is special assistant to the IU Vice President for Research.