Crowded Star Clusters Defy Simple View Of Gravity
February 21, 2013

Crowded Star Clusters Defy Simple Understanding Of Gravity

April Flowers for - Your Universe Online

On large astronomical scales, gravity remains the dominant force acting on heavenly bodies, from asteroids and exoplanets to solar systems and supermassive black holes. But when it comes to young stars in clustered galaxies, researchers have found that the dynamics of these crowded environments cannot be fully accounted for by simple understanding of gravity.

A new study, led by the“¯Kavli Institute for Astronomy and Astrophysics (KIAA)“¯at Peking University in Beijing, analyzed Hubble Space Telescope images of the star cluster NGC 1818 in the Large Magellanic Cloud — a satellite galaxy of the Milky Way. The team found more binary star systems towards the edge of the cluster than in the center. This distribution was surprising — the exact opposite of their expectations, in fact — and perhaps the result of complex, supra-gravitational interactions between stars within young clusters.

The findings of this study will be published in the March 1 issue of The Astrophysical Journal and is already available online.

KIAA astrophysicist Richard de Grijs explains that in the dynamic environment of a star cluster, high-mass stars are thought to gravitate toward the center of a cluster when they give a ℠kick´ to lower-mass stars and lose energy. In a process known as “mass segregation,” the high-mass stars then sink into the center of the cluster, while the lower mass star gains energy and takes up an orbit at a greater distance from the center. However, when the KIAA research team closely examined the binary star systems within NGC 1818, the picture they found was a bit more complex.

A majority of stars in the cluster formed pairs called binary stars. These stars are initially located so close to one another that they interact, which often results in the destruction of the binary systems. Others, however, switch partners. The research team assumed that the process that leads a cluster's most massive stars to fall towards the center would apply to binaries because the combined mass of the stars in a binary pair have, on average, more mass than a single star.

The astronomers were initially baffled when they discovered an increasing number of binary systems farther from the core of the star clusters. They classified the binary systems into two types, "soft" and "hard."  The soft binary systems, in which the two stars orbit each other at large distances, are destroyed due to close encounters with other stars near the cluster's center. However, the hard binaries, in which the stars orbit each other in much closer proximity, were far more likely to survive close encounters with other stars. This piece of observational data also helps explain why the team found more binaries the farther they looked from the center of the cluster.

Scientists have never before mapped the radial distribution of binary systems in dense star clusters like the youngster NGC 1818 cluster, which is thought to be some 15 to 30 million years old. This type of mapping is difficult to perform in any case because there are no young clusters to study in our own Milky Way galaxy, meaning that researchers generally have to rely on data from more distant galaxies which are harder to observe. The results of this new survey provide insights into theoretically predicted processes that govern the evolution of star clusters.

“The extremely dynamic interactions among stars in clusters complicates our understanding of gravity,” team member Chengyuan Li said. “One needs to investigate the entire physical environment to fully understand what´s happening in that environment. Things are usually more complex than they appear.”