The Milky Way's Restless Globular Clusters

PureInsight | February 17, 2003

[] Recently, astronomers have observed many unusual phenomena on a vast celestial scope. These observations demonstrate that our universe has been going through turbulent changes. To mark the beginning of 2003, Science magazine issued a special edition illustrating the great changes in globular clusters, which is one of the Milky Way's compact swarms of stars.

Globular clusters are believed to be the most ancient form of star clusters. One of the ways to measure the age of the universe is to measure the age of the oldest star in a globular cluster. From that, astronomers are able to calculate a conservative estimate of the age of the universe. Many extraordinary changes have been observed around these globular clusters. Astronomers pointed out that the night sky would look spectacular from a planet near the core of a cluster. The Science magazine article "The Milky Way's Restless Swarms of Stars," by Professor Robert, focuses on illustrating the splendid spectacles observed from these star clusters in the galaxy.


Galaxy's star cluster 47, Tucanae Core Source: NASA

Instead of the paltry few stars we see within several light-years of our sun, those on a planet near the core of a cluster would face a vista of 100,000 stars or more. Astronomers would have crisp views of binary partners that whip around each other in hours or minutes, pulsars that spin nearly 1000 times every second, and perhaps a nest of neutron stars or the birth of a sizable black hole at the center. Of course, such a planet almost certainly does not exist. Stars in the hearts of globular clusters interact so closely and so frequently, in astronomical terms, that planetary systems cannot survive the chaos. Indeed, stars themselves are not immune. Many get banished to the cluster's outskirts or are ejected into deep space after intense gravitational encounters. Some stars even collide, making bizarre new objects that are at least a billion times more likely to arise in globular clusters than elsewhere in the galaxy. It's no wonder that scores of observers and theorists peer beyond our sedate galactic neighborhood to study these angry swarms of stellar bees.

Detailed surveys by the Hubble Space Telescope (HST) and the Chandra X-ray Observatory as well as computer models have revealed that the dynamics of the cluster and the evolution of its stars are inextricably linked. Stars are so close that any changes in their mass or size influence how they interact with neighbors.
The binary star system is a system that has attracted quite a bit of interest from astronomers. They compare the binary star system to a molecular system with two atoms. Although the two systems vary tremendously in size as the binary system is made up of stars, not atoms, scientists believe the two systems have identical physical properties. The density of the overall star cluster influences the behavior of the binary star system. When the cluster expands and the two stars are not too close, the probability for binary stars to collide with other stars is low. Nevertheless, the attraction among stars in the cluster inevitably shrinks it. When a binary star system gets close to another star, the other star can obtain energy from the binary star system, which diminishes the binary star's inter-spinning orbits. Sometimes, a third star can even replace one of the stars in a binary system and cast it out of the cluster. Scientists refer to this process of casting out from a cluster as "star evaporation." This kind of replacement makes the cluster denser, preventing collapse of the cluster core. Yet, from a long-term perspective, this also endangers the existence of the cluster. The life of a cluster can be one billion or even ten billion years, depending on its original mass and the number of binary star systems within it.

Calculations indicate that if there are 5% to 10% binary star systems in a cluster, the evaporation process mentioned above can prevent cluster collapse. Theoretical forecasts state that when the cluster core collapses, the evaporation process should quickly enable the core to rebound, thus preventing the collapse of the cluster. Theoretically, we should not be able to see the collapse of any cluster. But scientists have observed quite a few instances of collapsing clusters among the star clusters in the galaxy. Observations found that 20% of the galaxy's star clusters have undergone core collapse. Serious questions arise reagrding the difference between actual observations and theoretical forecasts. So far, no physics theory can account for this kind of core collapse.
Globular star clusters have given astronomers an opportunity to conduct research on the behavior of dense stars. The reason why some of these star clusters have collapsed is still a mystery to science.


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