Different structures of star clusters are due to different levels of dynamical aging.
Like batches of cookies, stars are born together in groups. These star clusters, containing as many as 1 million members, evolve over time largely through a gravitational pinball where more massive stars are segregated from lower mass stars. Heavy stars tend to progressively sink toward the central region of the star cluster, while low-mass stars can escape from the system.
For the first time, the Hubble Space Telescope has been used to measure the effects of this dynamical aging on star clusters. They are all located 160,000 light-years from Earth in a satellite galaxy, the Large Magellanic Cloud (LMC). The diminutive galaxy is an ideal target because it hosts a selection of easily observed star clusters covering a wide range of ages.
Francesco Ferraro of the University of Bologna in Italy and his team used Hubble to observe five aging LMC star clusters — all born at about the same time but with different sizes — and succeeded in ranking them in terms of the level of dynamical evolution, which affects their shape.
Source: Hubblesite Press Release