A globular cluster is spherical, and a galaxy is more or less flat, but both follow the same laws of gravity....So why?
Well, according to Jérome Perez and Fabrice Roy (2 french mathematicians) it's at their formation :
Galaxies form from a pastery made of blobs (or clumps-stars and gas already linked to each other by their gravity) while globular clusters are merely from their birth a group of individual stars, without agregats.
The flatness of galaxies is often attributed to their fast rotation. Mistake, elliptical galaxies hardly rotate.
Let's assume stars orbiting a central point but at many inclinations. All together those orbits form a sphere. Let's imagine that under the influence of a perturbation, the orbit of one of those stars crosses (exceeds) the limit of this sphere. We can be sure the whole group of stars will little by little assume a flatter shape. The direction of the flatness will be the same as the initial perturbation (It's called radial orbit instability). For that to occure, the galaxy must be non-homogenous and contain clumps of stars in its center. Those gas and star clumps are not present in globular clusters.
The 2 mathematicians refer to a phenomenon (Antonov instability) (more and more instability as we get deeper inside a denser and denser system).
Above a certain threshold, there's a sudden phenomenon: the heart collapases on itself. (Black holes).
In globular clusters, there's no important density contrast (because of the lack of primordial clumps), which prevents material from collapsing.
Those researchers are trying to find counter examples: extremely elliptical galaxies, the flattened globular cluster, or one with a massive black hole; but so far haven't found any.
It's from november's Ciel et Espace

Raoul