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Your point is a good one, Renato. Surrounding the technical distinction between open and globular (permanently bound or not) is a maze of contributing factors that varies considerably around just two entities: system particle density and system energy density. 'Particle' density means a vast range in mass bridging the number of atoms per cubic centimeter ascending up to the number of stars per cubic parsec (3.26 cubic light years). On these scales, atoms and stars are mathematically considered 'particles' for computation models. In both cases individuals are tiny compared with the sample volume of space they occupy, and interact as point sources with specific properties that affect other particles in the same way. Think of a swarm of bees flying through a giant gnat cloud and you get the drift.
Turning this into numbers can get moderately complex, but it's piffle compared with calculating the effects of energy density. Gravity, light, kinetics (the energy of large objects moving in a direction), thermal (tiny objects moving at random), magnetic fields, even sonic turbulence. Space has sound, and there are screams in space. We can't hear them, but we can see them—shock waves of stellar gas expulsions give planetary nebula their shapes; turbulent shock waves make nebulae like M17, Orion, and Carina look like shredded cirrus clouds at sunset; and spiral arms are density concentrations not much different than the gong of a bell rung once a long time ago. The speed of sound in a collapsing supernova is 72,000 kms/sec. If we could hear these, Kepler's Music of the Spheres would be a music of rips, tears, shrieks, wheezes, gurgles, and a loud bang when a supernova shock front hits a snoozy little hydrogen cloud out there peaceably doing not much. These descriptions would horrify a young Ph.D with fresh diploma in hand, but after reading page after page of what these folks write, I'll take bees, gnats, John Cage, and pine boughs in a crosswind (the behavioural analogy of spirals in galaxies), any day of the week.
Whether the descriptions be florid or mathematical, the basic problem remains: particle densities span a large continuum, and so do energy densities. You can have low-particle, low energy density at one end and find yourself in the middle of an intergalactic void; and high-particle, high-energy density on the other and be in the middle of a supernova. Star clusters occupy a span somewhere in the middle. On one side are associations so tenuous they barely hang together (Lupus-Centaurus or Chamaeleon), and a Class I globular like NGC 2808 so tight no force in the universe can sunder it. N2808 could eat 100 simultaneous supernovae and spit out carpet tacks. The 'open' or 'globular' appellations are merely a lone reliable milepost on the long road between not much and a lot, nowhere and singularity, roots and wings.
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