Che cos'è un Globular Cluster?
A globular cluster is a gravitationally bound sphere of anywhere from ten thousand to several million stars, packed into a volume a few dozen light years across. The Milky Way has roughly 150 of them, orbiting in its halo rather than in its disc — and they are older than the disc is.
The density is difficult to convey. Near the core of one, stars are separated by fractions of a light year rather than the several light years typical around the Sun, and the night sky from a planet there would hold thousands of stars brighter than Venus, bright enough to read by. That crowding has consequences: stars physically interact, exchange partners, and occasionally collide outright, producing blue stragglers — stars that look far too young for a cluster this old because they were assembled from two older ones.
They are also clocks. Every star in a globular cluster formed at essentially the same time from the same material, so the cluster is a snapshot of one generation ageing together, and the point where its stars turn off the main sequence gives the age directly. The answer keeps coming out near 12 to 13 billion years — which for a while gave the embarrassing result that the oldest clusters appeared older than the universe. Better measurements of both closed the gap, but the constraint remains one of the sharpest independent checks on cosmology there is.
Where they came from is still argued. They are old, poor in heavy elements, and distributed in a spherical halo rather than a disc, so they formed before their host galaxy settled into its present shape — and some, like Omega Centauri, contain multiple stellar populations and look more like the stripped cores of small galaxies that the Milky Way swallowed than like clusters at all. The model uses a Plummer profile, the standard analytic description of these systems, with every star on its own randomly inclined orbit.