Why Do Globular Clusters Maintain Their Shape Over Billions of Years?

[Eridanus1]

they are the oldest things in the galactic halo, having formed near the birth of the milky way. So with all the 10 odd billions of years of existence, i think they all should have gone supernova and destroyed the shape of the cluster.

So how do they stay gloubular in their shape?

 

[Phobos]

(1) Only large stars explode as a supernova. Less massive stars like our sun and smaller do not do this (e.g., our sun will swell to a red giant, becoming a "planetary nebula" and then shrink to a white dwarf). Large stars are also short-lived. So, such ancient star clusters contain lots of lower mass (longer lived) stars.

(2) It's not much of a "shape" to begin with. It's a collection of stars held by each other's gravity (like a mini-galaxy). The shape is constantly changing anyway.

 

[SpaceTiger]

they are the oldest things in the galactic halo, having formed near the birth of the milky way. So with all the 10 odd billions of years of existence, i think they all should have gone supernova and destroyed the shape of the cluster.

As was already said, this is not an issue. However, globular clusters do undergo a process known as "core collapse", in which the central part gets ever denser with time and the outer parts get less dense. The end result of this process is that the cluster dissipates, but for most globular clusters, this timescale for it is much longer than 10 billion years.

So how do they stay gloubular in their shape?

It turns out that for a group of objects moving under the influence of gravity, the minimum energy configuration (and, therefore, the preferred one) is spherical...unless it has a significant amount of rotation, then it goes to a disk.