In a living spiral galaxy, like the Milky Way, the rich gas inside enables the ongoing formation of new stars.
When enough gas gets concentrated in a single location, it collapses under its own gravity.
Various matter clumps will grow, faster and faster, leading to new stars and star clusters.
This can be triggered by internal dynamics, an external gravitational influence, or a merger with another galaxy.
Galaxies that are relatively isolated form new stars at a slow, constant rate: for much longer than the Universe’s current age.
But once a galaxy’s gas is gone, star formation ceases, as there’s no material left to fuel future stellar generations.
When a galaxy enters a rich, massive cluster, it has to contend with two murderous factors.
A single major merger can use up all the gas in both progenitor galaxies, leading to a red-and-dead elliptical galaxy.
Even without one, the intracluster medium is rich in matter, and speeding through it can strip out a galaxy’s gas.
Without that gaseous presence, new stars can no longer form.
Gas-free galaxies are most commonly found in clusters, with the pile-up of matter being the culprit.