It consists largely of glowing gases from the star's outer layers ejected during the final stages of its life when it had run out of the fuel necessary to sustain its core fusion processes.

[7] Partly because of their small total mass, open clusters have relatively poor gravitational cohesion.

Consequently, open clusters tend to disperse after a relatively short time, typically some 10 million years, because of external gravitational influences amid other factors.

Theoretical models predict that planetary nebulae can form from main-sequence stars of between 8 and 1 solar masses, which puts their age at 40 million years and older.

One such reason is that the planetary nebula phase for more massive stars belonging to younger clusters is on the order of thousands of years - a blink of the eye in cosmic terms.