They're mostly made up of Hydrogen and Helium, as these are the lightest and most abundant elements in the universe.

It seems the lightest most superficial entities are always the most abundant.

youngest, coolest, and most dense of all nebulae

They are dense too.

As early as 150 AD, Ptolemy wrote in his first treatise about five stars that "appeared nebulous"

They had less city lights to hinder their observations back then. Lucky gazers.

The Pillars of Creation

I call it a zombie hand rising from the grave.

Cos that's clearly what it looks like.