In fact, thehelmet is actually morelikeaninterstellarbubble, blown as a fast windfromthe bright, massive star near the bubble's center sweeps through asurroundingmolecularcloud.
He chose this because the supernova that created it blew a giant bubble, known as the Vela nebula, out of the surrounding interstellar medium, and that bubble is big enough to stand in for the infinite sheet of gas.