Assertion: nebula's expansion | /r/CapableOf | create sharp density gradients
Facets:
Source sentences:
text: Each star within an H II region ionises a roughly spherical region—known as a Strömgren sphere—of the surrounding gas, but the combination of ionisation spheres of multiple stars within a H II region and the expansion of the heated nebula into surrounding gases creates sharp density gradients that result in complex shapes.