It is generally believed that expanding superbubbles and mechanical feedback from massive stars trigger star formation , because there are numerous examples of superbubbles showing secondary star formation at their edges .
However , while these systems show an age sequence , they do not provide strong evidence of a causal relationship .
The W3/W4 Galactic star-forming complex suggests a three-generation hierarchy : the supergiant shell structures correspond to the oldest generation ; these triggered the formation of IC 1795 in W3 , the progenitor of a molecular superbubble ; which in turn triggered the current star-forming episodes in the embedded regions W3-North , W3-Main , and W3-OH .
We present UBV photometry and spectroscopic classifications for IC 1795 , which show an age of 3 – 5 Myr .
This age is intermediate between the reported 6 – 20 Myr age of the supergiant shell system , and the extremely young ages ( 10 ^ { 4 } -10 ^ { 5 } yr ) for the embedded knots of ultracompact H ii regions , W3-North , W3-Main , and W3-OH .
Thus , an age sequence is indeed confirmed for the entire W3/W4 hierarchical system .
This therefore provides some of the first convincing evidence that superbubble action and mechanical feedback are indeed a triggering mechanism for star formation .