求助：一个英语句子分析~Spiral galaxies are well supplied with the inter

求助：一个英语句子分析~
Spiral galaxies are well supplied with the interstellar gas in which new star form; as the rotating spiral pattern sweeps around the galaxy it compresses gas and dust, triggering the imformation of bright young star in its arms.
分号后半句“galaxy it”这两个词的关系是什么哇~即it指代什么~后面是否是省略了连接词的从句.
另外最后的triggering 的主语是哪个,是pattern还是galaxy