A small galaxy called Sagittarius passed close to the Milky Way four times in the past 6 billion years, which may have caused periods of intense star formation
Source:
https://www.newscientist.com/article/2244234-the-sun-may-have-formed-because-a-small-galaxy-passed-by-the-milky-way/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home