Will having a recommendation letter from someone in a different subfield hurt my chances of admission to PhD programs? I am applying to the graduate program in astronomy in US universities. In support of my application I have provided three recommendation letters.  Two are from my thesis advisors, while the remaining one is from my course instructor who works in condensed matter physics. He is well known in his field, but perhaps not in astronomy. I did very well in his course and he agreed to write a good (but not detailed) letter for me. Will this hurt my chances of admission?
Obviously, the better your letters of recommendation are, the better they support your application package.   Impersonal, though supportive, letters may not hurt your package, but they don't help as much as fantastic letters from people who know you well.  

So, not so much "hurt", but "not as much help as it could be", which some would say is equivalent to "hurt"