These questions have answers that are very location- and school-specific.
If you're looking at a course-based MSc degree, then the idea is just that you take so many graduate courses and at the end you are awarded a master's degree. Because it's course-based, there's no reason you can't do it part time. You just take the classes as you can. This is popular in the engineering or teaching professions, because the master's degree is a means to professional advancement. Some companies might even foot the bill.
It's less popular in physics. My experience is with the Canadian system where an MSc in physics is thesis-based (including so many units of coursework and a graduate thesis). Again, you can do it part-time, but this stretches out the time to completion.
Of course it's possible to do something like this with a full-time job, but that would keep you incredibly busy.