What happened to the six-year-old girls' mother? How did she disappear?

In Ch.1 we find out that that their mother died of a sudden heart attack. She was 15 years younger than Atticus, which made it very unexpected. Scout was only two when her mother died so her memories are vague at best and her trauma was minimal. Jem was six and he remembers more. Scout can tell that he misses their mother.