A single month can realign the stars. Stars can turn into meteors. Old stars are better than new ones. Warshawsky discovered that funds with at least 10 years of history held onto their top star ratings better than funds with just three years under their belt. Funds that have been around longer also tend to have lower annual expenses, which gives them an enduring performance advantage.
Bad ratings are sticky.
Blake and Morey found that Morningstar ratings are quite good at predicting which funds will perform poorly. That is, most funds with ratings of one or two stars tend to keep them. Five stars are no better than three. By contrast, Blake and Morey found that funds with five, four or three stars all perform about the same. But beyond that, I wouldn't worry a lot about whether the fund has three, four or five stars.