I was recently reading the latest edition of Halachically Speaking. It says:
A woman who is a widow r”l or divorcee still has an obligation to cover her hair.
But why is this so? Why should they be any different than unmarried women?
Presumably standard practice is that once a woman begins treating her hair as erva, she should continue doing so. (I believe I've heard this from Rabbis Broyde or Willig.)
Rabbi Moshe Feinstein does write that hair-covering while married is dat moshe, but hair-covering afterwards is dat yehudit. There is a great deal of discussion over what those terms mean, but most straightforward is the argument of Rabbi Yehuda Henkin, Rabbi Michael Broyde, and others that the former is completely objective, but the latter may have some cultural dependence.
(By the way, why do unmarried women not have to cover their hair? It appears their definition of erva is culturally subjective, hence not today.)
The obligation of hair-covering post-marriage is less than during marriage; thus (as indicated by rony), in one responsum Rabbi Moshe Feinstein allowed a young widow to uncover her hair as that was needed for her employment in an office (which sounds a bit like the Mad Men era, but I digress). In another responsum, he allows a young divorcee to uncover her hair to increase the likelihood of meeting an eligible fellow. There are actually two reasons why this is helpful:
This is a very (extremely?) delicate subject. We must first understand what stands behind it and this forum may not be the best place to discuss it. The idea of "tzniut" is very deep and comes to help us build a meaningful and lasting married relationship. The chinuch (training? education?) starts at a very young age much before the boy/girl gets to the age of marrying. There are different customs depending on the congregations. I am happy that I don't have to be a posek in this subject! The source to cover the hair for married window is deHoraitah (Shulchan Haruch Even Haezer 21:2) and according to the commentators it includes even divorced and widows. Harav Feinshtein was asked on the subject (Igrot Moshe, Even Haezer 1:57) regarding if the widow or the divorced may loose her place of work if she comes with her hair covered. He said that in the case of need she can go with her hair uncovered. What is "case of need"? What about a situation of a young widow that hopes to re-marry and feels that is she goes with her covered it will decrease her chances?