Lexicographic Order of Expression [Automata Theory]

what will be kleene star "expansion" of expression $$a^*b^*$$ in lexicographic order? I'm confused and I really want to clear my concepts so I can proceed further

The lexicographic order is defined on words, not on regular expressions. An interesting exercise is to describe the restriction of the lexicographic order to the language $$a^*b^*$$, assuming that $$a < b$$. Note for instance that $$1 < a < a^2 < \dotsm < a^n < \dotsm$$, but $$\dotsm .