Changing _alpha in AS

Hey everyone, got a quick and easy question.

I want to change the alpha transparency of a mc from 100 to 80% in AS, without using the timeline. I know how to make it immediately change, but is there somehow I can have it gradually change from 100 to 80% over the course of say… 30 frames without using the timeline?

Cheers, and thanks in advance! ^_^.

–Xe.