Flash Player 4.
my_mc._alpha
Property; the alpha transparency (value) of the movie clip specified by my_mc. Valid values are 0 (fully transparent) to 100 (fully opaque). Objects in a movie clip with _alpha set to 0 are active, even though they are invisible. For example, you can still click a button in a movie clip with the _alpha property set to 0.
The following statements set the _alpha property of a movie clip named star to 29.6875% when the button is clicked (there are 128 levels of alpha mapped on a range of 0-100):
on(release) {
star._alpha = 30;
}