As I understand it (lapsed Catholic), there are two senses to excommunication.

In one sense, it means that you are ineligible to receive the sacraments, and if you do receive them, they are invalid. This would be the case for someone with un-confessed sins, including heresy, or using birth control for that matter.

In another sense, it is the formal finding of excommunication by the church authorities. That's where a person is supposed to be banished from society.

I don't think that Galileo was ever excommunicated. Feyerabend claimed that the church fathers chose a wording in their finding that stopped one step short of accusing Galileo of heresy (for which they would have had to torture and murder him).