Postgres: Order by string column with known values

Postgres: Order by string column with known values

I have an events table with a state column. If everything goes according
to plan, the states can only be one of:
scheduled
invited
notified
started
ended
Is it possible to order by state and specify which value comes first,
second, third, etc...?
Bonus Points: Is there a way to do this easily in Rails 3?