Your confusion stems from the existence of two different bus services. Roissybus, operated by the local transport authority RATP, connects Opéra to the Roissy CDG airport terminals. Cars Air France, operated by the French national airline, connects Étoile to CDG.
From Étoile by bus, you can take Cars Air France line 2 (make sure you board a bus for Roissy, not for Orly). The bus departs from Avenue Carnot heading away from the Place. Buses run every half-hour on the quarter-hour during daytime and take approximately one hour with a risk of traffic jams.
You can also take the train: take RER line A towards Boissy-Saint-Léger/Marne-la-Vallée (any train heading east), and change at the second stop Châtelet–Les Halles for a line B train to Roissy–Aéroport Charles de Gaulle (this is not the train on the opposite platform, you need to go up and down the stairs). The journey takes a little under an hour, with at least a train every 15 minutes and a risk of traffic disruptions.
The airport bus is a bit more expensive and a bit slower than the train, but more comfortable. The risk of delays is roughly the same; traffic jams are very common at peak hour but are managed, whereas major train service disruptions are less common but can be very disruptive.