With Sterling's help, you escape from the Gortress with the freed Celestrians and finally return to the Observatory.
Talk to Apus Major
Exit the room of the Observatory that you start in, then make your way downstairs. If you talk to the other Celestrians, they will reveal that Aquila just left the Observatory after returning all seven fyggs to Apus Major.
In the first large room that you reach, go to the northeast corner and unlock the jail cell to open the red chests inside. One contains a Goddess Ring, and the other contains Orichalcum. Be sure to save the Orichalcum. It's very useful, but very rare, so you will want to save it for a good opportunity.
After you are done exploring, talk to Apus Major on the bottom floor of the Observatory. He will go the Starflight Express so you can go with him to the Realm of the Almighty.
Ride the Starflight Express
Go to the top level of the Observatory and go to the left of the stairs that lead to the Yggdrasil. Talk to Apus Major, who is standing beside the train. He asks you to enter the train, and he will follow you. Go into the train.
Talk to Sterling and say that you talked with Apus Major. Watch the cutscene. You will end up in the Realm of the Almighty.