Addendum to the excellent stellar manipulation information provide by Douglas.
This link
Based on simultaneous movement type game.
Create Nebula: (Correction to Douglas’ information on Create Nebula.)
Can only be issued at a star, but no star is necessary when the order is carried out in a remote system. (Just like a Create Black Hole.) Note the star where the order was issued is NOT affected by the creation of a Nebula in a remote system unlike the Create Black Hole were it deletes the star were the order is issued.
Ring-World and remote Creation of Nebula or Black hole:
The remote system use of Create Nebula or Create Black hole is not affected by the presents of a ring-world in the remote system. (I didn’t test with a sphere world.)
A system with a ring-world can be destroyed by creating a nebula or a black hole, by using a ship at a sun in another system to issue the stellar manipulation order after giving it orders to move to the target system.
Ring-World treated like a planet when destroyed by a create nebula:
Interestingly, if a ring-world is destroyed by creating a nebula, the ring-world will be reduced to an asteroid field just like any other planet that can be seen after the nebula cloud is destroyed.
(Not Tested, but...) Stellar manipulation of one sun in a multiple sun system is probably not affected by the present of a ring world or sphere world around the other sun or suns.