What does the star icon mean?

[aaronflavor]

What does the star icon below a city mean? I've seen it show up midgame in a few of my games. Unlike the beaker, hammer, and similar, I can see it even if its in my opponent's city, such as in this case from a hotseat game:

 

[LFaWolf]

That means it is the secondary capital of that civilization that has either Versailles or Forbidden palace built in it.

 

[Crighton]

thanks, I wondered about that too