Zetar

From Memory Alpha, the free Star Trek reference.

The planet Zetar lies in an unspecified region of space. The planet was subject to a massive cataclysmic event which destroyed all corporeal life there. When all life died, only a hundred or so of the resident civilization, the Zetarians, lived on as a pattern of lights and noncorporeal energy. (TOS: "The Lights of Zetar").