The Ionian Nebula is a supernova remnant recorded by the people of Kobol 4000 years ago. The nebula closely resembles the Eye of Jupiter supernova event in the episode "Rapture" that resulted when the star the Algae Planet orbited exploded. Felix Gaeta noted similarities in the remnant patterns and believes the Eye of Jupiter is a marker that points toward the Ionian Nebula; another clue to finding Earth.
A massive battle takes place inside the Nebula between the Battlestar Galactica and 4 Cylon Basestars. 1,700 civilians are killed when the civilian ship Pyxis is destroyed, with Space Park and Astral Queen heavily damaged (episode "He That Believeth in Me").
The Ionian Nebula is 13,000 light years from the Algae Planet.
NGC 2440 resembles the Ionian Nebula and is 4000 light-years from Earth.
It is the fourth and last nebula (of major importance, for being/containing markers or pauses on the way to Earth) seen in the series, following:
- the nebula surrounding New Caprica (causing interference on DRADIS, thus hiding the planet from the Cylons, becomes the reason for humanity settling on the planet - temporarily for a year peacefully, and four months with Cylons);
- and the Lion's Head Nebula.
Read more about this topic: List Of Battlestar Galactica (reimagining) Locations, I