Thus all life should be contained in a single region 25,000 LY from the center of the galaxy. Why is this important. Take a look...Wikipedia wrote:The galactic habitable zone
The location of a solar system within the galaxy must also be favorable to the development of life, and this leads to the concept of a galactic habitable zone.
To harbor life, a solar system must be close enough to the galactic center that a sufficiently high level of heavy elements exist to favor the formation of rocky planets. Heavier elements must be present, since they form complex molecules of life, such as iron as the foundation for hemoglobin and iodine for the thyroid gland.
On the other hand, the solar system must be far enough from the galaxy center to avoid hazards such as impacts from comets and asteroids, close encounters with passing stars, and outbursts of radiation from supernovae and from the black hole at the center of the galaxy. The effect of radiation from supernovae on living organisms is not clear. Presumably, large amounts of radiation, which occur near the center of a galaxy, make formation of complex molecules more difficult.
Also, studies have shown that regions in which the level of heavy elements, or metallicity, is very high seem to be more likely to harbor massive planets orbiting close to their star. The gravitational tidal forces induced by such planets would distort the orbit and surface shape of any Earth-mass planets, which could destroy them before life has a chance to form.[citation needed] For these reasons, there are many uncertainties in determining where the habitable zone in a galaxy may lie.
In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years (8 kiloparsecs) from the galactic core, containing stars roughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smaller GHZ – or none at all. Future technologies may enable us to determine the number and location of Earth-type planets in the Milky Way, greatly refining our understanding of the Galactic habitable zone.
Since 343 GS said this:
There has always been a question of how the halos could wipe the galaxy clean of all sentient life with only 7 that had a range of 25,000 LY. Based on this, it's apparent that the entire galaxy didn't need to be covered, just this center ring. Any farther out and there can't be life, any farther in and life is impossible due to comets and the like. And GS doesn't say that the pulse goes three radii of the galactic center, just that there isn't any life left afterwards within three radii of the galactic center.343 Guilty Spark wrote:(pauses) More or less. Technically, this installation's pulse has a maximum effective radius of twenty-five thousand light years. But, once the others follow suit, this galaxy will be quite devoid of life, or at least any life with sufficient biomass to sustain the flood. (pause) But you already knew that. I mean, how couldn't you?
So what does this mean? It means that parts of the galaxy are left unaffected by the halos due to there being no chance of indigenous life. Now there's people here with theories that the Forerunner escaped the galaxy. What if they are still in the galaxy, just outside of the galactic habitable zone in some self sustained bunkers of some kind? We know they have the tech to do that.