You know i was wondering how other species could have survived
the first Halo blast...
Therefor i made me a picture of our galaxy with numbers i found at wikipedia.
Our galaxy is about 100,000 light years wide according to wiki. When i remember
correctly one halo´s blast radius was 25,000 ly.
Here is a picture with 7 radiuses placed randomly just to get a feeling for it.
As you can see there is quite a lot of space untouched.
(You don't have to cover the center of the galaxy because through radiation
and a super blackhole)

If GS has sad the truth about leaving nearly no life left in the galaxy. I only see two
ways the rings could do that.
1) to cover up most of the space the power of all 7 add up and bring the affective
radius up to 33ly or
2) the Halo´s shoot in a kind of array. Combining their power to one super-blast that
covers the galaxy. (Like the Deathstar in Star Wars did)
So 2) gave me a new idea... maybe only one ring fired a 100,000 years ago? Maybe
thats what went wrong and only one in line fired and that was the one where the Forerunners
where... nobody is perfect you know
