Regarding the Gamma Ray Burst as a source of destruction in general and extinction per this thread in particular:
The latest issue of Scientific American has an article on GRB's (the cover article in fact), according to which (the latest theory) the bursts are part of a hypernova explosion of stars larger than 35 solar masses, and are tightly confined "jets" directed axially outward, north and south, along the axis of rotation. According to this model, the grb's are detectable by us only when the distant hypernova is oriented so that the axis of rotation points more or less directly at the earth. (Presumably in a galaxy so oriented but safely distant.) Also the total energy released is substantially smaller when "the signal" we see is the result of axial jets, than it would be if it were at that same level in all radial directions.
If it should turn out that the axes of rotation of stars in a galaxy are mostly aligned with the axis of rotation of the galaxy itself, then the grb jets will mostly be directed out of the plane of the ecliptic into relatively "empty" space, and only a very few stars radially nearby or those immediately above or below in the galactic disc will be subject to obliteration.
Thus, according to this model of grb's, the destruction wrought on the local galaxy should be substantially less than previously predicted.
Best, Jeff Davis
"Everything's hard till you know how to do it."