1. chain reaction the splitting of a nucleus into two smaller nuclei whose combined mass is less than that of the original nucleus 2. critical mass the joining of two small nuclei to form one nucleus with a larger mass than the combined mass of the original nuclei 3. fission a self-sustaining reaction that creates an on-going series of reactions 4. fusion the amount of fissionable material required to keep a reaction goin

the circled areas are referred to   as sunspots.   the sunspots are cooler than the   surrounding area.

sunspots are temporary phenomena on the photosphere of the sun that appear visibly as dark spots compared to surrounding regions. they correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)