The black star shows the tracks made over a 48 hour period by alpha rays emitted from a radioactive particle of plutonium lodged in the lung tissue of an ape (the particle itself is invisible). In living lung tissue, if one of the cells adjacent to the particle is damaged in a certain way, it can become a cancer cell later on, spreading rapidly through the lung, causing almost certain death.
photo by Robert Del Tredici
from his book entitled
At Work In The Fields Of The Bomb
(Harper and Row, 1987)