Searching for Supernova
Image taken from http://cfa-www.harvard.edu/cfa/oir/Research/supernova/newdata/waldo2.html
The first two pictures here are of the same part of the sky, taken at two different times. Can you see the difference between them? The third picture shows what happens when you subtract one image from the other - a bright point is left over. That point is a supernova! This process of image subtraction is carried out by automated computer programs, and is a common way of searching for supernovae.