In the spring of 1944, Tokyo residents experienced numerous aerial attacks from Allied bombers.
Air raid sirens warned citizens to get indoors and preceded strategic blackouts across the city.
But 28-year old Hisako Koyama saw these blackouts as opportunities.
Dragging a futon over her head for protection, Koyama would gaze at the night sky, tracking all sorts of astronomical phenomena.
However, her latest endeavor required the light of day.
By angling her telescope towards the sun, Koyama could project the star's light onto a sheet of paper, allowing her to sketch the sun's shifting surface.
She spent weeks recreating this setup, tracking every change she saw.
But while Koyama didn't know it, these drawings were the start of one of the most important records of solar activity in human history.
To understand exactly what Koyama saw on the sun's surface, we first need to understand what's happening inside the star.
Every second, trillions of oxygen molecules fuse into helium molecules in a process called nuclear fusion.
