Answer:
At first she was studying uranium and, later on, polonium and radium.
Step-by-step explanation:
Marie Curie is, to this day, one of the greatest scientists in history. She was the first woman to win a Nobel Prize and to teach at Sorbonne, in Paris.
While researching to obtain her Ph.D.'s, Marie began to deal with uranium. She was trying to figure out why the rays emitted by uranium were not x-rays. While doing that, she found out that uranium minerals pitchblende and torbernite contained another chemical element that was more active than uranium itself.
From pitchblende, Marie was able to discover two new chemical elements: polonium and radium. The word radioactivity was coined after observing the element. Marie noticed that radium produces heat and luminosity without any chemical reaction taking place.