Henrietta Swan Leavitt was a very clever lady who lived a long time ago. She loved looking at the stars and wanted to learn more about them. She took a job as a "computer" at Harvard, where she spent many hours looking through a telescope and writing down what she saw.
She noticed something very interesting about a group of stars called Cepheid variables. These stars were very special because they got brighter and dimmer in a pattern. Henrietta noticed that the brighter Cepheid variables took longer to go from bright to dim and back to bright again. And the dimmer ones took less time to do the same thing.
By measuring the time it took for each of the Cepheid stars to get brighter and dimmer, Henrietta made a big discovery. She found out that the brighter stars took longer to get bright and dim than the dimmer stars. This discovery helped other scientists figure out how far away stars were from Earth.
It's kind of like if you are walking and you see two streetlights. One streetlight seems very far away because it takes a long time for its light to reach you. The other streetlight seems close because its light gets to you very quickly. In the same way, Henrietta helped scientists figure out which stars were close and which were far away.
Even though Henrietta couldn't see these stars up close, she used her brain and a lot of hard work to make an amazing discovery that helped us learn more about the stars and the universe.