With a calm expression and wistful gaze, Cecilia Payne-Gaposchkin could have been just another passerby, yet she was the brilliant mind who unlocked one of the universe’s greatest secrets. In 1925, through her groundbreaking doctoral thesis *Stellar Atmospheres*, Cecilia revealed that hydrogen is the most abundant element in the cosmos, and that stars, including our Sun, are primarily composed of this simple gas—a revelation so profound her work was later hailed as “the most brilliant PhD thesis ever written in astronomy.” But in her time, her discovery was dismissed. Born in England in 1900 when educating girls was seen as a waste, Cecilia defied expectations, earned a scholarship to Cambridge, and then crossed the ocean to become the first person to receive a PhD in astronomy from Radcliffe College. She later broke another barrier as Harvard’s first female full professor. Yet her name rarely appears alongside Newton or Einstein, and the cosmic truth she unveiled is often attributed to science itself, not to the woman who first proved it. Her story mirrors that of other overlooked pioneers like Rosalind Franklin, who helped reveal DNA’s structure, and Lise Meitner, who illuminated nuclear fission. This is for them—for the women history tried to silence even as they transformed our understanding of the world. It was Cecilia Payne who first proved the stars are made of hydrogen, reminding us that the universe is also made of women like her.