One-and-a-half seconds after the light bounced back to Earth; much of the atomic bomb exploded over rest continued onward, traveling all the Hiroshima on the morning of August 6, way to the Sun, and then indefinitely 1945, the flash of light from the explo-beyond. The glare would have been visision reached the Moon. Some of the ble from Jupiter.

In the perspective of the galaxy, it was the most insignificant flicker. Our Sun, alone, explodes the equivalent of many millions of such bombs every second. As Albert Einstein and other physicists had long recognized, E=mc2 does not apply just on Earth. It was just a quirk that the accelerated technology and pressures of wartime led to the equation’s first applications being focused on the development of weaponry.

Ever since the discovery of radioactivity in the 1890s, researchers had suspected that uranium or a similar fuel might be operating in the wider universe, and in particular, in the Sun to keep it burning. Something that powerful was needed because Charles Darwin’s insights as well as findings in geology had shown that the Earth must have been in existence—and warmed by the Sun—for billions of years. Coal or other conventional fuels would not have supplied enough energy to do that.

Astronomers, however, couldn’t find any signs of uranium in the Sun. Every element gives off a distinctive visual signal, and the optical device called the spectroscope (because it breaks apart the light spectrum) allows them to be identified. But point a spectroscope at the Sun, and the signals are clear: There is no uranium or thorium or other known radioactively glowing element up there.

What did seem to leap out, in readings from distant stars as well as the Sun, was that there was always iron inside these celestial bodies: lots and lots of metallic bulky iron. By the time Einstein was finally able to leave his job at the Swiss patent office, four years after publishing the 1905 paper setting forth his famous equation, the best evidence suggested that the Sun was about 66 percent pure iron.

This was a disheartening result. Uranium could pour out energy in accord with E=mc2, because the uranium nucleus is so large and overstuffed that it barely holds together. (According to Einstein’s equation, mass and energy are interchangeable: The energy [E] in any substance can be found by multiplying its mass [m] by the speed of light [c] squared.) Iron is different. Its nucleus is one of the most perfect and most stable imaginable. A sphere made of iron, even if it were molten or gaseous or ionized iron, could not pour out heat for thousands of millions of years. Suddenly the vision of using E=mc² and related equations to explain the whole universe was blocked.

The individual who broke that barrier—letting E=mc² slip the surly bonds of Earth—was a young Englishwoman named Cecilia Payne, who loved seeing how far her mind could take her. Unfortunately, the first teachers she found at Cambridge University when she entered in 1919 had no interest in such explorations. She switched majors, and then switched again, which led to her reading up on astronomy, and when Payne started anything, the effects were impressive. She terrified the night assistant at the university’s telescope her first night there, after she’d been reading for only a few days. (He "fled down the stairs," she recalled, gasping, "‘There’s a woman out there asking questions.’") But she wasn’t put off, and a few weeks later, she recalled in her autobiography,* "I bicycled up to the Solar Physics Observatory with a question in my mind. I found a young man, his fair hair tumbling over his eyes, sitting astride the roof of one of the buildings, repairing it. ‘I have come to ask,’ I shouted up at him, ‘why the Stark effect is not observed in stellar spectra.’" This time her subject did not flee. He was an astronomer himself, Edward Milne, and they became friends. Payne tried to pull her art student friends into her astronomical excitements, and even though they might not have understood much of what she was saying, she was the sort of person others liked being around. Her rooms at Newham College were almost always crowded. "When safely lying on her back on the floor (she despises armchairs)," a friend wrote, "she will talk of all things under the sun, from ethics to a new theory of making cocoa."

Ernest Rutherford, whose work helped reveal the structure of the atom, was then a key figure at Cambridge. With men he was bluff and friendly, but with women he was bluff and close to thuggish. He was cruel to Payne at lectures, trying to get all the male students to laugh at this one female in their midst. It didn’t stop her from going—she could hold her own with his best students in tutorials—but even 40 years later, retired from her professorship at Harvard University, she remembered the rows of braying young men, nervously trying to do what their teacher expected of them.

But also at the university was Arthur Eddington, a quiet Quaker who was happy to take her on as a tutorial student. Although his reserve never lifted—tea with students was always in the presence of his elderly unmarried sister—the 20year-old Payne picked up Eddington’s barely stated awe at the potential power of pure thought. He liked to show how creatures who lived on a planet entirely shrouded in clouds would be able to deduce the main features of the unseen universe above them. There would have to be glowing spheres up in space, he imagined them reasoning, for a ball of vaporized elements sufficiently large and sufficiently dense would compress the elements inside it to start a nuclear reaction that would make it light up—it would be a sun. These glowing spheres would be dense enough to pull planets swinging around them. If the beings on Eddington’s mythical planet ever did find that a sudden wind had blown an opening in their blanket of clouds, they would look up to see a universe of glowing stars with circling planets, just as they’d expected.

It was exhilarating to think that someone on Earth might solve the problem presented by the presence of so much iron in the Sun, and so be able to fulfill Eddington’s vision. When Eddington first assigned Payne a problem on stellar interiors, which might at least be a start toward achieving this, "the problem haunted me day and night. I recall a vivid dream that I was at the center of [the giant star] Betelgeuse, and that, as seen from there, the solution was perfectly plain; but it did not seem so in the light of day."