In November 1891, Maria Sklodowska, 23, rode 40 hours on a train from her native Warsaw to Paris, seeking a college education. She traveled fourth class, which meant that she had to bring her own food and a stool to sit on. Twelve years later, she would be known as Marie Curie and would win a Nobel prize for proving that some kinds of atoms are radioactive, emitting mysterious rays. In eight more years, her scientific research would earn her a second Nobel, making her the first of only four people to win the award twice. And she would raise two daughters, one of whom would go on to win a Nobel of her own. Along the way, Curie would become one of the most celebrated scientists in the world, within her field and among ordinary people. All of that success turned on a risky decision that she made in 1897, two years after marrying Pierre Curie, a French physicist, and just two months after giving birth to their first daughter. At the time, Marie Curie was working toward a doctorate in physics at the Sorbonne, France's most prestigious university. She needed to choose a research project for her dissertation. As a neophyte, Curie could have played it safe and studied X-rays. They had been discovered just two years earlier and were wildly popular with researchers. Instead, Curie boldly chose Becquerel rays, which most scientists ignored. While looking for X-ray sources, the French physicist Henri Becquerel had found that uranium compounds emitted only a weaker ray. Curie decided to search for other substances that emitted these rays, a phenomenon she soon would dub "radioactivity." Within six years, Curie's findings had wowed her dissertation committee, won her and husband Pierre the 1903 Nobel prize in physics (shared with Becquerel) and put Marie on course for a solo Nobel prize in chemistry in 1911. The first Nobel was for the couple's work on the nature of radioactivity; the second was for Marie's isolation of two radioactive elements: radium, which quickly came into use as a cancer treatment, and polonium. The new understanding of radioactivity also set the stage for the Curies' daughter, Irene Joliot-Curie, and her husband, Frederic Joliot-Curie, to share the 1935 Nobel prize in chemistry. "The greatest scientific deed of her life -- proving the existence of radioactive elements and isolating them," Curie's friend Albert Einstein said, "owes its accomplishment not merely to bold intuition but to a devotion and tenacity in execution under the most extreme hardships imaginable, such as the history of science has not often witnessed." In an age when the male-dominated scientific establishment made it clear that women were not welcome, Marie Curie entered the vanguard of physicists and chemists who were changing the world's ideas about the nature of atoms. She showed, for example, that radioactivity could signal the presence of previously unknown kinds of atoms. She also made a brilliant and revolutionary hypothesis that other researchers proved true. She suggested that radioactivity was atomic, not chemical. In other words, she said the rays emanating from substances she studied were created by changes within individual atoms and not by chemical reactions between atoms. In Curie's lifetime, scientists went from disagreeing about the reality of atoms to understanding their details, such as that the nucleus inside an atom can be divided into smaller parts. And they discovered that some atoms will release some of the protons and neutrons that make up their nuclei, giving off energy -- in the three forms of radioactivity called alpha, beta and gamma rays. When atoms do this, they "decay" into different elements. For example, when uranium-238 decays, it emits an alpha particle made of two neutrons and two protons and becomes an atom of thorium. As the decay process continues, all original uranium ultimately transmutes to lead, which is not radioactive and persists unchanged. Understanding these seemingly esoteric ideas about the inner life of atoms has made it possible to measure the age of fossils and rocks, create radioactive tracers that let scientists follow the movement of molecules in living things, design cancer treatments using tumor-destroying radiation, develop nuclear power and invent the atomic and hydrogen bombs. A HARD-FOUGHT ADVANCE "No one should think that Marie Curie's life was an easy, triumphal march," says her biographer, Susan Quinn, who believes that Curie displayed great courage in ignoring stereotypes about women to pursue a scientific career. "Think how brave she was traveling alone by train from Poland to Paris to go to the Sorbonne," Quinn says. "At that time, women alone were generally considered prostitutes. Her decision to live alone in the Latin Quarter was daring." Curie worked hard when she conducted research. Under conditions that no regulatory agency would allow today and no modern scientist would accept, Curie shoveled tons of uranium ore that had been delivered from a mine to her lab and would be purified to obtain radioactive materials weighing less than one-tenth of a gram. The shed in which she worked was poorly ventilated in summer and freezing in winter. "Students today have a hard time understanding the sheer physical burden," says Harold Goldwhite, a chemist and an editor of Creations of Fire, a history of chemistry. "It was backbreaking, tedious scut work to get what she needed for her analytical work. She had to have amazing strength." Indeed, overwork and long-term exposure to radioactive materials ultimately destroyed her health. Curie lost her husband and most important scientific collaborator in 1906, when he was run over by a horse-drawn wagon. Then 38, she wrote, "I lost my beloved Pierre, and with him all hope and all support for the rest of my life." To feed herself and two small daughters, Curie took over Pierre's Sorbonne faculty position, making headlines in the Paris newspapers. Artists, journalists and fashionable ladies joined students Nov. 5, 1906, to hear the first woman on the Sorbonne faculty give a lecture. "I think that the single bravest moment of Curie's life was when she told the Nobel committee that she would come to Sweden to accept the prize in 1911," Quinn says. When the prize was announced, French newspapers were bursting with stories that the widowed Curie had been having an affair with a married physicist, Paul Langevin, and scandal did not sit well with the Swedes. One xenophobic scandal sheet, called l'Oeuvre, even published excerpts of a letter Curie had written to Langevin. Its editor, Gustave Tery, attacked Curie for being emancipated, foreign and intellectual and "for pushing the father of a family to destroy his home." Tery's articles led Langevin to challenge him to a duel. Neither was hurt. Other members of the fractious French press took sides for or against Curie and also dueled. The Nobel committee abhorred such publicity. Svante Arrhenius, a Swedish Academy of Sciences member, wrote to Curie suggesting that she not accept the prize until her name was cleared. Curie disagreed and wrote back that "the prize has been awarded for the discovery of radium and polonium. I believe that there is no connection between my scientific work and the facts of private life." STRENGTH OF CHARACTER What gave Curie such determination? Quite possibly, it was her family and the circumstances of her youth. Curie was born Nov. 7, 1867, into a family that was part of the Polish intelligentsia fighting rule by czarist Russia. Some family members and friends were exiled to Siberia for their activities, and, as a young woman, Maria risked deportation for teaching peasant children to read in their native, forbidden Polish. At home in Warsaw, Maria's parents taught their five children illegal songs and read them works by banned Polish writers. At school, Curie's teachers switched swiftly from Polish to Russian whenever a government official appeared. Years later, she recalled that children "knew that a single conversation in Polish, or an imprudent word, might seriously harm not only themselves but also their families." Always a brilliant student, Maria initially hoped to attend university in France, but her family's financial problems kept her in Poland after secondary school. Undaunted, in the early 1880s, she participated in Warsaw's Flying University, an underground women's school. Women attending the university met secretly in homes and other clandestine locations to learn from Polish scholars out of sight of the Russian rulers. Meanwhile, Maria and her older sister, Bronia, schemed about how each could obtain a Sorbonne education: Marie would work as a governess in the hinterlands to support Bronia's medical schooling, and Bronia would return the favor when she finished her studies. NATURAL COLLABORATORS Maria became Marie when she moved to Paris. Ignoring the city's distractions, she soared academically. Initially, she planned to return to Poland, hoping to use her education to help her country. When she met Pierre in 1894, Marie was seeking lab space, and neither was looking for romance. But Pierre, who once had written that "women of genius are rare," was struck by the ambitious student. He used every rational argument he could muster to persuade her that they should pursue their scientific interests together. Pierre, eight years older and already an established scientist, had worked on the magnetic properties of various substances and designed scientific instruments that drew the attention of the leading English scientist, Lord Kelvin. Pierre and his brother, Jacques, had studied how heated mineral crystals behave and discovered what now is known as the piezoelectric effect: When certain crystals are squeezed, they produce an electric current. Pierre invented a device to measure piezoelectricity that later proved important for Marie's studies. One reason that the Curies succeeded as collaborators was because "Pierre was Pierre," says Helena Pycior, a science historian at the University of Wisconsin-Milwaukee. In Creative Couples in the Sciences, she wrote, "To Pierre, collaboration with a family member was a natural way of practicing science, since from his earliest years science had been a family affair."

The two researchers maintained a balance of skills and temperament. Pierre was more the physicist, Marie the chemist.

"Pierre was a slow thinker who framed scientific conclusions soberly . . . . Marie moved quickly from experiments to bold, published hypotheses," according to Pycior.

Pierre was scrupulously fair about giving Marie credit for her work. In their joint papers, who did what is stated clearly. When Pierre learned that Marie had not been nominated with him for the 1903 Nobel prize, he wrote to a Swedish Academy of Sciences member, "I very much wish to be considered together with Madame Curie with respect to our research on radioactive bodies." "In what appears to have been a concerted effort to deprive her of the prize," Quinn wrote in Marie Curie: A Life, four members of France's Academy of Sciences wrote a nominating letter that "completely ignored Marie Curie's contribution." THE EARLY EXPERIMENTS Becquerel, who shared the 1903 Nobel prize with the Curies, was interested in phosphorescence, in which certain materials absorb light and then glow. Because X-rays caused some materials to glow, Becquerel decided to test whether phosphor- escent substances also emitted X-rays. During his experiments, Becquerel stored a wrapped photographic plate in a dark closet along with a copper cross and some uranium salts, which he thought (wrongly, it turned out) were phosphorescent. When he developed the plate, Becquerel found the white image of a cross on a dark background. Because this exposure occurred without sunlight, Becquerel deduced that the uranium salts must have produced rays that penetrated the wrapping and exposed the plate, except where the cross had blocked them. Thus, Becquerel discovered radioactivity, but he misunderstood its nature, thinking it a form of phosphorescence. Not for several more years would scientists show that radioactivity is governed by what happens inside the atomic nucleus, while phosphorescence is caused by the activity of electrons outside the nucleus. Marie Curie asked a simple question: Does anything other than uranium salts emit these newfound rays? From experiments of Becquerel and Lord Kelvin, she knew that the rays caused air to conduct electricity. Thus, to determine the rays' presence, she used Pierre's piezoelectric instrument to measure tiny amounts of electricity. Marie tested various substances, such as gold and copper, but found nothing conclusive until she tried pitchblende, a black, pitchy natural substance mined as uranium ore. Pitchblende was more active than other uranium compounds or even pure uranium. A thorium-containing mineral also emitted rays, evidence that the phenomenon was not limited to uranium compounds. Recognizing that Marie's findings were important, Pierre put aside his research to provide his physics expertise. The pitchblende work was presented to the French academy in April 1898. In her subsequent published paper, Curie assumed rightly that the activity she saw was caused by elements unknown to science. After years of complicated chemical processing, she found them: polonium, which she named for her beloved Poland, and radium, named for its ability to emit rays. By the time Marie defended her doctoral dissertation in 1903, the physics world was abuzz with talk of transmutation. Ernest Rutherford, the great New Zealand-born physicist, had found alpha and beta rays. Rutherford and Frederick Soddy, a British chemist, showed that atoms were not always stable and that transmutation of one unstable element into another explained radioactivity. As radioactive elements go through transmutation, they spontaneously release various combinations of nuclear components until stability is reached and radioactivity is lost. In a poetic coincidence, the Curies met Rutherford at a dinner party in Paris the night Marie was awarded her doctorate. Rutherford, who became a lifelong friend and competitor of Marie's, saw Pierre take out a tube of glowing radium. "The luminosity was brilliant in the darkness, and it was a splendid finale to an unforgettable day," Rutherford recalled. Despite Curie's love of the lab, she was never able to spend as much time there as she wanted. One distraction was World War I, during which her humanitarian interests took her to the French battle front to teach skeptical military doctors how to use X-rays. After the war, she traveled to raise money for the radium institutes she created in Paris and Warsaw. Missy Meloney, editor of one of the largest American women's magazines, promoted Curie's cause so strongly in the United States that hundreds of thousands of dollars were raised in this country to buy the extremely costly radium. When Curie died July 4, 1934, of leukemia induced by decades of exposure to radioactive materials, her scientific legacy had been established. Her Institut de Radium in Paris was a vibrant radioactivity research center, and key discoveries had been made there under her leadership. Marguerite Perey, who became the first female member of France's Academy of Sciences, discovered the element francium in Curie's lab. Curie herself was never elected to the academy, one of the few scientific honors denied her. Some of the last work Curie oversaw at the Institut de Radium was that of her daughter and her husband, Irene and Frederic Joliot-Curie. The couple created artificial radioactivity in 1934 by bombarding aluminum, a stable element, with alpha particles to produce a radioactive version of phosphorus that does not occur naturally. For this, they received a Nobel the next year. The evident ease with which atoms could be manipulated, and even split, led Frederic Joliot-Curie to a stunning realization. "Scientists, building up or shattering elements at will," he wrote, "will be able to bring about transmutations of an explosive type." The "atomic age" that Marie Curie helped to build was about to dawn. Anna Maria Gillis is a freelance science writer in Bethesda. CAPTION: Marie Curie in 1921. CAPTION: Missy Meloney, left, editor of an American women's magazine that championed Marie Curie, poses with Curie, flanked by her daughters, Irene, left, and "Petite." CAPTION: Marie Curie gives one of her last lectures at the Conservatoire des Arts et Metiers de Paris. She stands behind a bench equipped for scientific demonstrations. Only a scattering of women's faces is visible in this undated photograph. CAPTION: Marie Curie in the Paris laboratory where she discovered the nature of radioactivity and the elements, polonium and radium. Photo is undated.