Astronomer Edwin Hubble

During the past 100 years, astronomers have discovered quasars, pulsars, black holes and planets orbiting distant suns. But all these pale next to the discoveries Edwin Hubble made in a few remarkable years in the 1920s. At the time, most of his colleagues believed the Milky Way galaxy, a swirling collection of stars a few hundred thousand light-years across, made up the entire cosmos. But peering deep into space from the chilly summit of Mount Wilson, in Southern California, Hubble realized that the Milky Way is just one of millions of galaxies that dot an incomparably larger setting.

Hubble went on to trump even that achievement by showing that this galaxy-studded cosmos is expanding–inflating majestically like an unimaginably gigantic balloon–a finding that prompted Albert Einstein to acknowledge and retract what he called “the greatest blunder of my life.” Hubble did nothing less, in short, than invent the idea of the universe and then provide the first evidence for the Big Bang theory, which describes the birth and evolution of the universe. He discovered the cosmos, and in doing so founded the science of cosmology.

Hubble’s astronomical triumphs earned him worldwide scientific honors and made him the toast of Hollywood during the 1930s and 1940s–the confidant of Aldous Huxley and a friend to Charlie Chaplin, Helen Hayes and William Randolph Hearst. Yet nobody (except perhaps Hubble) could have imagined such a future when the 23-year-old Oxford graduate began his first job, in New Albany, Ind., in 1913.

Hubble majored in science as an undergraduate at the University of Chicago. A tall, powerfully built young man, he excelled at basketball and boxing (fight promoters reportedly tried to talk him into turning pro), and his combination of academic and athletic prowess earned him a Rhodes scholarship to Oxford. In England, Hubble kept up his muscular pursuits: he fought, ran track and played on one of the first baseball teams ever organized in the British Isles.

His official academic focus shifted, thanks to a promise made to his dying father that he would study law rather than science (he also took up literature and Spanish). On his return to America, he took a position as a high school Spanish teacher. Though he was popular with students–especially, according to Hubble biographer Gale Christianson, with the girls, who were evidently charmed by his affected British diction and “Oxford mannerisms”–Hubble longed to return to science.

After a year, he signed on as a graduate student at Yerkes Observatory in Wisconsin and embarked on the work that would one day make him famous: studying faint, hazy blobs of light called nebulae (from the Latin word for cloud) that are visible through even a modest telescope.

Hubble’s skills as an astronomer were impressive enough to earn him an offer from the prestigious Mount Wilson Observatory. World War I kept him from accepting right away, but in 1919 the newly discharged Major Hubble–as he invariably introduced himself–arrived at observatory headquarters, still in uniform but ready to start observing with the just completed 100-in. Hooker Telescope, the most powerful on earth.

Up on the mountain, Hubble encountered his greatest scientific rival, Harlow Shapley, who had already made his reputation by measuring the size of the Milky Way. Using bright stars called Cepheid variables as standardized light sources, he had gauged the galaxy as being an astounding 300,000 light-years across–10 times as big as anyone had thought. Yet Shapley claimed that the Milky Way was the whole cosmic ball of wax. The luminous nebulae were, he insisted, just what they looked like: clouds of glowing gas that were relatively nearby.

Hubble wasn’t so sure. And in 1924, three years after Shapley departed to take over the Harvard Observatory, Hubble found proof to the contrary. Spotting a Cepheid variable star in the Andromeda nebula, Hubble used Shapley’s technique to show that the nebula was nearly a million light-years away, far beyond the bounds of the Milky Way. It’s now known to be the full-fledged galaxy closest to our own in a universe that contains tens of billions of galaxies. “I do not know,” Shapley wrote Hubble in a letter quoted by biographer Christianson, “whether I am sorry or glad to see this break in the nebular problem. Perhaps both.” (Hubble was not entirely magnanimous in victory. To the end he insisted on using the term nebulae instead of Shapley’s preferred galaxies.)

Hubble’s scientific reputation was made almost overnight by his discovery that the universe is vast and the Milky Way insignificant. But he had already moved on to a new problem. For years, astronomers had noted that light from the nebulae was redder than it should be. The most likely cause of this so-called red shifting was motion away from the observer. (The same sort of thing happens with sound: a police car’s siren seems to drop in pitch abruptly as the car races past a listener.)

Hubble and his assistant, Milton Humason, began measuring the distances to these receding nebulae and found what is now known as Hubble’s Law: the farther away a galaxy is from Earth, the faster it’s racing away. Could it be that the universe as a whole is rapidly expanding? That conclusion was extraordinary, almost mind-blowing, yet seemed inescapable.

When Einstein heard of Hubble’s discovery, he was elated. More than a decade earlier, his new general theory of relativity had told him that the universe must either be expanding or contracting, yet astronomers had told him it was doing neither. Against his better judgment, Einstein had uglied up his elegant equations with an extra factor he called the cosmological term–a sort of antigravity force that kept the universe from collapsing in on itself.

But suddenly, the cosmological term was unnecessary. Einstein’s instincts had been right, after all. His great blunder had been to doubt himself, and in 1931, during a visit to Caltech, the great and grateful physicist traveled to the top of Mount Wilson to see the telescope and thank Hubble personally for delivering him from folly.

With the greatest scientific superstar of the age paying him homage, Hubble became a popular superstar in his own right. His 1936 book on his discoveries, The Realm of the Nebulae, cemented his public reputation. Tourists and Hollywood luminaries alike would drive up the mountain to marvel at the observatory where Hubble had discovered the universe, and he and his wife Grace were embraced by the elite of California society.

Hubble’s last great contribution to astronomy was a central role in the design and construction of the Hale Telescope on Palomar Mountain. Four times as powerful as the Hooker, the Hale would be the largest telescope on Earth for four decades. It would have been even longer, but its completion was interrupted by World War II. So was Hubble’s career. The ex-major signed on as head of ballistics at Aberdeen Proving Ground in Maryland. (At one point the eminent astronomer spent an afternoon test-firing bazookas, at great personal risk, to pinpoint a design flaw.)

Hubble finally got his hands on the Hale when it went into service in 1949. It was too late; he had suffered a major heart attack, and he never fully regained the stamina it took to spend all night in a freezing-cold observatory. No imaginable discovery, however, could have added to his reputation.

The only recognition that eluded him was a Nobel Prize–and not for lack of effort on his part. He tried everything. In the late 1940s he even hired a publicity agent to promote his cause. Alas, there was no prize for astronomy, and by the time the Nobel committee decided astronomy could be viewed as a branch of physics, it was too late. Insiders say Hubble was on the verge of winning when he died, in 1953.

Hubble would have been consoled by the fact that his name adorns the Hubble Space Telescope, which probes the cosmos to depths he could not have imagined but would have fully appreciated. Whatever marvels the Hubble telescope reveals, they’re all played out on the stage Edwin Hubble first glimpsed from a lonely mountaintop in California.

TIME senior writer Michael D. Lemonick is the author of Other Worlds: The Search for Life in the Universe