In 1929, an astronomer named Edwin Hubble made a startling observation from a mountaintop observatory in southern California. Based on his readings and calculations, he concluded that the further a galaxy is from us, the faster it appears to be receding into space.

Ultimately, his work, based on the findings of previous scientists, including Harvard Observatory "human computer" Henrietta Swan Leavitt, demonstrated a relationship between the speed of cosmic objects and their distance from Earth. The finding amounted to groundbreaking evidence that the universe is expanding.

Hubble began his post-graduate career at southern California's Mount Wilson Observatory in 1919, an exciting time in the field, according to Vassar College assistant professor of astronomy Ed Buie.

"There were a bunch of newly commissioned telescopes in California, and folks were starting to get involved in really ramping up their efforts as far as observing extragalactic sources, things that are beyond our galaxy," says Professor Buie.

The catch is that many stargazers weren't convinced that anything existed beyond the "island universe" of the local galaxy, then estimated to measure a maximum of 300,000 light-years in diameter. The lack of clarity was highlighted by what became known as the Great Debate of 1920 between astronomers Harlow Shapley and Heber Curtis. Shapley insisted our own known galaxy was large enough to encompass all observable phenomena. Curtis, meanwhile, argued that other galaxies existed beyond our own Milky Way.

Wading into the issue, Hubble trained Mount Wilson's 100-inch telescope on the Andromeda "nebula"—a term then used to define any of the non-identifiable clouds of light in the sky—with the hope of finding an extraterrestrial marker on which to pinpoint his studies.

Such “standard candles” were known to exist, thanks to the decade-old work of Harvard College Observatory’s Henrietta Swan Leavitt, who catalogued a category of stars that waxed and waned in brightness, known as Cepheid variables. Leavitt discovered a direct relationship between a Cepheid's luminosity and its waxing and waning period: A brighter star will go through a longer period—a relationship now known as Leavitt's Law.

Others soon realized that by calculating the distance to nearby Cepheids and correlating those numbers to their apparent magnitudes, the distance to any such star could be determined by its period of luminosity.

"The pulsation of the Cepheids is tightly related to how far they are away from us," says Buie. "And because you have this tight relationship between how these stars are pulsating and how far away they are, you can use that to plot this relationship out for fairly distant evolved stars."

In the fall of 1923, Hubble identified the first Cepheid in the Andromeda nebula. After locating a few more of these standard candles, and determining their magnitudes from their pulsation periods, he calculated Andromeda to be nearly 1 million light-years away—well beyond even the most generous estimates of our galaxy's reach.

Although Hubble didn't officially report his findings for another year, his peers quickly learned of his discoveries and realized the debate about the existence of other galaxies was settled: They were there. Upon receiving a letter detailing Hubble's work, Shapley reportedly told a colleague, "Here is the letter that destroyed my universe."

A few years later, Hubble zeroed-in on another matter of great interest in his field.

Around 1912, American astronomer Vesto Slipher began attempts to calculate the "radial velocity"—the speed of an object through space relative to the viewer—of Andromeda and other nebulae. His method lay in examining the readouts of nebulae wavelengths along the visible light spectrum and determining how far the wavelengths shifted compared to the expected results of its component elements.

"The best way to think of this is if you've ever heard an ambulance or a fire truck approaching, and as it's approaching the pitch of the alarm starts to rise and then as it zooms past you the pitch starts to fade away," explains Professor Buie.

"A similar thing happens with light: If a photon is coming toward you, its wavelength will be smushed to smaller wavelengths, so it will appear bluer. And if it is traveling away from you, then its wavelength will be stretched and it will appear redder."

Slipher largely found the light wavelengths shifted to the red end of the spectrum, indicating that these nebulae were moving away from us at speeds from several hundred to more than 1,000 kilometers per second. This marked some of the earliest evidence of the expansion of the universe, although astronomers and mathematicians spent the next decade-plus trying to demonstrate empirical proof of this theory.

As described in Edwin Hubble, The Discoverer of the Big Bang Universe, Hubble in 1928 recruited Mount Wilson assistant Milton Humason to record the “redshifting” of more distant nebulae while he set to work calculating the distances of these clusters.

The following year, Hubble published his results in the paper "A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae." Although he cautioned that more research was needed, the paper caused a stir by announcing a "linear correlation between distances and velocities."

Hubble’s data demonstrated that the outward velocity of an object in space was equal to the distance from Earth times a constant of proportionality, now known as the Hubble constant, which he set at 500 kilometers per second per megaparsec (3.09 x 1022 meters). In other words, a galaxy located one megaparsec from Earth would be receding at a rate of 500 km/s, while another galaxy 10 megaparsecs from Earth would be clipping along at 10 times that speed.

Two years later Hubble and Humason published a follow-up paper, "The Velocity-Distance Relation among Extra-Galactic Nebulae." Having examined even more distant clusters, which revealed velocities that approached 20,000 km/s, the pair confirmed the conclusion that was already transforming the field: The farther an object is, the faster it appears to be moving away.

Despite coming up with numbers that showed remote galaxies becoming ever more remote, Hubble resisted settling on a definitive cause for this phenomenon. It took later generations of observers to confirm an expanding universe was behind the observation.

He was also off with some of his calculations. For example, current measurements estimate the Andromeda galaxy to be about 2.5 million light-years away, not 1 million, as Hubble had originally calculated. And the exact figure of the Hubble constant is in the range of 70 km/s/mpc, not 500, as he had proposed in his 1929 paper.

Hubble's status as the first to discover the velocity-distance relation has also been called into question, with Belgian priest and mathematician Georges Lemaitre now known to have arrived at a similar conclusion in a then little-seen 1927 paper.

Nevertheless, Hubble's impact on the field was monumental for those who followed his well-publicized footsteps.

"He almost created a whole new branch of astronomy, extragalactic astronomy, because now there is a lot of interest as far as trying to better understand what this universal expansion is, what it looks like, and to better quantify that," says Professor Buie. “After him you have people that are trying to measure ever more distant objects, and all of these things help to better understand this expansion rate of the universe."

Hubble’s work enabled successors to make the imaginative leaps that address some of our most vexing questions about space, including the age of the universe. His legacy inspired the powerful Hubble Space Telescope that went into orbit in 1990. The telescope named in honor of the groundbreaking astronomer has since provided some of the most iconic images of the universe.