The Discovery of Moving Galaxies


By Emily LevesqueUniversity of Washington

Vesto Slipher, an astronomer working at Lowell Observatory in Flagstaff, Arizona, observed the spectra of mysterious spiral nebulae in 1912, well before astronomers had established that they were actually external moving galaxies. Slipher became the first person to discover galactic redshifts, finding, to his surprise, that almost all of the spiral nebulae he observed were speeding away from Earth.

An illustration of the concept of expansion of the universe
Georges Lemaître proposed the idea that the universe’s expansion had to have started from a primordial point. (Image: Andrea Danti/Shutterstock)

Milton Humason’s Rise to the Stars

Edwin Hubble used observations of Cepheids to determine galaxy distances, and observations of redshifts to determine these same galaxies’ velocities. Measuring galactic velocities proved to be exceptionally difficult, some of the most challenging observations of the era that pushed the limits of existing telescopes. In the end, most of these observations were carried out by a man named Milton Humason.

Humason’s first job at Mount Wilson involved trekking equipment up the mountain while the observatory was being built; he later worked there as a busboy and a janitor. His curiosity, intellect, and love of the mountains drove him to begin asking questions about the work done by the observatory’s night assistants. 

Night assistants were permanent observatory staff members who lived year-round on the mountain and specialized in the technical tasks of observing—operating the telescope, preparing and sometimes loading the photographic plates, and assisting astronomers with the physical side of their observations. 

Humason proved to be a top-notch night assistant, and Hubble eventually hired him as a night assistant at the largest telescope at Mount Wilson, the 100-inch. Eventually, Humason was carrying out most of the observations for Hubble’s program while Hubble analyzed the plates.

Workers working on the telescope
Milton Humason carried out his observation using the largest telescope at Mount Wilson, the 100-inch one. (Image: George Ellery Hale/Public domain)

Centering on Moving Galaxies

One job of an observer in those days involved guiding the telescope. Guiding means peering through a small eyepiece and adjusting the position of the telescope bit by microscopic bit to keep it perfectly positioned on the right patch of sky. When taking spectroscopic data, this meant keeping a faint star or galaxy perfectly centered for hours at a time, often during freezing cold nights. 

Humason eventually got so good at this that he could capture reliable spectra of galaxies so dim that they were invisible to the human eye. He developed a tactic of centering the telescope on a nearby bright star and then moving it by a carefully measured amount, based on old photographic plate images, until he knew the unseen galaxy must be centered in the telescope. 

His eye may not have been able to collect enough light to see these galaxies, but the huge mirror of the telescope could, and he often emerged from his observations with sharp and clear spectra of impossibly faint—and distant, and fast-moving—galaxies.

This article comes directly from content in the video series Great Heroes and Discoveries of AstronomyWatch it now, on Wondrium.

Einstein’s Research

Hubble ultimately combined the galaxy velocities from Humason’s observations with Slipher’s published results when devising his law describing the expansion of the universe. However, Hubble was far from the only astronomer thinking about the expansion of the universe.

Theoretical research had already begun to hint at the idea of an expanding universe. The most groundbreaking theory of the past 10 years had unarguably been Albert Einstein’s elegant series of equations describing the relationship between gravity, space, and time. 

Other physicists working with these equations soon realized that they implied that the universe was expanding, and that this expansion might even be measurable. Einstein himself recognized this as a possibility, though he disliked the idea. In the mid-1920s, he was working on a correction to his equations—what he referred to as a ‘cosmological constant’—since he thought that the idea of a universe expanding indefinitely sounded implausible.

Image of Georges Lemaître
Belgian priest and astronomer Georges Lemaître’s wrote a paper connecting the motion of these galaxies with the possibility of an expanding universe. (Image: Unknown/Public domain)

Georges Lemaître

One person interested in both Einstein’s discovery and Slipher’s new observations was Belgian priest and astronomer Georges Lemaître. After hearing about Slipher’s strange measurements about newly discovered galaxies speeding away from our own, Lemaître wrote a paper connecting the motion of these galaxies with the possibility of an expanding universe. 

His paper was published in 1927, in an obscure Belgian journal and was largely ignored by other astronomers for a few years. This proved especially disappointing since his paper contained an early derivation of the same equation that made Hubble famous just two years later.

Hubble’s 1929 paper had reported and plotted a linear velocity-distance relation, based on observations of 24 galaxies and described by a clear and simple equation.

Today, Lemaître’s name may no longer be as famous as Hubble’s, but the physics community in 1931 recognized the importance and impact of what he’d done. 

Common Questions about the Discovery of Moving Galaxies

Q: Why did Edwin Hubble hire Milton Humason?

Milton Humason had shown his talent and curiosity, so Edwin Hubble hired him as a night assistant.

Q: Why was Albert Einstein trying to correct his own equations in the mid-1920s?

Though Albert Einstein recognized that moving galaxies and space expanding is a possibility, he also thought that the universe expanding indefinitely was implausible. He was in search of what he called the cosmological constant.

Q: Who was Georges Lemaître?

Georges Lemaître was a Belgian priest and astronomer. After hearing about Slipher’s strange measurements about newly discovered galaxies speeding away from our own, Lemaître wrote a paper connecting the motion of these galaxies with the possibility of an expanding universe. 

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