Beyond the Milky Way: Expanding the Cosmic Horizon

For most of human history, the Milky Way was the universe. Every star, every nebula, every faint smudge of light in the night sky was assumed to belong to one single cosmic system. Then, in the span of just a few decades in the early 20th century, a series of remarkable discoveries shattered that assumption entirely. Two astronomers – Harlow Shapley and Edwin Hubble – reshaped our understanding of where we are and how large the cosmos truly is. Their work turned the Milky Way from “everything” into just one galaxy among billions.

The universe before the 20th century 🔗

Before the 1920s, most astronomers believed that the Milky Way constituted the entire universe. Fuzzy patches of light visible through telescopes – called nebulae – were widely thought to be clouds of gas or dust within our galaxy. The idea that any of these objects might be separate star systems, comparable in size to the Milky Way, seemed far-fetched. The prevailing model, shaped largely by Dutch astronomer Jacobus Kapteyn, placed the Sun near the centre of a relatively small galaxy, perhaps 30,000 light-years across. This comfortable picture was about to be upended.

Henrietta Leavitt and the cosmic yardstick 🔗

No discussion of early 20th-century cosmology is complete without Henrietta Swan Leavitt. Working at the Harvard College Observatory as a “computer” – a person tasked with examining photographic plates – Leavitt made one of the most consequential discoveries in the history of astronomy. In 1908, she noticed that a particular class of pulsating stars called Cepheid variables displayed a clear pattern: the brighter a Cepheid was, the longer its pulsation period. She published her findings more fully in 1912, establishing what is now known as the period-luminosity relation, or Leavitt’s Law.

The significance of this relationship was profound. If you could measure a Cepheid’s pulsation period, you could calculate its true brightness. Comparing that true brightness to how bright the star appeared from Earth then gave you its distance. For the first time, astronomers had a reliable “standard candle” – a tool to measure distances far beyond what stellar parallax could achieve. Leavitt’s discovery gave Shapley, Hubble, and others the means to measure the Milky Way and, eventually, the space beyond it.

Harlow Shapley: remapping the Milky Way 🔗

Harlow Shapley began his astronomical career almost by accident. Born on a farm in Missouri in 1885, he originally intended to study journalism at the University of Missouri but found the journalism school had not yet opened. He chose astronomy instead and went on to earn a PhD from Princeton under Henry Norris Russell. By 1914, he was at Mount Wilson Observatory in California, working with one of the most powerful telescopes in the world.

Using globular clusters to measure the galaxy 🔗

Shapley turned his attention to globular clusters – dense, spherical collections of thousands of stars that orbit the Milky Way. He applied Leavitt’s period-luminosity relation to the Cepheid variables found within these clusters, calculating their distances with far greater precision than anyone had managed before. He noticed something striking: the globular clusters were not evenly distributed around us. They were concentrated in one part of the sky, forming a loose halo around a central point that was tens of thousands of light-years away from Earth.

Shapley concluded that this central point was the true centre of the Milky Way and that the Sun was located far from it – roughly 50,000 light-years away (later refined to about 26,000 light-years). He also estimated the galaxy to be about 300,000 light-years in diameter, a figure that was too large but still far bigger than anyone had previously accepted. While his size estimate was later corrected to about 100,000 light-years, the core insight held: the Sun is not at the centre of the galaxy. This was a major extension of the Copernican principle – the idea that Earth holds no special or central position in the cosmos.

Shapley’s error about spiral nebulae 🔗

Despite getting the big picture of the Milky Way right, Shapley made one major mistake. He believed that his enlarged Milky Way was essentially the whole universe. The so-called “spiral nebulae” – objects like the Andromeda Nebula – were, in his view, relatively small gas clouds located within the galaxy’s boundaries. He partly relied on measurements by astronomer Adriaan van Maanen, who claimed to have detected the rotation of a spiral nebula. If those measurements were correct, the nebulae would have to be nearby objects; otherwise, their edges would be moving faster than the speed of light. Those measurements, as it turned out, were wrong.

The Great Debate of 1920 🔗

The question of whether the universe extended beyond the Milky Way came to a head on April 26, 1920, at the National Academy of Sciences in Washington, D.C. In what became known as the Great Debate, Shapley faced off against astronomer Heber Curtis of the Lick Observatory.

Shapley argued that spiral nebulae were part of the Milky Way, which he described as an enormous system encompassing everything visible. Curtis took the opposing view: that spiral nebulae were independent “island universes” – galaxies comparable to our own, located at vast distances. Curtis pointed to several lines of evidence, including the distinct spectral signatures of spiral nebulae, their high velocities of recession, and the fact that they were never seen along the plane of the Milky Way (a feature now explained by the galaxy’s own dust blocking our view).

Neither man had data strong enough to settle the argument definitively. Shapley was correct about the galaxy’s large size and the Sun’s off-centre location. Curtis was correct that spiral nebulae were external galaxies. As the American Physical Society has noted, both men were partly right and partly wrong – and the outcome was essentially a draw at the time. The resolution would come only a few years later.

Edwin Hubble: proving the existence of other galaxies 🔗

Edwin Hubble arrived at Mount Wilson Observatory in 1919, just as the 100-inch Hooker Telescope – the world’s most powerful – became operational. Armed with this instrument, Hubble set out to settle the nebula question once and for all.

Cepheids in Andromeda 🔗

In 1923, Hubble trained the Hooker Telescope on the Andromeda Nebula and resolved individual stars within it. Among them, he identified Cepheid variable stars. Using Leavitt’s period-luminosity relation, he calculated the distance to Andromeda and found it to be roughly one million light-years away – far beyond the outermost boundaries of even Shapley’s enlarged Milky Way. (Modern measurements place Andromeda at about 2.5 million light-years.)

Hubble announced his findings on January 1, 1925, at a meeting of the American Astronomical Society. The implications were staggering: Andromeda was not a gas cloud within our galaxy. It was an entirely separate galaxy, a vast island of stars in its own right. The Milky Way was suddenly no longer the totality of the cosmos but merely one galaxy among many. When Shapley received a letter outlining Hubble’s evidence, he reportedly told a colleague that it was the letter that destroyed his universe. To his credit, Shapley accepted the new evidence and shifted his position.

The expanding universe 🔗

Proving the existence of other galaxies was transformative enough. But Hubble’s next discovery was even more radical.

Redshift and recession 🔗

Building on earlier work by astronomer Vesto Slipher, who had measured the light from distant nebulae and found that most of it was shifted toward the red end of the spectrum – a phenomenon known as redshift – Hubble set out to connect galaxy distances with their speeds of recession. By 1929, after measuring distances and redshifts for dozens of galaxies, he found a clear relationship: the farther away a galaxy is, the faster it is moving away from us.

This proportional relationship between distance and velocity is now known as Hubble’s Law (recently renamed the Hubble-Lemaître Law, acknowledging Belgian physicist Georges Lemaître, who had independently reached a similar conclusion in 1927). It was not that galaxies were flying through space away from some central point. Rather, space itself was expanding, carrying the galaxies along with it.

Einstein’s “greatest blunder” 🔗

The idea of an expanding universe had been lurking in the mathematics of Einstein’s general relativity for over a decade. Einstein’s own equations predicted that the universe should be either expanding or contracting. But Einstein, like most scientists of his era, believed in a static universe. To make his equations compatible with this belief, he introduced a fudge factor called the cosmological constant. When Hubble’s observations demonstrated that the universe was indeed expanding, Einstein reportedly called the cosmological constant the greatest mistake of his career. He visited Mount Wilson in 1931 to meet Hubble and publicly acknowledged that expansion was real.

The road to the Big Bang 🔗

If the universe is expanding, then running the clock backwards leads to a startling conclusion: at some point in the distant past, all matter and energy must have been compressed into an unimaginably small, hot, dense state. This insight, first articulated by Lemaître and later developed by physicists like George Gamow, eventually became the foundation of the Big Bang theory. Hubble’s observational evidence – showing that galaxies recede faster with distance – provided the first empirical support for this model of cosmic origins.

From one galaxy to trillions 🔗

The shift in understanding that occurred between roughly 1918 and 1929 is one of the most dramatic in the history of science. In little more than a decade, the known universe went from being a single galaxy, perhaps 30,000 light-years across, to a cosmos filled with billions of galaxies stretching across billions of light-years – and growing.

Modern estimates suggest there may be two trillion or more galaxies in the observable universe. Each of these contains hundreds of billions of stars. Our Sun, once thought to sit near the heart of everything, turned out to be an unremarkable star in the outskirts of one ordinary galaxy among countless others.

A philosophical shift 🔗

This transformation was not merely astronomical – it was deeply philosophical. It extended the Copernican revolution to its logical conclusion. Copernicus removed Earth from the centre of the solar system. Shapley removed the Sun from the centre of the galaxy. Hubble removed the galaxy from the centre of the universe. Each step displaced humanity further from any claim to a privileged cosmic position. And yet, each step also expanded the scope of what we could know, observe, and investigate. The universe became larger, stranger, and more awe-inspiring with every new measurement.

The legacy of Shapley and Hubble 🔗

Both Shapley and Hubble left behind legacies that extend far beyond their individual discoveries. Shapley went on to become director of the Harvard College Observatory, where he built one of the world’s leading astronomy programmes and championed public science education. He also studied the distribution of galaxies in the universe, contributing to early efforts at large-scale cosmic mapping.

Hubble’s name became synonymous with modern cosmology. The Hubble Space Telescope, launched in 1990, has carried his legacy forward by peering deeper into the universe than any ground-based telescope could, capturing images of galaxies billions of light-years away and helping to refine measurements of the universe’s expansion rate. In a fitting tribute, the telescope used Cepheid variables – the same tools Hubble and Leavitt relied on – to pin down the Hubble constant with unprecedented precision.

Their combined work also illustrates how science progresses. Shapley and Curtis each got important things right and important things wrong. Hubble built on earlier findings by Leavitt, Slipher, and Lemaître. No single person made the entire breakthrough alone. It was the accumulation of observations, arguments, errors, and corrections that ultimately reshaped our picture of the universe.

What it means for us today 🔗

We now live in a universe that is approximately 13.8 billion years old, expanding at an accelerating rate driven by a mysterious force called dark energy. The observable universe spans about 93 billion light-years in diameter and contains structures – galaxy clusters, filaments, and enormous voids – that were unimaginable a century ago. All of this traces back to the foundational work done in the 1920s, when a few determined astronomers looked through their telescopes and realised that everything they thought they knew about the size and nature of the cosmos was incomplete.

The story of how we moved from “our galaxy is the universe” to “our galaxy is one of trillions” is not just a story about telescopes and stars. It is a story about the willingness to follow evidence wherever it leads – even when it demolishes comfortable certainties.

What do you think? Does knowing that the universe contains trillions of galaxies change how you think about humanity’s place in the cosmos? And what might future discoveries – perhaps enabled by telescopes like the James Webb Space Telescope – reveal that could once again transform our understanding of the universe?

References

1. https://www.britannica.com/biography/Harlow-Shapley
2. https://science.nasa.gov/people/edwin-hubble/
3. https://www.britannica.com/biography/Henrietta-Swan-Leavitt
4. https://en.wikipedia.org/wiki/Period-luminosity_relation
5. https://fi.edu/en/news/case-files-harlow-shapley
6. https://en.wikipedia.org/wiki/Copernican_principle
7. https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
8. https://www.aps.org/publications/apsnews/200004/history.cfm
9. https://carnegiescience.edu/news/carnegie-science-celebrates-edwin-hubbles-discovery-universe
10. https://www.pnas.org/doi/10.1073/pnas.1424299112
11. https://science.nasa.gov/mission/hubble/science/science-highlights/discovering-a-runaway-universe/