At the beginning of the 20th century, astronomers were perplexed by the vast number of nebulous objects scattered across the night sky. French astronomer Charles Messier had cataloged over 100 such objects in the late 1700s, most of which were identified as star clusters, nebulae, supernova remnants, and glowing gases. However, 40 of these objects were mysterious whirlpools, collectively known as the spiral nebulae. Unlike the other Messier objects, they were scattered across the sky, which led to further confusion. Astronomer Vesto Slipher, using spectroscopy, discovered that the light from all of the spiral nebulae was redshifted, suggesting they were moving away from Earth.
Image credit: NASA, ESA, Hubble Heritage Project (STScI, AURA)
Edwin Hubble theorized that the spiral nebulae were extragalactic, as they were not confined to the plane of our galaxy. To verify this, he set out to measure the distances to stars within these nebulae, focusing on Cepheid variables—pulsating stars whose brightness correlates with their pulsation period, allowing astronomers to calculate distances. By measuring the distance to Cepheid V1 in Andromeda, Hubble settled the debate on whether these spiral nebulae were merely cosmic clouds or entire galaxies. He determined that Andromeda was over 2 million light-years away, which was 20 times the diameter of the Milky Way. This revelation, made 100 years ago, opened humanity’s eyes to the vastness of the universe, marking a pivotal moment in the understanding of space.
This discovery set the stage for a century of astronomical exploration with ever more powerful scientific tools, culminating in the Hubble Space Telescope, which has been key in measuring the age of the universe to be 13.8 billion years—partly through Cepheid star measurements.
Image credit: NASA, ESA, Hubble Heritage Project (STScI, AURA)
The most important star for humanity is the Sun, followed by one located in the Andromeda galaxy—though it’s 2.2 million light-years away and too faint to be seen with the naked eye. A century ago, Edwin Hubble’s discovery of this star, V1, opened the door to a much larger universe and revealed that the Milky Way galaxy was only one of hundreds of billions. This moment was a defining moment in humanity’s curiosity about its own origins. To mark this centennial, Carnegie Science and NASA celebrated Hubble's discovery at the 245th meeting of the American Astronomical Society in Washington, D.C.
Hubble, using the 100-inch Hooker Telescope at Mount Wilson Observatory in California, discovered V1 in 1923. The Cepheid variable, a type of pulsating star, served as a "milepost marker" to measure cosmic distances. By the early 20th century, astronomer Henrietta Swan Leavitt had shown that the pulsation period of Cepheid variables was directly tied to their luminosity, providing a method for measuring distances in space.
Many astronomers, including Harlow Shapley, believed that the Milky Way’s edge marked the universe’s edge. However, Hubble determined that V1, located within the Andromeda "nebula," was far beyond the Milky Way, proving that the universe extends far beyond our galaxy. In a moment of realization, Hubble scribbled an exclamation mark on the photographic plate of Andromeda, confirming his groundbreaking discovery.
This revelation dramatically reshaped the field of cosmology. Hubble's contemporary, Shapley, who had argued for a much smaller universe, was devastated upon receiving Hubble’s findings. He later remarked, “Here is the letter that destroyed my universe.” Just three years prior, Shapley had argued that the Milky Way encompassed the entire universe and that the "spiral nebulae" were simply distant star clusters.
Hubble's legacy lived on in the Hubble Space Telescope, launched 70 years later. The telescope has since explored the cosmos, uncovering active stars, colliding galaxies, and runaway black holes, all revealing the dynamic interplay between matter and energy.
Though Hubble initially believed galaxies came in only a few specific shapes—spirals, ellipticals, and irregular galaxies—modern observations, particularly with the Hubble Space Telescope, have revealed a much more complex picture of galaxy evolution. The most significant of Hubble’s findings was that the farther away a galaxy is, the faster it seems to be receding from Earth, supporting the idea of an expanding universe. This phenomenon, based on Hubble’s observation of galaxy redshifts, was later formalized into Hubble–Lemaître’s law, named after both Hubble and Belgian physicist Georges Lemaître.
While Hubble never fully embraced the runaway universe hypothesis, he helped establish the framework that led to the modern understanding of an expanding universe. His findings aligned with the work of Albert Einstein, who had theorized that the universe could not remain stationary due to gravity's pull.
The Hubble Space Telescope has now been in operation for over three decades, advancing the study of the universe. It has made groundbreaking discoveries about dark energy and dark matter, phenomena that continue to challenge our understanding of cosmology. NASA’s upcoming Roman Space Telescope is expected to provide new insights into the expanding universe.
Edwin Hubble’s discovery marked the beginning of humanity’s exploration of the vastness of the universe, shifting perceptions of Earth's place in the cosmos.
Image credit: NASA, ESA, Hubble Heritage Project (STScI, AURA)
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