Here in 2025, most of us are familiar with the stunning wide-field images from the Hubble Space Telescope, showing hundreds of galaxies scattered across the cosmos. We now understand that the universe contains billions of galaxies, each made up of billions of stars, and that it all began with the Big Bang roughly 13 billion years ago. What’s truly remarkable is that, despite astronomy being practiced for thousands of years, these discoveries were made only within the last century. It’s fascinating to think that in 1915, when Einstein introduced his General Theory of Relativity — a theory still used to describe the structure of the universe — these facts about the cosmos were completely unknown.
One man, Edwin Hubble (1889-1953), kick-started the revolution that led to these discoveries. Hubble was born in Missouri and studied Mathematics and Astronomy at the University of Chicago. He switched fields to get a law degree at Oxford University, before moving back to astronomy, first at Yerkes Observatory in Wisconsin, and later at the Mount Wilson Observatory in California.
Before Hubble’s work, many astronomers believed that the Milky Way galaxy encompassed the entire universe. Nebulae — fuzzy, cloud-like objects in the night sky — were believed to be a part of the Milky Way. What is today called the Andromeda Galaxy (M31) was called the Andromeda Nebula. In the 1920s, using the 100-inch Hooker Telescope at Mount Wilson Observatory in Southern California, Hubble observed Cepheid variable stars in the Andromeda Nebula. By measuring their brightness and applying period-luminosity relationship first developed by Henrietta Leavitt, he calculated their distance and found they were far too distant to be part of the Milky Way. This proved that Andromeda was a separate giant stellar system like the Milky Way, leading to the realization that the universe contains countless galaxies beyond our own. Hubble’s classification of galaxies based on their shapes — elliptical, spiral, irregular — was continued with modifications in later decades.
Hubble’s other groundbreaking contribution came in 1929 when based on meticulous measurements of distances and velocities of numerous galaxies, he concluded that other galaxies are moving away from us and the farther away they are, the faster they recede. The ratio of the speed of the galaxies to their distance was found to be constant (Hubble’s constant, currently estimated at 70 kilometers per second per megaparsec — a megaparsec is equal to 1 million parsecs or approximately 3.26 million light-years). This relationship provided the first observational evidence for an expanding universe and laid the foundation for the Big Bang theory.
Through his discoveries, Hubble transformed cosmology from a speculative field into an observational science. His work not only changed our view of the cosmos but also our place within it — demonstrating that Earth and the Milky Way are just small parts of an immense, ever-expanding universe. Edwin Hubble’s legacy continues with the Hubble Space Telescope, which was launched by NASA in 1990. The HST has further expanded our understanding of the universe and over three decades has delivered dozens of iconic images of the universe.