Edwin Hubble — The Man Who Discovered the Expanding Universe
In an era when astronomers believed our galaxy was the whole universe, Hubble proved Andromeda was a separate galaxy and discovered that the universe itself is expanding. Here's his story, along with the space telescope that carries his name.
When our galaxy was thought to be the whole universe
Well into the early 20th century, many astronomers believed the Milky Way was the only galaxy in existence. Hazy, spiral-shaped objects like Andromeda, visible through telescopes, were assumed to be gas clouds within our own galaxy, and the idea that they might be independent "island universes" beyond it remained a fringe view for decades.
The man who settled this debate was the American astronomer Edwin Hubble (1889–1953). He had an unusual background — he even qualified as a lawyer — but he eventually returned to astronomy and continued his research at Mount Wilson Observatory, home to the world's largest telescope at the time.
Andromeda was a galaxy beyond our own
In 1923, while photographing the Andromeda Nebula over long exposures with Mount Wilson's 100-inch (2.5m) telescope, Hubble spotted a "Cepheid variable" — a star whose brightness varies on a predictable cycle. Cepheid variables have a precise relationship between their pulsation period and their true brightness, which makes them a special kind of star useful for measuring distance.
The calculation showed that the Andromeda Nebula lay far beyond the diameter of our own galaxy. That was decisive proof that Andromeda wasn't a gas cloud inside the Milky Way, but an entirely separate galaxy of its own. This discovery established, for the first time, that the universe contains not just our galaxy but countless others.
The universe is expanding — Hubble's Law
Hubble didn't stop there. In 1929, analyzing the spectra of light from many galaxies, he discovered that the farther away a galaxy was, the more its light was shifted toward the red end of the spectrum — a "redshift." Much like the lower pitch of a receding ambulance siren, this was a signal that galaxies were moving away from us.
He went further still, confirming a consistent relationship: the farther a galaxy's distance, the faster it recedes — what we now call "Hubble's Law." This was the first observational evidence that the universe isn't static but expanding in every direction, and it later became the central pillar supporting the Big Bang theory.
A space telescope carries his name
Hubble's two discoveries — that the universe contains countless galaxies, and that it is expanding — became the starting point of 20th-century cosmology. In his honor, the space telescope launched in 1990 was named "Hubble."
As it happens, one of the most important tasks the Hubble Space Telescope has carried out is re-measuring, far more precisely, the very question Hubble himself first tackled: how fast is the universe expanding (the "Hubble constant")? A question one man began is still being answered by a telescope that carries his name.