Milky Way

In a groundbreaking discovery, Edwin Hubble demonstrated that there are many galaxies in the universe besides our Milky Way. Before this discovery in the 1920s, scientists and the public believed that our Milky Way was the only galaxy in the universe.

The key to Hubble's discovery was a special type of star called Cepheid Variable Stars. These stars fluctuate in brightness over a period of time. There is a relationship between the period of these fluctuations and the star's true brightness. That is, by knowing how fast a Cepheid star blinks, we can estimate its true brightness. By comparing a star's true brightness with its apparent brightness from Earth, we can determine how far away that star is. This acts as a "cosmic milestone" that helps astronomers measure distances in space.

In 1924, Hubble observed the Andromeda Nebula, then thought to be a small "nebula" or star cloud, with the 100-inch Hooker Telescope at Mount Wilson Observatory. He discovered a Cepheid variable star in it. When he calculated the distance to this star, he realized that it was beyond the boundaries of our Milky Way. The Andromeda Nebula was much farther away than the diameter of the Milky Way as it was then known. This discovery proved that the Andromeda Nebula was not just a star cloud, but a complete galaxy similar to our own.

This discovery revolutionized our view of the universe. With it, humanity realized that the Milky Way was just one of billions of galaxies in a vast universe.  After Hubble's discoveries, he also devised a scheme for classifying galaxies based on their shape (the Hubble sequence). These discoveries became the cornerstone of modern cosmology.