What do distant galaxies, interstellar clouds of gas and dust, and faint star clusters have in common? When seen through a small or medium-sized telescope, all tend to look like faint, unresolved clouds. The brightest of these so-called nebulae (the Latin word for “clouds”) can even be glimpsed with the naked eye. The Orion Nebula, for instance, can be found in the sword of the constellation Orion on any clear winter evening, and the grand Andromeda Galaxy is a treat in the autumn sky.
By the beginning of this century, more than 10,000 nebulae had been catalogued. A number had been identified as interstellar clouds or star clusters, but most still remained a mystery.
Not until Edwin Hubble, observing with the 100-inch telescope on Mt. Wilson in the 1920s, spotted individual stars in the Andromeda Galaxy (then known as the “Great Nebula”) did their true nature come out.
Hubble found that these nebulae were huge galaxies in their own right, enormously expanding our view of the size and contents of the universe.