Telescopes from the Ground Up

1845: Cloudy objects
are really spirals

Get to the root of it

Early telescopes had revealed that the cloudy band in the night sky, known as the Milky Way, was made up of millions of stars. As telescopes grew more powerful, astronomers found that other cloudy patches in the sky, called nebulae, were also made up of stars. Yet some of the nebulae remained hazy. Were these objects also groups of stars that were just too far away to see clearly? It was a puzzle.

In 1845, an Irish nobleman named William Parsons, the third Earl of Rosse, turned his huge reflecting telescope toward a cloudy nebula called M51. The telescope’s 72-inch mirror, the largest in the world, made it powerful enough to distinguish detail in the nebula that no one had ever seen before: the nebula had a spiral shape.

Artists’ impression of M51 as seen by early telescopes Artists' impression of M51 as seen by early telescopes.

Over the next five years, Lord Rosse discovered fourteen more spiral nebulae. He hired several astronomers to assist in and to confirm his observations. One assistant, John L.E. Dreyer, began collecting a list of these cloudy patches. His list, the New General Catalogue of Nebulae and Star Clusters, contained more than 7000 entries, and is still used today.

Gas or galaxies?

In the decades after Lord Rosse’s discovery, astronomers found many other spiral nebulae. They began to debate whether the strange, cloudy spirals were truly part of our Milky Way galaxy.

If these spirals were just clouds of gas, they could be within our galaxy. However, if they were made up of stars too distant to see even with the most powerful telescopes available, they had to be located far away, well beyond our galaxy. This debate would not be settled until the 1920s.

Though they didn’t realize it, astronomers had discovered the existence of other galaxies. The Whirlpool nebula, in which Rosse discovered the first spiral shape, would eventually be recognized as the Whirlpool Galaxy.

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