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Like a Huge Spiral, the Milky Way Floats in Space

For thousands of years, people have been puzzling over the milky strip that extends across the entire firmament.

In the modern era, Galileo Galilei discovered that this Milky Way consists of countless stars. However, it was not until the 20th century that astronomers succeeded in deciphering its form and its true nature.

"My third observation relates to the nature of the Milky Way (...) No matter which part of it one targets with the telescope, one finds a huge number of stars, several of which are quite large and very striking; yet, the number of small stars is absolutely unfathomable." These words were written in 1610 by a man who with his self-constructed telescope studied unknown lands that were not of this world. It was this work that earned him a place in history: Galileo Galilei.

The land that he described is literally out of this world, and the document bears the title Sidereus Nuncius ("Starry Messenger"). In it, the Italian mathematician and astronomer presents his observations of the satellites of Jupiter, the Earth's moon and also the Milky Way. Until then, their nature had remained a mystery, and had above all been the subject of mythology. The Greek natural philosopher Democritus had already claimed in the 5th century BC that the diffusely glowing strip in the sky - known by the African !Kung bushmen as the "backbone of the night" - consisted of countless weak stars.

After the discovery made by Galilei, however, nearly 150 years would pass before this celestial structure would again became the subject of scientific study. Thomas Wright of County Durham believed that stars were arranged in a flat region similar to a grindstone, which extended over the entire sky. For him, the Milky Way was nothing other than the projection of this grindstone. The German philosopher Immanuel Kant seized on this theory - and came very close to discovering the truth.

In his General Natural History and Theory of the Heavens, published in 1755, he explained the Milky Way as an extended and very diluted layer of stars. The Sun, the Earth and all the other planets were part of this layer - but not at its centre. Depending on the line of sight, along the plane of the layer or vertically out of it, we would see different numbers of stars

But how were the astronomers to find out whether the apparent view of the Milky Way in the sky reflected its actual spatial structure? Stellar statistics devised at the end of the 18th century by Friedrich Wilhelm Herschel promised a solution: Herschel recorded the coordinates and brightness of all the stars that he could see through his telescope.

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