In 1924, the American astronomer Edwin Hubble began studying spiral nebulae. Back then, astronomers thought spiral nebulae were cloud-like eddies within the Milky Way. They didn’t know there were other galaxies in the universe, and the Milky Way was thought to be a simple stream of stars in an otherwise vacant cosmos.

Hubble found that there were stars in the "Andromeda nebula." By carefully measuring their brightness he concluded that the nebula was really a huge island of stars far beyond the Milky Way. The nature of spiral nebulae was finally resolved and the Andromeda nebula was re-classified as a galaxy.

By 1929, observations of other spiral nebulae led Hubble to conclude that the Milky Way was surrounded by galaxies, and that these galaxies were all moving away from us. In fact, the farther away the galaxies were, the faster they were receding.

Growth rate

Knowing the rate of expansion is key to understanding the age, size and destiny of the universe. Astronomers are constantly seeking to refine the Hubble constant, a ratio of speed to distance that measures the expansion rate.

Stretching universe

According to Einstein’s theory, the space between galaxies is what expands: the galaxies do not actually move through space. To picture this, imagine lampposts fixed to an expanding highway. As the road stretches, the space between lampposts increases.
Canadian Heritage Information Network
Australian Museums & Galleries Online, Australia; Centre of the Universe; Gemini Observatory, Hawaii; Glenbow Museum; The Manitoba Museum; National Research Council Canada; Planétarium de Montréal

© Canadian Heritage Information Network, 2003

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