[2][3] Hubble proved that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way.

[4] He used the strong direct relationship between a classical Cepheid variable's luminosity and pulsation period[5][6] (discovered in 1908 by Henrietta Swan Leavitt[7]) for scaling galactic and extragalactic distances.

[11] A decade before, the American astronomer Vesto Slipher had provided the first evidence that the light from many of these nebulae was strongly red-shifted, indicative of high recession velocities.

[15] In his younger days, he was noted more for his athletic prowess than his intellectual abilities, although he did earn good grades in every subject except spelling.

[16] Hubble's studies at the University of Chicago were concentrated on mathematics, astronomy and philosophy, which resulted in a Bachelor of Science degree by 1910.

[20] Hubble was a dutiful son, who despite his intense interest in astronomy since boyhood, acquiesced to his father's request to study law, first at the University of Chicago and later at Oxford.

Hubble volunteered for the United States Army and was assigned to the newly created 86th Division, where he served in the 2nd Battalion, 343rd Infantry Regiment.

Shortly before his death, Hubble became the first astronomer to use the newly completed giant 200-inch (5.1 m) reflector Hale Telescope at the Palomar Observatory near San Diego, California.

[24] Edwin Hubble's arrival at Mount Wilson Observatory, California, in 1919 coincided roughly with the completion of the 100-inch (2.5 m) Hooker Telescope, then the world's largest.

Using the Hooker Telescope at Mount Wilson, Hubble identified Cepheid variables, a standard candle discovered by Henrietta Swan Leavitt.

In 1931 he wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift-distance relation:[32] Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae.

The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.Today, the "apparent velocities" in question are usually thought of as an increase in proper distance that occurs due to the expansion of the universe.

According to Allan Sandage, Hubble believed that his count data gave a more reasonable result concerning spatial curvature if the redshift correction was made assuming no recession.

To the very end of his writings, he maintained this position, favouring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature.

Earlier, in 1917, Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting.

According to a Los Angeles Times article reporting on Hubble's remarks, "The nebulae could not be uniformly distributed, as the telescope shows they are, and still fit the explosion idea.

[45][46][47] Hubble's papers comprising the bulk of his correspondence, photographs, notebooks, observing logbooks, and other materials, are held by the Huntington Library in San Marino, California.

However, the observational astronomer Sidney van den Bergh published a paper[49] suggesting that while the omissions may have been made by a translator, they may still have been deliberate.

On March 6, 2008, the United States Postal Service released a 41-cent stamp honoring Hubble on a sheet titled "American Scientists" designed by artist Victor Stabin.

[46] His citation reads:[61] Often called a "pioneer of the distant stars", astronomer Edwin Hubble (1889–1953) played a pivotal role in deciphering the vast and complex nature of the universe.

In the 1980 documentary Cosmos: A Personal Voyage by astronomer Carl Sagan, Hubble's life and work are portrayed on screen in episode 10, "The Edge of Forever".

The play Creation's Birthday, written by Cornell physicist Hasan Padamsee, tells Hubble's life story.