How Hubble proved other galaxies exist: one star, V1

Edwin Hubble proved that other galaxies exist by finding a Cepheid variable star, later named V1, in the Andromeda “nebula” in October 1923. Its 31.4-day pulse put it about 1 million light-years away, more than three times Harlow Shapley’s estimate for the diameter of the entire Milky Way.

How did Hubble spot V1?

Only just: the discovery almost slipped by. On the night of October 5–6, 1923, in poor conditions at the 100-inch Hooker telescope on Mount Wilson, Hubble took a 45-minute exposure of an Andromeda spiral arm and marked three suspected novae with an “N”. Comparing the plate with older ones, he saw that one “nova” brightened and dimmed far too quickly. He crossed out its N and wrote “VAR!”.

Film copies of that plate later flew on the shuttle missions that launched and serviced the Hubble Space Telescope.

When was the discovery announced?

In stages, which is why sources give different dates. In 1924 Hubble sent Shapley, who had argued that spiral nebulae lay inside our galaxy, a letter with V1’s light curve. On November 23, 1924, the New York Times became the first publication to report the discovery. The result was formally announced at the American Astronomical Society meeting in Washington, held December 30, 1924 – January 1, 1925, where Henry Norris Russell read Hubble’s paper aloud on New Year’s Day in his absence.

“Here is the letter that destroyed my universe.” — Harlow Shapley, as he reportedly told a colleague

Hubble’s distance was an underestimate. V1 sits in Andromeda’s outer regions about two million light-years away, and in 2010–11 the Hubble Space Telescope returned to photograph the star that made its namesake famous.