A hundred years ago today, the New York Times reported that Edwin Hubble had made a big discovery. He confirmed that many of the spiral objects known as nebulae were really separate galaxies – “island universes” of stars far outside the Milky Way.

The story ran on page six, and it misspelled Hubble’s name. But it was the first public announcement of one of the greatest scientific findings of the 20th century. The discovery dramatically expanded the known universe. Instead of consisting of the Milky Way alone, the universe contained billions of galaxies, spread across billions of light-years of space.

Hubble studied a class of stars in two spiral nebulae – Andromeda and M33. Called Cepheid variables, the stars pulse in and out. Measuring the length of its pulses and other details reveals the star’s true brightness. From that, astronomers can calculate the star’s distance.

Hubble calculated that Andromeda was about 860,000 light-years away, and M33 a bit farther. Modern calculations show that they’re about three times farther than Hubble thought. Still, the concept was proved: the universe extends far beyond our galactic home – a concept revealed a century ago.

Andromeda – now known as the Andromeda Galaxy – is in the east-northeast at nightfall. Under dark skies, it’s visible to the eye alone – 2.5 million light-years away.

We’ll talk about the most famous Cepheid tomorrow.

Script by Damond Benningfield