Beyond the Milky Way

Natural History, Dec, 1999 by Richard Panek

Edwin Hubble's announcement seventy-five years ago opened the doors to a whole new view of our universe.

Now that the century and the millennium are ending, seemingly by popular demand in December 1999 rather than in December 2000, what better way for sky watchers to celebrate than by marking the seventy-fifth anniversary of one of the most significant astronomical discoveries of the past hundred--indeed, thousand--years? On January 1, 1925, Edwin Hubble made public his finding that at least one "island universe" or galaxy of stars, lies outside our own Milky Way.

Throughout the nineteenth century, astronomers had debated the nature of nebulae--smudges of light at the farthest limits of telescopic sight, many of them in the shape of spirals. Were these fuzzy patches part of our galaxy, and was our galaxy the universe in its entirety? Or were these nebulae vast star systems entirely separate from, yet equal in magnitude to our own galaxy? By the turn of the twentieth century, the vast majority of astronomers had agreed: the Milky Way is all there is.

Hubble himself disagreed, at least regarding spiral nebulae, as he made clear in his 1917 doctoral thesis. But it wasn't until October 1923 that he found decisive evidence to support his suspicion. With the 100-inch Mount Wilson telescope (then the most powerful astronomical instrument in history) focused on the Andromeda Nebula, Hubble detected a Cepheid variable--a star whose regular periods of brightening and fading correspond to its absolute magnitude and that is therefore useful in measuring galactic distances. According to the information available at the time, Hubble calculated that this spiral nebula lay one million light-years from Earth--or a distance more than three times the most generous estimates of the diameter of the Milky Way itself.

In February 1924 Hubble wrote to his former Mount Wilson colleague Harlow Shapley, then the director of the Harvard College Observatory and a leading proponent of the single-galaxy view of the universe, "You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula (M31)" (see "The Shapley-Curtis Debate," Natural History, May 1995). Years later, a student of Shapley's recalled the astronomer's receiving this pivotal piece of correspondence, reading it quickly, then holding it out and sighing, "Here is the letter that has destroyed my universe."

Everybody else's as well. On New Year's Day 1925, an astronomer friend presented Hubble's paper on the topic (he himself was absent) to the American Association for the Advancement of Science meeting in Washington, D.C. This was one of those truly defining moments that divide history neatly in two. Before then, the common conception of the universe began and ended with our galaxy--period. After that date, the universe would consist of however many galaxies astronomers could find.

As it happens, M31 will occupy an appropriate place of honor in the night sky as this century and this millennium draw to a close. On New Year's Eve, step outside an hour or so after nightfall and look up--straight up. There, at the zenith, will be the Andromeda Galaxy. (For locations in midnorthern latitudes, such as New York City, M31 will be virtually at zenith at about 6:00 P.M.) At magnitude 3.5, it's the most distant celestial object visible to the naked eye--although binoculars do help.

Hubble, in fact, had underestimated the distance: Andromeda is now believed to be 2.2 million to 2.9 million light-years away (while the approximate diameter of the Milky Way has shrunk to only 100,000 light-years). And during this past decade alone thanks to the flood of fresh data from the aptly named Hubble Space Telescope, the total number of known galaxies has increased exponentially--from 10 billion to 125 billion, and counting.

Both as a next-door neighbor, galactically speaking, and as a Milky Way look-alike, Andromeda has long been a sentimental favorite among nighttime observers. But Andromeda also deserves to come first in the hearts of sky watchers if only because, in the ongoing galactic census that regularly redefines our place in the universe, it was first.

Richard Panek is the author of Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (Penguin, 1999).