One hundred years ago, galaxies outside of the Milky Way did not exist.  Or rather, they existed, but astronomers had not seen them exactly.  The Universe was, for all intents and purposes, much smaller.

The Stars that Guided Us

For millennia, humans have looked up at the night sky and experienced a primal sense of awe.  The moon waxes and wanes through its phases, and the stars arc through the darkness in numbers too high to count.  The Milky Way spills across the heavens.  Quite reasonably, our distant ancestors watched the show and concluded that we were at the center of all that cosmic action.

A few places remain from which to see the Milky Way as our ancestors saw it.

Astronomers from multiple early civilizations mapped the stars, collecting them into constellations, and the stars effectively returned the favor by providing virtual maps in the sky to guide people on their journeys.  A solitary star in the northern sky was especially helpful to the explorers, staying in one spot as if to organize the others around it – thank you, Polaris, for showing our forebears the way.

Ancient observers noticed that a few “stars” moved in eccentric courses among the myriad twinkling lights; the Greeks called them planetes, or “wanderers.”  Philolaus, who lived in the late 5th and early 4rd century B.C.E., proposed a universe with a “Central Fire” around which the Sun, Earth, the planetes, and stars revolved.  A century or so later, Aristarchus of Samos was likely the first to posit that the Earth and the other planets orbited the Sun itself, although he believed the Sun and “fixed stars” were locked in place.

(Image credit: Australia Telescope National Facility)

The Growing Universe

The Universe grew a little bigger when Nicolaus Copernicus came along.  As early as 1514, he had worked out the basics of a hypothesis that placed the Sun at the center of the Universe.  Copernicus hesitated to permit the publication of his fully evolved theory until shortly before his death, in 1542, and De revolutionibus orbium coelestium appeared in print the next year.

Hans Lippershey (1570 - 1619)

For the Universe to grow larger still, astronomers would need to get closer to the stars.  But how could they?  The answer came with the invention of the telescope, the first of which was probably made by Hans Lippershey in Holland, in 1608.  Soon after, basing his instrument on Lippershey’s design, Galileo Galilei was peering into the night sky to discover wonders previously unseen.

The advent of the telescope accelerated the stream of discoveries dramatically.  Astronomers quickly learned that the visible Universe was larger and more complex than had been believed, but its borders seemed to coincide with the Milky Way.  They studied and catalogued all manner of celestial objects, including fuzzy smudges of light that appeared in telescopic lenses in ever greater numbers.  By 1784, Charles Messier had catalogued 102 such “nebulae,” and astronomers would add thousands more to the list into the 20th century.

Galaxies Come into Focus

In the early 1900s, the only known galaxy was still the Milky Way and, as far as could be determined, it was the Universe – or so most astronomers continued to believe.  There were a few who looked at the spiral nebulae that had been so carefully catalogued and wondered if they might actually be “island universes” similar to our Milky Way, but they lacked observational data (and telescopes powerful enough) to advance such radically new hypotheses very far.  That was about to change.

The Hooker Telescope vastly expanded the observational capabilities of astronomers. (Image credit: Mt. Wilson Observatory)

In 1917, the 100-inch Hooker Telescope was installed at the Mount Wilson Observatory in California.  Within a few years, the Hooker allowed Edwin Hubble to prove with certainty that the Great Andromeda Nebula was actually a galaxy in its own right lying outside the bounds of the Milky Way.  Not only that but, in 1929, Hubble and Milton Humason determined that the Universe was expanding in size, confirming Albert Einstein’s earlier General Theory of Relativity.

Edwin Hubble determined that the Andromeda Galaxy existed outside the bounds of the Milky Way. [Image credit: ESA/Hubble & Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)]

It wasn’t long before astronomers began reclassifying many of the nebulae discovered by their predecessors as galaxies scattered across a very much larger Universe than had previously been known.  How many galaxies are out there?  Using deep field images from the Hubble Space Telescope and observations by colleagues around the world, astronomers from the University of Nottingham recently estimated that there may be between one and two trillion galaxies in the Universe!

Hubble Deep Field image of galaxies in Ursa Major, including some over 12 billion light years away. (Credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA/ESA)

And how large is the Universe?  Check out the SpaceRip video below to get a handle on some possible answers to that biggest of all questions.