by Shane L. Larson
Science is a powerful method to know the world. Without science, the Cosmos would be an impenetrable mystery to us; we would live our lives without the benefit of knowing how to make fire, how to cross the water or fly in the sky, or how to combat fevers and illness. Early in our education, we are often instructed in The Scientific Method. I remember long hours as a student, filling up lab notebooks with precisely organized and formatted records of investigations into the mysteries of Nature, followed by my professors mercilessly docking me points for not completely writing out an equipment list or forgetting to record a step of the procedure in my notebook — points lost for not following explicitly the formal steps listed in The Scientific Method. I’m sure those days of bleeding red ink on my lab notebook taught me to be more careful in my scientific records, but they did little to inspire me about the way to do science.
Now, many years into my career as a professional scientist, I still think I practice The Scientific Method, but in reality I find the process of science is more organic, moving forward on leaps of inspiration, mixed together with careful planning, fraught with confusion and difficulties, and punctuated with debate and argument. Science is a very human endeavour, and as such has all the elements of every story ever told: ignorance, curiosity, hubris, depression, discovery, elation, and ultimately wisdom. One such story, spanning more than 400 years, is about how we have come to understand our own galaxy.
From remote dark skies, far from the bustling life of modern cities, you can see the Milky Way, arching overhead. Nebulous and shimmering, it has evoked wonder and questions for many thousands of human generations. While today the nature of the Milky Way has passed from our scientists into the collective knowledge base of our species, there was a time when the Milky Way was a tremendous mystery upon which we hung stories, myths and legends. The peoples of the Indian sub-continent called the Milky Way Akash Ganga, the Ganges of the heavens. Life along the Ganges is intimately tied to the river, and it is sacred to the Hindu people; perhaps staring at the vast gossamer river in the sky touched some deep chord in their minds, igniting the idea that we here on the Earth are connected to the sky. The !Kung bushmen tribes of the Kalihari desert call the Milky Way The Backbone of Night, because it looks like a ghostly arch, soaring overhead and holding up the sky.
Our knowledge of the Cosmos is like an archway — it is built up stone by stone, and supported by keystones, essential pieces of knowledge that define how we think about Nature. The Milky Way has come to be a keystone, a focal point of attention that has guided us in our long journey to understanding the Cosmos. In our understanding of the galaxy, that journey began 400 years ago with a 45 year old professor from Padua, Italy, who turned a spyglass to the heavens — Galileo Galilei. His telescope was a simple device, of poor imaging quality compared to the cheapest pair of binoculars you might find at a discount store today. But it could show more than just the eye alone. Peering through his telescope at the diaphanous mist of the Milky Way, Galileo was presented with a staggering wonder — the galaxy was comprised of innumerable stars, packed so closely together that the eye could not resolve them, instead seeing only a nebulous fog. The Universe had suddenly gotten much larger.
There is a bit of folklore that in those early days, Galileo doubted what the telescope was showing him. How could he be certain that what he saw when he looked to the skies was real, and not some phantasm born of his mind’s inability to interact with his new-fangled optical device? To answer this question, he dutifully did what Galileo is known so well for — he conducted an experiment. Setting up a coin across a courtyard, he viewed and sketched the coin through his telescope, noting every detail he could see. Then, leaving his telescope behind, he walked right up to the coin and sketched it again viewing it from a distance of only a few inches. After much examination, he convinced himself the telescope was not lying to him, and all the wonders he had seen were real.
He published his first telescopic observations of the heavens in 1610, in a book called Sidereus Nuncius — “The Starry Messenger.” Despite having convinced himself of the truth, he had less luck convincing others of the utility of the telescope. He complained in a letter to Kepler,
“…I think, my Kepler, we will laugh at the extraordinary stupidity of the multitude. What do you say to the leading philosophers of the faculty here, to whom I have offered a thousand times of my own accord to show my studies, but who with the lazy obstinacy of a serpent who has eaten his fill have never consented to look at planets, nor moon, nor telescope? Verily, just as serpents close their ears, so do these men close their eyes to the light of truth. These are great matters; yet they do not occasion any surprise.”
Even in Galileo’s day, communicating science was hard. But the Age of Enlightenment was soon upon the world; modern astronomy was born in this time, a direct descendant of Galileo’s stargazing. Telescopes began to proliferate. Larger telescopes were built, new designs were invented, observatories were constructed, and astronomers were appointed. We began to plumb the heavens, trying to see all that we could see.
As we looked deeper into the sky, we came to understand that there was far more to the Milky Way than even Galileo knew. The first person to map the Milky Way was William Herschel. In 1784, Herschel used his telescope to count stars in every direction on the sky. From those studies, he produced a map, eerily accurate to what we know today, concluding that the Earth and the Sun are near the center of a flat disk of stars (you can read a detailed description of Herschel’s method in this paper). But as telescopes scanned back and forth across the sky, observers would occasionally see things that were not stars — new, dim fuzzy objects. Some were round, some were oblong, some were completely irregular. The looked for all the world like the Milky Way looked before Galileo’s telescope — thin, white, diaphanous fogs among the stars. They were generically named nebulae, Latin for “clouds.” Herschel himself catalogued more than 2400 of these in an epic survey of the sky conducted with telescopes he built.
This first reconnaissance of the sky brought to the forefront of our minds questions we had asked before: how big is the Cosmos? where did it come from? what is our purpose in it? At this time, most astronomers had decided that the entire Universe was the Milky Way. They had no reason to believe (nor ability to measure) that distances in excess of thousands of lightyears were reasonable. Thus all the nebulae, since they were parts of the Universe, must reside within the Milky Way itself. One prominent view of the day was the “nebular hypothesis,” which supposed that stars and planets formed from gravity acting to collapse vast clouds of gas. The detection of nebulae among the stars of the Milky Way could explain where all the stars in the galaxy came from.
Today we now accept that the nebular hypothesis is correct, but in the 19th Century there were those who certainly did not. Among those who disliked the notion was William Parsons, the Third Earl of Rosse. He believed that like the Milky Way, the nebulae should resolve themselves into innumerable faint stars, if you could just look with a powerful telescope. So in 1845 he built one of the largest telescopes the world had ever seen. More than six feet in diameter, the structure of the telescope had to be held up by a castle wall, and was colloquially known as “The Leviathan of Parsonstown.”
One of the great truths of telescopes is that bigger telescopes can see more, because they collect more light. By all accounts, the Leviathan could see more than any telescope of the time — it revealed details in the nebulae that had never been seen before. In 1845, shortly after it was built, Lord Rosse pointed the Leviathan toward a distant nebula, just under the handle of the Big Dipper, known as Messier 51 (“M51”). In a moment that must have been utterly breathtaking, Lord Rosse realized that he could see spiral structure in the nebular cloud. He promptly declared that M51 was an “island universe,” another galaxy like the Milky Way.
This ignited “The Great Debate” in astronomy, which persisted for more than 80 years before it was resolved. Arrayed against each other were the scientists who believed the Milky Way was the entire Universe, versus those who contended that the Milky Way was but one of a vast number of other galaxies. Both sides had good arguments that supported their position, but there was no way to decide that one group was more right than the other — the observations simply weren’t good enough. To resolve this debate we had to know two things: the size of the Milky Way itself, and the distance to the spiral nebulae. Astronomers noodled this over in vain for decades to no avail.
In the end, the solution to the problem was discovered in 1912 at the Harvard College Observatory by astronomer Henrietta Swan Leavitt. She had discovered a type of star now known as a Cepheid variable that changes its brightness in a regular way over time (a variable star). Leavitt demonstrated that for Cepheid variables, if you could accurately measure the time it takes the star to get dim then bright again, you could use the change in brightness to determine the distance to the star. This was the first, robust method for using telescopic observations of stellar brightness to determine distances through the galaxy; Leavitt’s discovery transformed astronomy. Sadly, Leavitt died of cancer at the age of 53 in 1921, before The Great Debate was resolved.
Knowing that bigger telescopes see more, the Mount Wilson Observatory built a 100” telescope overlooking Los Angeles in 1917; it would be the largest telescope in the world for more than 30 years. In 1924, Edwin Hubble announced that he had used the 100” telescope to detect Cepheid variables in several spiral nebulae. Using Leavitt’s discovery, he was able to determine the distance to the spiral nebulae, discovering that they were vastly farther away than astronomers had imagined. The Universe was suddenly a very big place!
How big? The Milky Way galaxy is about 100,000 lightyears across — it takes light 100,000 years to travel from one side to the other. The disk, which we see edge on as the faint river of light in the night sky, is on average only about 10,000 lightyears thick. By contrast, Hubble was observing the Andromeda Nebula, which is 2.5 million lightyears away! It would take 25 Milky Way galaxies laid edge to edge to span the gulf of space to our closest neighbor, and there are galaxies further still. While these vast distances startled astronomers, The Great Debate was, for all practical purposes, resolved instantly. The data was clear and unambiguous. Leavitt’s great breakthrough in the discovery of the Cepheid variables was a singular event — it resolved an argument that had plagued and befuddled us for almost a century. Astronomers shook hands, dusted off their chaps, and moved on to new, equally difficult mysteries, suddenly revealed by uncountable galaxies far, far away.
How do we study those galaxies that are so far, far away? We build bigger telescopes. We look at the Milky Way up close, and assume galaxies far away are similar. We spend time being confused. We argue. We make inspirational breakthroughs, and eventually, we understand. This is the nature of science.
This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage. The introductory post of the series, with links to all other posts may be found here: http://wp.me/p19G0g-dE