Astronomical Games: September 2009

Into the Great Milky Yonder

Our galactic home in the cosmos

The vault is sown thick with them, the vault is alive with them, trembles with them, quivers with them! And through their midst rises a broad belt of their like, uncountable for number—rises and flows up into the sky, from the one horizon, and pours across and goes flooding down to the other—a stupendous arch, made all of glittering vast suns diminished to twinkling points by the awful distance—and where is that colossal planet of mine? It's in that Belt—somewhere, God knows where!

—Mark Twain (1835–1910), "Three Thousand Years Among the Microbes"

IN THE early morning hours of January 17, 1994, the Los Angeles basin was struck by a fairly strong earthquake. It was nothing like the earthquakes that have struck Asia in recent years; this one measured about 6.9 or 7.0 magnitude.

But this one I was in the middle of. This one I was actually awake for.

I was prepared for a big earthquake to last a long time. Even so, this one seemed to last an awful long time—sometime between 30 seconds and a minute. (The truth was closer to 30 seconds.)

My most vivid memory after the initial shaking was done—there were countless aftershocks that rattled the area and our nerves—was the sound of rain. It took some seconds before that sound registered with some surprise, because I remembered the previous afternoon was cloudless. And then it occurred to me that our water heater was in a closet, on the top floor of our town house, above our bedroom, which was above our dining room.

A quick check of our bedroom closet revealed a waterfall leading from the bare light bulb in the ceiling down to the floor. I ran up to the water heater closet, and turned the valve off as quickly as I could. The damage was done, however, although we had escaped in one regard: The water from the water heater had broken a quarter-sized hole in the light bulb in the closet, and this water had flowed in a stream straight down to the floor of the closet, sparing everything else in the closet but the carpeted floor.

It had then passed through the flooring and into the ceiling of the dining room below, and that was where the sound of rain and falling plaster was coming from. So we spent the rest of the morning cleaning up that mess. Later, we replaced the minimal strap holding the water heater upright with something much more robust, and also replaced the burst pipe with flex piping.

The reason I bring this up is that as a consequence of having to deal with the waterfall in our house, I had completely missed on one of the outstanding suburban astronomical observations. Because the earthquake had knocked power out over a wide swath of the Los Angeles area (including our neighborhood), residents who were not otherwise occupied were treated to an unpolluted view of the Milky Way over the city.

Now, astronomy is not one of your most popular pursuits. As represented in the magazine rack, it falls well short of more enduring activities such as golf, yachting, and video gaming. So apparently, it turns out, many of the people who viewed the silvery stream of stars arcing over the night sky did not appreciate the impressive view they'd been afforded. Some of them even thought that the silver cloud might have caused the earthquake. (I guess something had to take the blame.)

I happen to think education is the antidote to fear and loathing, so let's take a closer look at our unfairly maligned galactic home, the Milky Way.

Early civilizations didn't know much more about the Milky Way than their non-astronomically inclined Los Angeles counterparts. The very name "Milky Way" is a direct translation of the Latin Via Lactea, which in turn derives from the Greek galaxias, from a Greek root meaning "milk." This is, of course, also where we get our own word "galaxy." In Greek myth, the Milky Way was milk spilled from the breast of Hera, while she suckled the infant Heracles (Hercules).

The Greeks and Romans were not the only ones to liken the galaxy to milk. In ancient Egyptian myth, it was a pool of cow's milk, deified as the goddess Bat, stretched across the heavens.

In Chinese, the Milky Way has long been known as Yinhe, the Silver River. In one story, it is said to forever separate a lowly herder, represented by Altair, in Aquila the Eagle, and a young maiden, represented by Vega, in Lyra the Lyre, after their tryst was discovered by the maiden's grandmother (or the Goddess of Heaven, depending on which version you read). And indeed, the Milky Way does split the sky between Altair and Vega. Once a year, on the seventh day of the seventh month, the lovers are allowed to meet when a bridge of birds forms across the Silver River.

The Cherokee of the southeastern U.S., in one folktale, believed the Milky Way to be a trail left behind by a dog who stole some cornmeal and was found out and chased. As a result, one Cherokee name for the Milky Way is gili ulisvsdanvyi, meaning "the place where the dog ran."

What none of these myths contained was any hint that the Milky Way was composed of stars, which stands to reason. Stars are bright points of light, individually seen. The Milky Way, even to people of extraordinarily acute eyesight, was a faint gauze of light without any points to speak of. Naturally, there were bright stars in the Milky Way, or possibly in front of it, but these were clearly distinct from it, and in no way explained its spread out light.

To be sure, there were some inspired guesses. The Greek philosopher Democritus (c. 460–370 BCE) suggested that the Milky Way was composed of stars that were too faint to be seen individually. He also thought that there were many other worlds in the universe, some of them inhabited, an idea echoed many centuries later by the Dutch polymath Christiaan Huygens (1629–1695), and he is perhaps best known as the originator, along with his mentor, Leucippus, of the Greek atomic theory, which stated that all matter was made up of atoms. For these and other ideas, Democritus is often seen, from our modern perspective, as a prescient but persecuted scientist.

The problem with this point of view is that Democritus also thought a great many things that sound a bit nutty to modern ears. For instance, he did not conceive of atoms of hydrogen, or helium, or oxygen, because those elements are modern ones that Democritus could have had no knowledge of. Instead, he conceived of atoms of the Greek elements—fire, earth, water, and air—and also believed that fire atoms were pointy tetrahedra, air atoms were rounded off icosahedra, and so on.

Because he lacked what we would consider sufficient material evidence for these ideas, other Greek minds could be forgiven for thinking that Democritus had "atoms on his brain," to a certain extent. The fact that he also thought the Milky Way was made up of individual stars might therefore have been attributed to his tendency to see indivisible building blocks everywhere. If there were some place in the sky where a line of clearly seen stars could be seen trailing off into the Milky Way, that might have been somewhat convincing. But there is no such place in the sky, no convenient spectrum of stellar brightness, with the brightest stars on one end and the Milky Way on the other. As a result, Democritus's intriguing ideas were relegated to the philosophical wayside.

The turning point came in 1609, when the Italian mathematician, physicist, and astronomer Galileo Galilei (1564–1642) made his first telescope. It was not the first telescope ever made; at the very least, he was preceded by the Dutch lensmaker Hans Lippershey (1570–1619). But he was the first we know of who turned his telescope to the sky and recorded for all posterity what he saw.

Certainly he was among the first one to make a telescope good enough to be worth pointing at the sky. The heavens are a challenging target for a telescope. The terrestrial world has landmarks, making it easier to find targets. It's also much brighter, so that the low light-gathering power of early telescopes was not severely tested. For the early telescopic astronomers, however, the heavens were field upon field of indistinguishable points of light. A telescope had to be made to more exacting standards to be skyworthy. Judging by his reports, Galileo's passed muster.

Of course, it was subpar by today's standards. His telescopes magnified at most 30 times, and one of the drawbacks of even that modest power was that the field of view was tiny: about the width of a strand of spaghetti held at arm's length. Even so, it was enough to reveal many celestial highlights for the first time, some of which he wrote up in his 1610 newsletter, Sidereus Nuncius (Latin for Starry Messenger). Among these sights were the four big satellites of Jupiter (sometimes called the Galilean satellites in his honor): Io, Europa, Ganymede, and Callisto. Later, he also discovered mountains on the Moon, the phases of Venus, and the rings of Saturn—although he never managed to figure out what the rings actually were.

From the perspective of our story here, however, the most important discovery he made was that the night sky was filled with stars that were too dim to be made out by the unaided eye. This was a serious blow to a human sensibility that saw no point in there being stars too dim to see—serious enough of a blow for many to contend that they weren't really there and were simply an artifact of Galileo's telescope.

What's more, these heretofore unseen stars were almost uncountably more numerous in the Milky Way, which turned out not to be an infinitely unresolvable glow of light, but a massive collection of these dimmer stars.

This was the first solid indication that the stars might not be distributed haphazardly across the celestial globe, but might instead be structured. This structure would cause some parts of the sky, as seen from the Earth, to be more densely packed with stars than others. The fact that the Milky Way stretched across the sky, dividing it into two roughly equal halves, suggested that the Earth and the rest of the solar system was embedded in the Milky Way.

This idea was taken up by the English astronomer and mathematician Thomas Wright (1711–1786), who suggested that the visible universe (that is, the Milky Way) constituted a spherical shell of enormous proportions. Assuming that the Earth was ensconced within this shell, we would naturally see the universe as a flat layer of stars, much as we intuitively see the Earth as flat, since we're basically right on its surface. The fact that the brightest stars did not appear as part of this layer was attributed to the finite thickness of this shell, and the plausible idea that these bright stars were closer to us, and would therefore appear more scattered around the sky.

Another speculation, made in 1755 by the German philosopher Immanual Kant (1724–1804), proposed that the Milky Way was akin to the solar system, in that it was a flat, circular rotating body, much like the solar system but composed of stars and almost impossibly more vast. He even put forth the daring notion that some of the nebulae—cloudy hazes of light, like the Milky Way but much smaller—were other galaxies, but seen from the outside from a great distance away.

Either of these notions of the Milky Way, if true, would explain a strange paucity of dimmer stars, which I alluded to at the end of the previous essay. If stars aren't arranged uniformly around us, but are mostly arranged in a flat shell (especially as they get further and further away from us), then there would be fewer stars at a distance.

This observation made the suggestions more compelling, but they were still merely suggestions. To make them a bit more concrete, some investigation into the actual structure had to be made. To that end, the English musician and astronomer William Herschel (1738–1822) decided to survey the galaxy. He didn't attempt to catalogue every star visible through his telescopes. He dedicated the better part of his life to astronomy, but even with that in mind, such a comprehensive census would have been impossible: His largest telescopes were capable of seeing over a billion stars. Instead, he selected a number of what he hoped were representative portions of the sky, and counted the number of stars visible in each.

In 1785, based on his survey, Herschel decided that the Earth and Sun were at, or nearly at, the center of a giant irregular pancake of stars. He assumed that the stars were approximately identical in brightness, so that the dimmer a star appeared to be, the further it was in truth. He also assumed that the galaxy was more or less in one piece. After much observation, he was able, finally, to arrive at a tentative sketch of the galaxy, reproduced in Figure 1.

Herschel's pointillist Milky Way
Figure 1. The Milky Way, as mapped out by William Herschel. The Sun lies just left of center, marked by a larger star.

Herschel's diagram is not the smooth, oval shape we've come to expect in galactic depictions. Instead, it's jagged around the edge, with frequent incursions of empty space. Herschel believed these to be regions of the Milky Way—"holes in the heavens," he called them—that were devoid of dimmer stars, which he concluded were further away. But why? Why should certain parts of the Milky Way be poor in stars and others not?

The first major step toward an answer was made by a poor Tennessee boy whose father died before he was born, who received little in the way of formal education, and ended up by being possibly the greatest observational astronomer in the age of the telescope. His name was Edward Emerson Barnard (1857–1923), and he was born in Nashville in the years of foment leading up to the American Civil War.

For all his lack of schooling, he was an inquisitive boy, and fast developed an aptitude in photography. When he was nine, he took his first job, as an assistant to a photographer. In his teen years, he developed a keen interest in astronomy that was to extend the rest of his life. When he was 18, he bought himself a telescope, with which he discovered a comet five years later, the first of what would eventually be 14.

Near the end of the 19th century, Barnard joined the faculty of the University of Chicago, as professor of astronomy. There he combined his love of astronomy with his photography background, and began studying the Milky Way in great detail through astrophotography. Not only did astrophotos introduce an aspect of objectivity to the study of the skies, but through long exposures, they could reveal much dimmer objects than could be seen visually.

Barnard was particularly interested in regions of the sky where stars seemed virtually to be absent. These corresponded to Herschel's incursions, and at first it seemed that they could represent exactly what they appeared to be: a relative paucity of stars.

But in too many of these cases, these holes in the heavens seemed to butt right up against the most densely parts of the galaxy. It seemed improbable to Barnard that stars could truly segregate themselves that sharply in the long run. How much more likely, he gradually realized, that these apparent holes were actually dark clouds, or nebulae, which obscured our view, so that only the stars in front of them could be seen. In cases where the dark nebula was close to us, only a relative few stars would stand superimposed, and they would look especially sparse.

However, Barnard's suggestion was only that. Bright nebulae are obvious because they glow and can be seen against the black of night. Dark nebulae, however, if they existed, would be black on black and couldn't easily be seen as such. Barnard's examination of astrophotos made his idea plausible, but stronger evidence was needed.

It was the German astronomer Max Wolf (1863–1932) that supplied the necessary evidence. Through a careful statistical analysis, he found that whenever one found one of these dark holes, one was much more likely to find a bright nebula. This firmly established for the first time that the dark holes were overwhelmingly likely to be nebulae, too. Back in America, Barnard began amassing a catalogue of these nebulae, and by the end of his life he had compiled a list of over 300 of them.

Barnard's dark nebulae explained the incursions in Herschel's map of the galaxy, but they made it difficult to explore the true shape of the galaxy. If the edges of the galaxy lay beyond the reaches of the dark nebulae, they could be at any distance whatsoever. After all, there might lie dark nebulae even where there were evidently crowds of stars; they might simply be too far to detect.

The difficulty with identifying the structure of the Milky Way, of course, is that we're embedded in it. One might as well try to figure out the shape of a crowd of people while standing in it. The Milky Way is a little easier in some ways because stars are basically points, while people have significant width (some more significant than others), blocking out more distant people. On the other hand, people are only a couple of meters tall at the most, and it's a simple matter most of the time to get up high enough above a crowd to see its extent; doing the same for the galaxy was plainly impossible.

What about other galaxies? Might we not be able to discover something about our own galaxy by looking at others?

To be sure, in the 18th and 19th centuries, it wasn't clear that there were other galaxies. There were faint gauzy clouds in the sky that might be other conglomerations of stars, but on the other hand they might be collections of gas and dust, or something else entirely. Confusing the matter was that some of the nebulae (Latin for "clouds") turned out, under close examination, to be clearly made out of stars, though far too few to rival the Milky Way, while others remained stubbornly cloudy no matter how powerful a telescope was turned on them.

In 1845, six decades after Herschel's groundbreaking survey, William Parsons, Lord Rosse (1800–1867) built the world's largest telescope out of a single 72-inch disc of metal. (Actually, because the metal tarnished so rapidly, he had two 72-inch mirrors built, and alternately swapped one in while the other one was being repolished, moving back and forth in cycles lasting about one year.) The mirror was so heavy that several assistants were required to operate it. Its large size did, however, enable the sky to be examined in unprecedented detail and brightness. Rosse was interested at the time in verifying Kant's suggestion that the nebulae were distant galaxies, and to that end, he turned his telescope to them, hoping to discern evidence of rotation.

One target, known simply as M51, did seem to show rotation. Rosse sketched what he saw through the eyepiece, and his drawing shows a distinct whirlpool, along with what seems to be evidence of motion across the sky. Largely as a consequence of Rosse's sketch, M51 came to be called the Whirlpool Nebula. (See Figure 2.)

Rosse's Whirlpool sketch
Figure 2. Lord Rosse's sketch of M51, the Whirlpool Nebula. The bright spot at right is a companion; both are now known to be galaxies.

Further studies even appeared to reveal individual stars in the nebulae, further bolstering the case for Kant's hypothesis. But Rosse's observations were made at the very limits of his ability to see. M51 does indeed turn out to be a spiral galaxy, somewhat smaller than our Milky Way but still containing tens of billions of stars. But Rosse also believed he was able to resolve M42, the Great Orion Nebula, into individual stars, something which the older Herschel vigorously disputed, and which turns out, with modern evidence, to be untrue. The academic debate turned a trifle ugly, with each party casting aspersions on the optical manufacturing abilities of the other. History has looked kindly upon the telescopes of both men, but as far as the nature of the galaxy was concerned, something a bit less subjective than borderline observations at the eyepiece was needed.

Herschel drew the Sun near the center of the galaxy because that's what his star counts indicated, and because it jibed with the rough appearance of the galaxy. Here and there the Milky Way is denser than in other places, but overall, the Milky Way itself gave no reason to suggest that there was more of it on one side of us than the other. Of course, that could be because there isn't more of it on one side, but it could also be because of Barnard's dark nebula.

Fortunately, there exist other celestial objects that were far enough away to be potentially useful markers of the Milky Way, but not so far that their true nature couldn't be made out with telescopes. These were the globular clusters, roughly spherical conglomerations of tens to hundreds of thousands of stars, scattered across the night sky. The first one to be recognized as such was one later called M22, in the constellation of Sagittarius the Archer; it was discovered by a German amateur astronomer named Abraham Ihle, in 1665. Herschel himself discovered plenty of them, eventually cataloguing about 70 of them, about half of them original discoveries. He was also the first to call them globular clusters.

In 1914, the American astronomer Harlow Shapley (1885–1972) embarked on a systematic study of these clusters. He had been intrigued by the apparent asymmetry in the way these clusters were distributed across the sky; there were more of them in the half of the sky centered around Sagittarius than in the other half. That might be an indicator that the center of the Milky Way lay in that direction, but it might also just be random variation. One thing it was not likely to be was the veil of an overwhelmingly large dark nebula.

To find out for sure, though, he had to know more about the globular clusters than their direction from us; he had to know their distance, to divine their three-dimensional arrangement around us. But how could he tell if a given cluster was far away and bright, or close up and dim?

In this case, Shapley was able to make use of a recent discovery, made by the astronomer Henrietta Swan Leavitt (1868–1921) about a certain kind of variable star called a Cepheid, after a particular exemplar in the constellation Cepheus the King. Leavitt had a sharp mind, but she had the misfortune of being born a woman in an age when females were reckoned to have no scientific bent. So even though she graduated from Radcliffe with a bachelor's degree in 1892, at the age of 23, rather than conducting research, as she would have done decades later, she instead found employment as a human computer at the Harvard College Observatory, carrying out simple but tedious measurements and calculations that her male colleagues considered beneath their station.

It was in the midst of these computations that she discovered that the period of a Cepheid—that is, the length of time it takes to first brighten and then dim back down to its original brightness—was directly related to its luminosity. This relationship held the key to measuring the distance to Cepheids. By measuring their period, one could determine how bright they really were, and by comparing that with how bright they appeared in photographs, one could determine in turn just how far they were. Before Leavitt's discovery, the distance to only the most nearby of stars could be measured, using the laborious parallax technique. Now, a whole class of variable stars could be plumbed, with only a few weeks of brightness measurements. Leavitt's reward for her startling insight was to be forgotten in her own time while the astronomer Edwin Hubble (1889–1953) received credit for discoveries made using her technique. (Hubble himself generously maintained that she should have won the Nobel Prize for her findings.)

At any rate, Shapley reasoned that he could use Cepheids in a globular cluster to identify the distance to the entire cluster, assuming that those stars were pretty much all of a distance from us. After four years of study, in 1918, he was able to determine that the clusters were indeed centered around a point significantly far from us, in the direction of Sagittarius, as the original distribution suggested. In fact, Shapley deduced that the center of the galaxy was about 50,000 light-years away from us. Later, more precise measurements have cut that number approximately by half.

A few years later, incidentally, Shapley was embroiled in a heated though civilized debate with another astronomer, Heber Curtis (1872–1942) over the nature of spiral nebula such as M51. Their debate centered on an even larger nebula, the Andromeda Nebula, also known as M31. Shapley argued, on the basis of photographs that seemed to show rotation in the Andromeda Nebula, that it had to be close enough to be within the Milky Way, and therefore not a galaxy itself; at this point, it wasn't yet clear if there even were other galaxies besides the Milky Way. Curtis, for his part, showed evidence that there were as many novae (a kind of stellar explosion) in the area of the sky covered by M31 as there were in the rest of the sky combined, and that those novae were much dimmer than those in the rest of the galaxy—just as you'd expect if M31 were a faraway galaxy rather than a nearby nebula.

The Shapley-Curtis debate was considered a draw at the time, but over the next few years, it became evident that the apparent nebular rotation in M31 was merely that, and that it really was a spiral galaxy of great size—larger, as it turns out, than the Milky Way itself.

And what of the Milky Way? Is it, like M31, a spiral galaxy, too? Or is it an amorphous blob, like many other galaxies turned out to be? As part of his argument in the debate, Curtis pointed out that there were dark blobs in detailed photographs of M31, which—if it were indeed a galaxy—would be perfect analogues to Barnard's nebulae.

That was merely suggestive, but here, again, 20th-century technology came to the aid. When radio astronomy was developed a decade or so later, it was discovered that although Barnard's nebulae might be opaque in visible light, they were essentially transparent in radio waves, which are just like light waves, only of a much lower frequency. As radio telescopes grew ever more refined and supplemented by infra-red astronomy (using waves of intermediate frequency), it became possible to map out the distribution of stars within the Milky Way, even beyond those nebulae.

a map of the Milky Way
Figure 3. The Milky Way, as mapped out by modern studies. The Sun lies at center, about 26,000 light-years away from the galactic center. (Image courtesy NASA.)

The stars turn out to be grouped into spiral arms, just like those in M31, and of roughly the scale determined by Shapley's study of globular clusters—a finding which required the remarkable ingenuity of numerous scientists, and which couldn't have been further from the minds of millions of Angelenos who looked upward in the early morning hours of January 17, 1994, and saw only a ghostly electromagnetic menace.

Copyright (c) 2009 Brian Tung