Your Place in the Universe Page 9

  Astronomy was in need of a pioneer, and it found one in Henrietta Swan Leavitt. She had a job as a computer (back when “computer” meant a person crunching numbers by hand and not a machine that does it for you) at the Harvard College Observatory and was tasked with and/or particularly interested in the Magellanic Clouds, two cloudlike (hence the name) nebulae in the Southern Hemisphere. The clouds have been known to various cultures for millennia, but European astronomers only became aware of them in the 1400s, in part thanks to records kept during the world-girdling voyage of Magellan. Somehow his name stuck.

  The Clouds contain billions of individual stars, dense stellar clumps, and smaller knots of clouds. Leavitt was particularly focusing on some specific stars within the Clouds that varied in brightness, called, well, variable stars, and here's the game she played.

  If you look at a collection of stars, with some looking brighter and others dimmer, it's impossible to tell if those differences are due to variations in their true brightness (i.e., if you could examine each star close up, you would find some are blazing hot and others are dim and quiet) or whether those stars are just at different distances.

  It's a mix of both in most situations—except maybe the Magellanic Clouds. They're too far away for parallax measurements, but hey, it looks like all that stuff is all clumped together in the same general vicinity, so maybe it's safe to assume that all the stars we'll find there ought to have about the same distance. (Very roughly, but hey, let's take things one step at a time and see how far we get.)

  Of all the different kinds of variable stars (and yes, there are multiple kinds of variable stars), Leavitt was measuring the light output from a class known as Cepheids, named after the prototypical example located in the constellation Cepheus. These are giant stars with brightnesses at least a few thousand times that of the sun, but that brightness is, as you might have guessed, variable, dimming and brightening considerably over the course of a few days or weeks.

  Painstakingly, Leavitt analyzed the photographic plates returned from a survey of the Clouds, comparing the same field at different nights, searching for any differences in the intensity of brightness from any of the pinpricks of light. It was mentally exhausting, repetitive work, but Leavitt excelled in dedication, and she identified nearly two thousand of these Cepheid creatures.

  It's usually the case in astronomy that the application of large data sets reveals hidden patterns and deeper workings. When you only have a single special case or a handful of examples, it's hard to make sense of the universe. When you have a couple thousand to toss around, it's still hard, but at least you have the serum of statistics to compel nature to reveal the truth.

  What Leavitt found by 1912—whether she was looking for it or not—was a remarkable pattern. If you assume that these particular Cepheids are roughly the same distance from us, then if they look brighter, they are brighter. And she must have stared in amazement at her charts and graphs when a simple relationship revealed itself to her: the brighter the Cepheid star, the longer its period between cycles of intensity.2

  I know, that doesn't sound that amazing, but it is. This one simple relationship is about to hurtle astronomers—and us—to unimaginable scales.

  Here's the deal with the deal. Measuring the parallax of these stars is essentially impossible. Measuring the brightness of these stars as they vary over time is merely insanely difficult—not impossible. And if you can connect the time variation to the true brightness (the astronomers among us switch to a slightly more technical term, the luminosity, to describe the brightness of a star if it can be measured from a fixed distance), then you can compare the true brightness to the brightness as the star appears in the sky.

  And following the recommendation of our ancestral astronomers, we can do a little bit of geometry to compute a distance: there is only one number that connects true to perceived brightness. This allows us to go way, way further than crude parallax measures can take us.

  Just how far? Well, that was the subject of an intense debate that I'm about to talk about. But all this history, as fun as it is to relate, is getting me hungry for some astrophysics. What is a Cepheid star, and how does it work?

  Here's the basic picture, as far as we understand it. Take a giant star and wrap it in a layer of helium. Not hard, since helium is the second most abundant element in the universe. The star will heat up the helium and ionize it, ripping its electrons off. This stripped helium is a little bit opaque—light has a hard time making it through.

  And so from our perspective the star looks a little dimmer, as the light from the surface is blocked by the enveloping helium. But that intense heat inflates the layer of helium, which causes it to expand, and in the inevitable process of the physics, it separates from the star and cools off.

  Now more cool and collected, the prodigal electrons return to their homes, turning the helium neutral and the gas transparent. From here on Earth, we get to see the full blast of the star's intensity.

  But cooling gases contract, right? So over time the helium collects near the surface, where it heats up, ionizes, and turns opaque, and—wax on, wax off—the cycle repeats again.

  We're pretty sure. On the plausibility scale this gets a pretty high score, but of course it doesn't quite explain all the observational data.

  Here's the hilarious part: it doesn't matter. We could be totally 100 percent clueless about how Cepheids work, with no understanding at all behind the cycles and variations. What matters, when it comes to cosmology, is that the relation between true brightness and period length holds fast. As long as the data support that relationship, we can use it to measure reliable distances. You don't have to know how your microwave oven works to heat up your ramen noodles, after all.

  And now back to your regularly scheduled exploration of the universe.

  Of course, we still didn't know the distance to the Magellanic Clouds. Leavitt only discovered a relative measure—the longer the period of variation in a Cepheid, the intrinsically brighter it is. And any place in the universe you can measure this variation, you can calculate a distance. But you need an anchor—a first step, using another method, to pin down the nearest Cepheid. From there you can step your way as far as your reliable brightness measures can take you.

  Thankfully, a few years after Leavitt's work, a nearby Cepheid was found nestled in the Milky Way, with a confirmed distance using other methods, and the first rung of a distance ladder to the universe could be built.

  But in the 1910s, not everybody was liking the taste of the Cepheid special sauce. With the benefit of hindsight and multiple decades of advancement, we now know that this brightness-period relation was on pretty solid ground, but of course at the time there were curmudgeonly skeptics—they thought there was too much fuzziness and uncertainty in the relations to use them reliably to gauge the spectacular distances under consideration. And good that those skeptics raised their contentious objections; they keep us all on our toes.

  And so the debate raged on: just how big is this place, and where do we sit? These discussions came to a heated conclusion in a set of lectures on “The Scale of the Universe” held at the Smithsonian Museum of Natural History in 1920. Apparently one of the organizers had suggested an alternative topic, Einstein's theories of relativity, but this was quickly discouraged because not enough people understood it to even make for a decent debate.3

  The two debaters, Harlow Shapley and Heber Curtis, represented the two main camps that astronomers had settled into over the past decade. It's important to relate this debate because (a) it provides a case study to set up how astronomers approach controversial issues, which will prove useful when I talk about present-day cosmological controversies, and (b) it's a fun story because Shapley and Curtis were both wrong.

  In one corner, we had the Shapley camp, who thought the Cepheids were pretty spiffy and argued that our galaxy is a few hundred thousand light-years across. Plus, by noting that globular clusters (deserted clumps of old stars, or old clumps
of deserted stars, take your pick) tended to be found on one side of the sky, we could imply via geometry that the sun is not at the center of the galaxy. But the galaxy, due to its great extent, filled up the entire universe, including all the mysterious spiral nebulae that dotted the evening skies.

  And in the opposite corner, there was the Curtis camp, who looked askance at this Cepheid tomfoolery and insisted that our galaxy is small. The best we could do with parallax and other methods was a rough estimate of thirty thousand light-years for the diameter of the Milky Way, and no matter the direction we look, we see the same kinds of stars, implying that we're roughly in the center. But those spiral nebulae surely reside outside our own galaxy. If we assume they're the same size as the Milky Way, then they can only be extragalactic to explain their extent on the sky. And sometimes we see stars flare up—a nova—inside these nebulae. But these novae are far dimmer than those outside of nebulae. Coincidence? I think not. The spiral nebulae are “island universes,” isolated homes to populations of stars far removed from us.

  Two arguments, both supported by solid lines of evidence, sound reasoning, and good old-fashioned hard thinking. We can't blame the Shapleyites or Curtisans for taking the positions they did. Both sides had weaknesses in their arguments, for sure, which their opponents exploited with relish. In a way, this single lecture event encapsulated the growing frustration with the cosmos. So much was known, but it fell short of being understood. We were squeezing useful information from our telescopes and photographic plates, but no consistent story was forthcoming.

  The universe was sending us mixed signals. Who could possibly sort it all out?

  I won't keep you in suspense longer than I have to: it was Edwin Hubble, in the Mount Wilson Observatory, with the one-hundred-inch Hooker telescope.

  Hubble found Cepheids, forty of the variable little suckers, embedded within the Andromeda Nebula, the largest (and hence thought to be closest) of the mysterious spiral nebulae. Nobody else had seen them because nobody else had a hundred-inch-wide telescope. But with Hubble at the controls of the biggest, baddest telescope ever made, sitting outside the not-yet-insanely-bright city of Los Angeles, Hubble could resolve features never before seen to humans.

  Hubble published his data and necessary analysis (remember, kids, it's important to show your work) in a very readable short paper in 1925, laying out his newfound vision for the cosmos: the Milky Way is indeed large, but it is very far away from Andromeda.4

  Hubble estimated the distance between our galaxies to be about nine hundred thousand light-years.

  We now know it to be three times greater.

  In a single well-written, well-argued, well-researched paper, Edwin Hubble completely repainted the portrait of our universe. And to do so, he needed a much bigger canvas.

  Shapley was right in his debate a decade earlier: the sun is not at the center of the galaxy. But he was also wrong: the Milky Way is much smaller than he calculated.

  Curtis was also right and also wrong: he got roughly the right size for our home galaxy but missed our location within it.

  It took more data and a new round of better instruments to finally sort through the confusion, and once again the implacable data showed that the universe obeys a sort of vicious and amped-up version of Copernicus's original thoughts: We are not special. We are not at the center. And the cosmos is far, far larger than we can be reasonably comfortable with.

  The Milky Way galaxy is but an island of stars—hundreds of billions of stars, but an island nonetheless—separated from our nearest neighbor by vast gulfs of almost absolutely nothing.

  Just a few centuries earlier, Brahe thought it absurd to place the stellar sphere seven hundred times farther than the orbit of Saturn. And here was Hubble, sitting underneath his gigantic telescope, recording the variations in brightness from a point of light sixteen trillion Saturn-orbits away from us.

  The Andromeda Nebula was no banal collection of gas and dust with a dash of errant stars loosely tossed within it. It was a galaxy in its own right, one of an uncountably large multitude, spread throughout the achingly large cosmos the way a child might leave toys scattered in a room.

  Nobody was really thrilled at the result. Hubble demonstrated that even the “large” universe as advocated by Shapley was too small. Let that sink in: the Cepheids used in Edwin's analysis were farther away than even the farthest possible thing a reasonable person could argue existed.

  And Andromeda wasn't alone. Now that Cepheids could be reliably used to measure extragalactic distances (and “extragalactic” was now a thing), many other distances were pegged to other nebulae-cum-galaxies. Andromeda has pride of place of being the first, but it was far from the last.

  But the fun didn't end there. Oh, no, child.

  Not satisfied with simply rescaling our universal yardsticks and taking the first crack at a truly cosmological birds-eye view of our home, Hubble took it one step further and completely revolutionized our understanding of the dynamics of the universe writ large.

  Remember how much intellectual inertia had to be overcome to finally conclude that the heavens were as violent, chaotic, and messy as the physics here on dear old Earth? Centuries, that's how long. In those intervening generations, scientists played a sort of slippery rhetorical bait and switch. Sure, stars can blow up, change their brightness, and even scoot around. Whatever. But the universe is eternal and never changing.

  Individuals may come and go, but life goes on, forever into the past and forever into the future. It's the way it has been, the way it is, and the way it always will be.

  Academics replaced the fixity of the firmament for a constancy of the cosmos. It's the same thinking, just with a few zeros tacked onto the end of all the numbers.

  And yes, Hubble had to go and pop that bubble too.

  In 1929, four years after his landmark presentation on the true distances available in our universe, he published another, highly readable paper in which he reported an interesting relationship between the distance to a galaxy and, of all things, its speed.5

  Remember the spectral Doppler technique used to measure the motions of stars? (If you don't, you weren't really paying attention to chapter 3, were you?) The beauty of that method is its brutal universality. Find a star, measure its speed toward or away from us. Boom, done. Move onto the next.

  Find a nebula, measure the light. Recognize any emission or absorption lines from your favorite element? Is the fingerprint correct but shifted left or right from its Earthbound counterpart? Congratulations, you've measured the speed of that nebula—even different parts of it!

  Is that “nebula” really a gigantic galaxy, home to hundreds of billions of stars, as big as or bigger than the Milky Way, located hundreds of thousands of light-years away? Who cares? It's emitting light, so we can take a spectrum, either from individual stars, if we're lucky enough to resolve them, or from the generic glow of the galaxy itself.

  Recognize the elements? Fingerprint shifted? You've measured the velocity.

  If it's shifted toward the blue, it's coming toward us. If it's shifted toward the red, it's moving away.

  Of course, just like with stars, this technique only returns the radial speed, the speed along our line of sight to the object. In other words, the speed along the in-out direction. Who knows what the up-down or left-right speed is. But hey, it's something, and we'll take it.

  Specifically, Hubble took it, twenty-four times. It was the best he could do through painstaking (Have I used that word enough to describe the procedure of astronomical observation? No, I haven't.) work collecting, measuring, recollecting, and remeasuring the tediously extracted distance measurements from the Cepheids, then matching those distances to velocity measurements taken by himself and previous astronomers.

  What he found was simple, surprising, and essentially inarguable: galaxies, on average, are receding away from us. And the farther away a galaxy sits from us, the greater its redshift, implying the faster it's receding from us. A
nd this relationship is linear: double the distance, double the speed. Quadruple the distance, quadruple the speed.

  Hubble could even provide a number for the recession rate: five hundred kilometers per second per megaparsec.

  As we'll soon see, the numbers and counting systems we're accustomed to quickly become too cumbersome to have any utility, a fact that astronomers quickly realized probably just after the phrase astronomically large came into circulation. The standard go-to is the light-year, which as you recall is the distance that light can travel in a year—5,878,499,810,000 miles, for the curious.

  But astronomers typically use a different measure: the parsec. Why? I honestly don't know—maybe because it sounds cooler, and maybe because the term light-year was initially used more for the benefit of the popular imagination and not for use by Serious Astronomers. But anyway, parsec is short for parallel arc second and comes in handy when measuring astronomically large distances. If you hold a pencil in front of your face while closing each eye individually, the pencil will appear to wiggle back and forth relative to the distant background. If you hold the pencil farther away, it will wiggle less.

  If you know how far apart your eyes are, and you measure the amount of wiggle, and you know about trigonometry, you can calculate a distance. This is the parallax method.

  That isn't so useful for interstellar measurements, so instead of alternating eyes, astronomers alternate seasons, repeating measurements when the Earth is at opposite sides of its orbit around the sun. This is the precise technique that Brahe used to cast doubt on the sun-centered model of the universe and Bessel used to give him a run for his money. And here comes the definition: a parsec is the distance an object has to be at in order to wiggle by one arc second (1/360th of a degree, for those not nautically inclined) when observed six months apart.