Part 6 (1/2)
The core of the sun, where all its thermonuclear energy is generated, is not a place to find low-density material. But the core comprises a mere 1 percent of the Sun's volume. The average density of the entire Sun is only one-fourth that of Earth, and only 40 percent higher than ordinary water. In other words, a spoonful of Sun would sink in your bathtub, but it wouldn't sink fast. Yet in 5 billion years the Sun's core will have fused nearly all its hydrogen into helium and will shortly thereafter begin to fuse helium into carbon. Meanwhile, the luminosity of the Sun will increase a thousandfold while its surface temperature drops to half of what it is today. We know from the laws of physics that the only way an object can increase its luminosity while simultaneously getting cooler is for it to get bigger. As will be detailed in Section 5, the Sun will ultimately expand to a bulbous ball of rarefied gas that will completely fill and extend beyond the volume of Earth's...o...b..t, while the Sun's average density falls to less than one ten-billionth of its current value. Of course Earth's oceans and atmosphere will have evaporated into s.p.a.ce and all life will have vaporized, but that needn't concern us here. The Sun's outer atmosphere, rarefied though it will be, would nonetheless impede the motion of Earth in its...o...b..t and force us on a relentless spiral inward toward thermonuclear oblivion.
BEYOND OUR SOLAR SYSTEM we venture into interstellar s.p.a.ce. Humans have sent four s.p.a.cecraft with enough speed to journey there: we venture into interstellar s.p.a.ce. Humans have sent four s.p.a.cecraft with enough speed to journey there: Pioneer 10 Pioneer 10 and and 11 11, and Voyager 1 Voyager 1 and and 2 2. The fastest among them, Voyager 2 Voyager 2, will reach the distance of the nearest star to the Sun in about 25,000 years.
Yes, interstellar s.p.a.ce is empty. But like the remarkable visibility of rarefied comet tails in interplanetary s.p.a.ce, gas clouds out there, with a hundred to a thousand times the ambient density, can readily reveal themselves in the presence of nearby luminous stars. Once again, when the light from these colorful nebulosities was first a.n.a.lyzed their spectra revealed unfamiliar patterns. The hypothetical element ”nebulium” was proposed as a placeholder for our ignorance. In the late 1800s, there was clearly no spot on the periodic table of elements that could possibly be identified with nebulium. As laboratory vacuum techniques improved, and as unfamiliar spectral features became routinely identified with familiar elements, suspicions grew-and were later confirmed-that nebulium was ordinary oxygen in an extraordinary state. What state was that? The atoms were each stripped of two electrons and they lived in the near-perfect vacuum of interstellar s.p.a.ce.
When you leave the galaxy, you leave behind nearly all gas and dust and stars and planets and debris. You enter an unimaginable cosmic void. Let's talk empty: A cube of intergalactic s.p.a.ce, 200,000 kilometers on a side, contains about the same number of atoms as the air that fills the usable volume of your refrigerator. Out there, the cosmos not only loves a vacuum, it's carved from it.
Alas, an absolute, perfect vacuum may be impossible to attain or find. As we saw in Section 2, one of the many bizarre predictions of quantum mechanics holds that the real vacuum of s.p.a.ce contains a sea of ”virtual” particles that continually pop in and out of existence along with their antimatter counterparts. Their virtuality comes from having lifetimes that are so short that their direct existence cannot ever be measured. More commonly known as the ”vacuum energy,” it can act as antigravity pressure that will ultimately trigger the universe to expand exponentially faster and faster-making intergalactic s.p.a.ce all the more rarefied.
What lies beyond?
Among those who dabble in metaphysics, some hypothesize that outside the universe, where there is no s.p.a.ce, there is no nothing. We might call this hypothetical, zero-density place, nothing-nothing, except that we are certain to find mult.i.tudes of unretrieved rabbits.
FIFTEEN.
OVER THE RAINBOW.
Whenever cartoonists draw biologists, chemists, or engineers, the characters typically wear protective white lab coats that have a.s.sorted pens and pencils poking out of the breast pocket. Astrophysicists use plenty of pens and pencils, but we never wear lab coats unless we are building something to launch into s.p.a.ce. Our primary laboratory is the cosmos, and unless you have bad luck and get hit by a meteorite, you are not at risk of getting your clothes singed or otherwise sullied by caustic liquids spilling from the sky. Therein lies the challenge. How do you study something that cannot possibly get your clothes dirty? How do astrophysicists know anything about either the universe or its contents if all the objects to be studied are light-years away?
Fortunately, the light emanating from a star reveals much more to us than its position in the sky or how bright it is. The atoms of objects that glow lead busy lives. Their little electrons continually absorb and emit light. And if the environment is hot enough, energetic collisions between atoms can jar loose some or all of their electrons, allowing them to scatter light to and fro. All told, atoms leave their fingerprint on the light being studied, which uniquely implicates which chemical elements or molecules are responsible.
As early as 1666, Isaac Newton pa.s.sed white light through a prism to produce the now-familiar spectrum of seven colors: red, orange, yellow, green, blue, indigo, and violet, which he personally named. (Feel free to call them Roy G. Biv.) Others had played with prisms before. What Newton did next, however, had no precedent. He pa.s.sed the emergent spectrum of colors back through a second prism and recovered the pure white he started with, demonstrating a remarkable property of light that has no counterpart on the artist's palette; these same colors of paint, when mixed, would leave you with a color resembling that of sludge. Newton also tried to disperse the colors themselves but found them to be pure. And in spite of the seven names spectral colors change smoothly and continuously from one to the next. The human eye has no capacity to do what prisms do-another window to the universe lay undiscovered before us.
A CAREFUL INSPECTION of the Sun's spectrum, using precision optics and techniques unavailable in Newton's day, reveals not only Roy G. Biv, but narrow segments within the spectrum where the colors are absent. These ”lines” through the light were discovered in 1802 by the English medical chemist William Hyde Wollaston, who naively (though sensibly) suggested that they were naturally occurring boundaries between the colors. A more complete discussion and interpretation followed with the efforts of the German physicist and optician Joseph von Fraunhofer (17871826), who devoted his professional career to the quant.i.tative a.n.a.lysis of spectra and to the construction of optical devices that generate them. Fraunhofer is often referred to as the father of modern spectroscopy, but I might further make the claim that he was the father of astrophysics. Between 1814 and 1817, he pa.s.sed the light of certain flames through a prism and discovered that the pattern of lines resembled what he found in the Sun's spectrum, which further resembled lines found in the spectra of many stars, including Capella, one of the brightest in the nighttime sky. of the Sun's spectrum, using precision optics and techniques unavailable in Newton's day, reveals not only Roy G. Biv, but narrow segments within the spectrum where the colors are absent. These ”lines” through the light were discovered in 1802 by the English medical chemist William Hyde Wollaston, who naively (though sensibly) suggested that they were naturally occurring boundaries between the colors. A more complete discussion and interpretation followed with the efforts of the German physicist and optician Joseph von Fraunhofer (17871826), who devoted his professional career to the quant.i.tative a.n.a.lysis of spectra and to the construction of optical devices that generate them. Fraunhofer is often referred to as the father of modern spectroscopy, but I might further make the claim that he was the father of astrophysics. Between 1814 and 1817, he pa.s.sed the light of certain flames through a prism and discovered that the pattern of lines resembled what he found in the Sun's spectrum, which further resembled lines found in the spectra of many stars, including Capella, one of the brightest in the nighttime sky.
By the mid-1800s the chemists Gustav Kirchhoff and Robert Bunsen (of Bunsen-burner fame from your chemistry cla.s.s) were making a cottage industry of pa.s.sing the light of burning substances through a prism. They mapped the patterns made by known elements and discovered a host of new elements, including rubidium and caesium. Each element left its own pattern of lines-its own calling card-in the spectrum being studied. So fertile was this enterprise that the second most abundant element in the universe, helium, was discovered in the spectrum of the Sun before before it was discovered on Earth. The element's name bears this history with its prefix derived from it was discovered on Earth. The element's name bears this history with its prefix derived from Helios Helios, ”the Sun.”
A DETAILED AND accurate explanation of how atoms and their electrons form spectral lines would not emerge until the era of quantum physics a half-century later, but the conceptual leap had already been made: Just as Newton's equations of gravity connected the realm of laboratory physics to the solar system, Fraunhofer connected the realm of laboratory chemistry to the cosmos. The stage was set to identify, for the first time, what chemical elements filled the universe, and under what conditions of temperature and pressure their patterns revealed themselves to the spectroscopist. accurate explanation of how atoms and their electrons form spectral lines would not emerge until the era of quantum physics a half-century later, but the conceptual leap had already been made: Just as Newton's equations of gravity connected the realm of laboratory physics to the solar system, Fraunhofer connected the realm of laboratory chemistry to the cosmos. The stage was set to identify, for the first time, what chemical elements filled the universe, and under what conditions of temperature and pressure their patterns revealed themselves to the spectroscopist.
Among the more bone-headed statements made by armchair philosophers, we find the following 1835 proclamation in Cours de la Philosophie Positive Cours de la Philosophie Positive by Auguste Comte (17981857): by Auguste Comte (17981857): On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are...necessarily denied to us.... We shall never be able by any means to study their chemical composition.... I regard any notion concerning the true mean temperature of the various stars as forever denied to us. (p. 16, author's trans.) (p. 16, author's trans.) Quotes like that can make you afraid to say anything in print.
Just seven years later, in 1842, the Austrian physicist Christian Doppler proposed what became known as the Doppler effect, which is the change in frequency of a wave being emitted by an object in motion. One can think of the moving object as stretching the waves behind it (reducing their frequency) and compressing the waves in front of it (increasing their frequency). The faster the object moves, the more the light is both compressed in front of it and stretched behind it. This simple relations.h.i.+p between speed and frequency has profound implications. If you know what frequency was emitted, but you measure it to have a different value, the difference between the two is a direct indication of the object's speed toward or away from you. In an 1842 paper, Doppler makes the prescient statement: It is almost to be accepted with certainty that this [Doppler effect] will in the not too distant future offer astronomers a welcome means to determine the movements...of such stars which...until this moment hardly presented the hope of such measurements and determinations. (Schwippell 1992, pp. 4654) (Schwippell 1992, pp. 4654) The idea works for sound waves, for light waves, and in fact, waves of any origin. (I'd bet Doppler would be surprised to learn that his discovery would one day be used in microwave-based ”radar guns” wielded by police officers to extract money from people who drive automobiles above a speed limit set by law.) By 1845, Doppler was conducting experiments with musicians playing tunes on flatbed railway trains, while people with perfect pitch wrote down the changing notes they heard as the train approached and then receded.
DURING THE LATE 1800 1800S, with the widespread use of spectrographs in astronomy, coupled with the new science of photography, the field of astronomy was reborn as the discipline of astrophysics. One of the pre-eminent research publications in my field, the Astrophysical Journal Astrophysical Journal, was founded in 1895, and, until 1962, bore the subt.i.tle: An International Review of Spectroscopy and Astronomical Physics An International Review of Spectroscopy and Astronomical Physics. Even today, nearly every paper reporting observations of the universe gives either an a.n.a.lysis of spectra or is heavily influenced by spectroscopic data obtained by others.
To generate a spectrum of an object requires much more light than to take a snapshot, so the biggest telescopes in the world, such as the 10-meter Keck telescopes in Hawaii, are tasked primarily with getting spectra. In short, were it not for our ability to a.n.a.lyze spectra, we would know next to nothing about what goes on in the universe.
Astrophysics educators face a pedagogical challenge of the highest rank. Astrophysics researchers deduce nearly all knowledge about the structure, formation, and evolution of things in the universe from the study of spectra. But the a.n.a.lysis of spectra is removed by several levels of inference from the things being studied. a.n.a.logies and metaphors help, by linking a complex, somewhat abstract idea to a simpler, more tangible one. The biologist might describe the shape of the DNA molecule as two coils, connected to each other the way rungs on a ladder connect its sides. I can picture a coil. I can picture two coils. I can picture rungs on a ladder. I can therefore picture the molecule's shape. Each part of the description sits only one level of inference removed from the molecule itself. And they come together nicely to make a tangible image in the mind. No matter how easy or hard the subject may be, one can now talk about the science of the molecule.
But to explain how we know the speed of a receding star requires five nested levels of abstraction: Level 0: Star Level 1: Picture of a star Level 2: Light from the picture of a star Level 3: Spectrum from the light from the picture of a star Level 4: Patterns of lines lacing the spectrum from the light from the picture of a star Level 5: s.h.i.+fts in the patterns of lines in the spectrum from the light from the picture of the star
Going from level 0 to level 1 is a trivial step that we take every time we snap a photo with a camera. But by the time your explanation reaches level 5, the audience is either befuddled or just fast asleep. That is why the public hardly ever hears about the role of spectra in cosmic discovery-it's just too far removed from the objects themselves to explain efficiently or with ease.
In the design of exhibits for a natural history museum, or for any museum where real things matter, what you typically seek are artifacts for display cases-rocks, bones, tools, fossils, memorabilia, and so forth. All these are ”level 0” specimens and require little or no cognitive investment before you give the explanation of what the object is. For astrophysics displays, however, any attempt to place stars or quasars on display would vaporize the museum.
Most astrophysics exhibits are therefore conceived in level 1, leading princ.i.p.ally to displays of pictures, some quite striking and beautiful. The most famous telescope in modern times, the Hubble s.p.a.ce Telescope Hubble s.p.a.ce Telescope, is known to the public primarily through the beautiful, full-color, high-resolution images it has acquired of objects in the universe. The problem here is that after you view such exhibits, you leave waxing poetic about the beauty of the universe yet you are no closer than before to understanding how it all works. To really know the universe requires forays into levels 3, 4, and 5. While much good science has come from the Hubble Hubble telescope, you would never know it from media accounts that the foundation of our cosmic knowledge continues to flow primarily from the a.n.a.lysis of spectra and not from looking at pretty pictures. I want people to be struck, not only from exposure to levels 0 and 1, but also from exposure to level 5, which admittedly requires a greater intellectual investment on the part of the student, but also (and perhaps especially) on the part of the educator. telescope, you would never know it from media accounts that the foundation of our cosmic knowledge continues to flow primarily from the a.n.a.lysis of spectra and not from looking at pretty pictures. I want people to be struck, not only from exposure to levels 0 and 1, but also from exposure to level 5, which admittedly requires a greater intellectual investment on the part of the student, but also (and perhaps especially) on the part of the educator.
IT'S ONE THING to see a beautiful color picture, taken in visible light, of a nebula in our own Milky Way galaxy. But it's another thing to know from its radio-wave spectrum that it also harbors newly formed stars of very high ma.s.s within its cloud layers. This gas cloud is a stellar nursery, regenerating the light of the universe. to see a beautiful color picture, taken in visible light, of a nebula in our own Milky Way galaxy. But it's another thing to know from its radio-wave spectrum that it also harbors newly formed stars of very high ma.s.s within its cloud layers. This gas cloud is a stellar nursery, regenerating the light of the universe.
It's one thing to know that every now and again, high-ma.s.s stars explode. Photographs can show you this. But x-ray and visible-light spectra of these dying stars reveal a cache of heavy elements that enrich the galaxy and are directly traceable to the const.i.tuent elements of life on Earth. Not only do we live among the stars, the stars live within us.
It's one thing to look at a poster of a pretty spiral galaxy. But it's another thing to know from Doppler s.h.i.+fts in its spectral features that the galaxy is rotating at 200 kilometers per second, from which we infer the presence of 100 billion stars using Newton's laws of gravity. And by the way, the galaxy is receding from us at one-tenth the speed of light as part of the expansion of the universe.
It's one thing to look at nearby stars that resemble the Sun in luminosity and temperature. But it's another thing to use hypersensitive Doppler measurements of the star's motion to infer the existence of planets in orbit around them. At press time, our catalog is rising through 200 such planets outside the familiar ones in our own solar system.
It's one thing to observe the light from a quasar at the edge of the universe. But its another thing entirely to a.n.a.lyze the quasar's spectrum and deduce the structure of the invisible universe, laid along the quasar's path of light as gas clouds and other obstructions take their bite out of the quasar spectra.
Fortunately, for all the magnetohydrodynamicists among us, atomic structure changes slightly under the influence of a magnetic field. This change manifests itself in the slightly altered spectral pattern caused by these magnetically afflicted atoms.
And armed with Einstein's relativistic version of the Doppler formula, we deduce the expansion rate of the entire universe from the spectra of countless galaxies near and far, and thus deduce the age and fate of the universe.
One could make a compelling argument that we know more about the universe than the marine biologist knows about the bottom of the ocean or the geologist knows about the center of Earth. Far from an existence as powerless stargazers, modern astrophysicists are armed to the teeth with the tools and techniques of spectroscopy, enabling us all to stay firmly planted on Earth, yet finally touch the stars (without burning our fingers) and claim to know them as never before.
SIXTEEN.
COSMIC WINDOWS.
As noted in Section 1, the human eye is often advertised to be among the most impressive of the body's organs. Its ability to focus near and far, to adjust to a broad range of light levels, and to distinguish colors are at the top of most peoples' list of eye-opening features. But when you take note of the many bands of light that are invisible to us, then you would be forced to declare humans to be practically blind. How impressive is our hearing? Bats would clearly fly circles around us with a sensitivity to pitch that extends beyond our own by an order of magnitude. And if the human sense of smell were as good as that of dogs, then Fred rather than Fido might be the one who sniffs out contraband from airport customs searches.