The discovery of the extremely dense white dwarf star turned out to be only the first volley in a startling stellar revolution. By the 1930s, working with the new laws of both quantum mechanics and relativity, theorists were astonished (and disturbed) to find that dying stars might face even stranger fates, if they had enough mass. Discovery of the white dwarf had opened up a whole can of cosmic worms.
In the early 1930s a young man from India named Subrahmanyan Chandrasekhar, while about to start graduate work at Cambridge University, calculated that if the mass of a white dwarf passes beyond a certain limit (now known to be 1.4 solar masses; that is, 1.4 times the mass of our Sun), it will collapse, its radius approaching zero as the star is overcome by the extreme pressure of gravity. What happens to the star? Chandrasekhar didn’t know. All he could say for sure was that a “star of large mass . . . cannot pass into the white-dwarf stage, and one is left speculating on other possibilities.”
The great Eddington declared that “there should be a law of nature to prevent a star from behaving in this absurd way!” But, with the discovery of a new atomic particle—the neutron—in 1932, others ventured that the star might end up as a relatively tiny ball of neutrons, not much wider than a city.
J. Robert Oppenheimer, who went on to become the father of the atom bomb, briefly dabbled in the subject, joining with two of his graduate students to ponder a neutron star’s range of stable masses. And in these deliberations he and another student, Hartland Snyder, in 1939 calculated that past a certain threshold of mass, the neutron star itself would not endure but instead face “continued gravitational contraction.” The neutrons could no longer serve as an adequate brake against collapse. Oppenheimer and Snyder found that the last light waves to flee get so drawn out by the enormous pull of gravity that the rays become invisible, and the star vanishes from sight. The star literally closes itself off from the rest of the universe. “Only its gravitational field persists,” reported Oppenheimer and Snyder. By 1968, astronomers regularly began calling these objects “black holes.”
Today astronomers recognize that galaxies are peppered with both black holes and rapidly spinning neutron stars (we know them as pulsars). And our understanding of such zany stellar outcomes commenced, in a way, with the discovery of Sirius’s faint companion, first spotted (maybe by accident, maybe not) more than 150 years ago.
CHAPTER SEVEN
The Star No Bigger Than a City
“Look happy dear, you’ve just made a Discovery!”
IN the fall of 1967, the first neutron star was detected, a discovery that came as a complete surprise to one and all. While the existence of such a compact star—a mere dozen miles wide—was not unforeseen (as pointed out in the previous chapter), no one imagined it would be emitting clocklike radio pulses. “No event in radio astronomy seemed more astonishing and more nearly approaching science fiction,” said the British radio astronomy pioneer James S. Hey. And it was a long road to that flabbergasting finding.
Subrahmanyan Chandrasekhar, while starting his research career at Cambridge University before moving to the United States, spent several years in the early 1930s trying to convince his colleagues in the British astrophysics community that if a star were massive enough it would never settle down as a white dwarf star in its old age. Instead, his calculations indicated that the dwarf would undergo further stellar collapse. While Chandra (as he was best known) never speculated on the other forms the star might take, others boldly did.
At a 1933 meeting of the American Physical Society, Walter Baade of the Mount Wilson Observatory in California and Fritz Zwicky at Caltech introduced the idea that such a massive sun might end up as a neutron star, a dense ball of packed neutrons not much wider than a city. This transformation would occur, they reported, in a spectacular stellar explosion they had christened a “supernova.”
Astronomers had long recognized that novae—“new stars”—occasionally appeared in the heavens. By the early twentieth century, they realized that this phenomenon involved some kind of outburst on the star. Moreover, they began to notice that there were two kinds. There were the “common” novae that appeared up to thirty times a year in both the Milky Way and other galaxies (now known to occur when a white dwarf steals mass from a companion—matter that compresses on the dwarf and eventually ignites in a thermonuclear blast). And then there was a special set, far more luminous and much rarer. In his native German, Baade first referred to them as Hauptnovae (chief novae). But both Zwicky and Baade translated that into English as “supernovae” during their lectures in Pasadena.
More than providing a name, Zwicky and Baade offered a reason for the spectacular flare-up. Neutrons had just been discovered by particle physicists in 1932, and even before that the Soviet physicist Lev Landau had suggested that the compressed cores of massive stars might be “forming one gigantic nucleus,” as he put it. Zwicky and Baade took the idea further by suggesting that under the most extreme conditions—during the explosion of a star—suns would transform completely into naked spheres of neutrons. The stellar core would somehow implode, pressing together all its positively charged protons and negatively charged electrons to form a compact ball of neutral particles.
This proposal was considered wildly speculative, and only a handful of physicists, including J. Robert Oppenheimer and his student George Volkoff, proceeded to investigate a neutron star’s possible structure, recognizing how nuclear forces would keep such stars from further collapse. For some three decades, neutron stars remained only theoretical inventions, which astronomers figured would never be seen even if they did exist, due to their extremely small size. Even the notable Princeton theorist John Archibald Wheeler was shortsighted at first. In 1964 he published an article on the neutron star, in which he said, “There is about as little hope of seeing such a faint object as there is of seeing a planet belonging to another star.” But Wheeler’s prediction was soon thwarted in a mere three years—thanks to a bit of serendipity.
A small platoon of students and technicians, led by Cambridge University radio astronomer Antony Hewish, had just completed the construction of a sprawling radio telescope near the university: more than two thousand dipole antennas, lined up like rows of corn and connected by dozens of miles of wire. Jocelyn Bell, a native of Ireland, was one of the laborers: “I like to say that I got my thesis with sledgehammering,” she once joked.
The telescope was designed to passively search for fast variations in the intensities of pointlike radio sources, such as quasars, while the celestial sky moved overhead. The data continually registered on a strip-chart recorder, and it was Bell’s job to analyze the long stream of paper—ninety-six feet (29 meters) each day—for her doctoral dissertation. Upon reviewing the first few hundred feet, she noticed, “There was a little bit of what I call ‘scruff,’ which didn’t look exactly like [manmade] interference and didn’t look exactly like [quasar] scintillation. . . . I began to remember that I had seen some of this unclassifiable scruff before, and what’s more, I had seen it from the same patch of sky.”
Jocelyn Bell helped build this radio telescope
that discovered the first pulsar in 1967.
(Graham Woan)
Eventually observing it with a higher-speed recording, Bell (later Bell Burnell upon marriage) came to see that the scruff was actually a methodical succession of pulses spaced 1.3 seconds apart. The unprecedented precision caused Hewish and his group to briefly label the source LGM, for “Little Green Men.” This was done only half in jest. At one point, some consideration was given to the possibility that the regular pulsations might be coming from an extraterrestrial-built beacon, which annoyed Bell a bit: “I was [then] two-and-a-half years through a three-year studentship and here was some silly lot of Little Green Men using my telescope and my frequency to signal the planet Earth.”
But within a few months, Bell uncovered three more rhythmical signals in different regions of the sky (along with getting engaged to be married between the second an
d the third). There was no more mention of outer-space aliens. It was highly unlikely, she said, that there were “lots of little green men on opposite sides of the universe” using the same frequency to get Earth’s attention. Carefully kept under wraps, the news was finally released in February 1968, and upon discovering a pretty, young woman was involved, the press went wild. “One of [the photographers] even had me running down the bank waving my arms in the air—Look happy dear, you’ve just made a Discovery!” Inspired by the name of the recently discovered quasars, a British science journalist dubbed the novel objects pulsars, for pulsating stars, a label that astronomers swiftly adopted.
In their Nature report, Hewish, Bell, and three colleagues pointed out that the exceedingly short span of the beep itself—around a hundredth of a second—meant that the source could span no more than 5,000 kilometers (3,100 miles, around the distance light can travel in a hundredth of a second, close to the width of the planet Mercury). This suggested the pulsar was either a white dwarf or neutron star.
The Cambridge team at first wondered whether the entire star was pulsating in and out, with the radiation then “likened to radio bursts from a solar flare occurring over the entire star during each cycle of the oscillation.” Within months, though, Cornell University theorist Thomas Gold developed the model that best explained a pulsar’s behavior: it was most likely a neutron star, whose highly magnetized body as it rapidly spins transfers the rotational energy into electromagnetic energy. This radiation is then beamed outward like a lighthouse beacon from its north and south magnetic poles. Depending on the pulsar’s alignment with Earth, we observe either one or two blips of radio energy with each pulsar rotation.
Jocelyn Bell in 1967, at the time she
was working on revealing
the first neutron star.
(Roger Haworth, Wikimedia Commons)
Since neutron stars can spin quite fast, Gold predicted that radio astronomers should also detect pulsars with shorter periods than those first discovered. This was successfully confirmed when astronomers found extremely fast-spinning pulsars within the Vela and Crab Nebulae—with periods of 0.089 and 0.033 seconds, respectively. Since each nebula was a supernova remnant, these finds also validated Zwicky and Baade’s original assertion that neutron stars would be found at the sites of stellar explosions. You can think of a pulsar as a stellar tombstone, which marks the spot where a giant star, too heavy to die quietly as a white dwarf, tore itself apart in a brilliant explosion.
Zwicky had imagined that the stellar explosion somehow created the neutron star. But astronomers later realized it was the other way around. Once it runs out of nuclear fuel, the massive star’s core collapses catastrophically under the force of gravity. A core that was once the size of the Moon is squeezed down in less than a second, cramming the mass of 1.4 to 2.5 suns into a space roughly as wide as Philadelphia. In this way the stellar protons and electrons merge to form a tight ball of neutrons, whose density is so great that a sugar-cube-sized portion would weigh as much as Mount Everest. The shock wave sent out from the collapse, along with a flood of neutrinos, then speeds through the remaining stellar envelope, emerging from the surface as the spectacular supernova.
Astronomers estimate that at least a few hundred million neutron stars now reside in the Milky Way, created over the eons since our galaxy’s birth. But the first one revealed, officially known as PSR 1919+21 for its celestial coordinates, will never be forgotten—not just for its discovery but for the controversy that later surrounded it. When the Nobel Prize in Physics was awarded in 1974 for pioneering work in radio astrophysics, including the discovery of pulsars, it was Hewish who walked up to the podium (along with Martin Ryle), but not Bell Burnell. Hewish had been skeptical about Bell’s “scruff” at first, believing at one point that it was either a stellar flare or manmade. It was only due to her persistence that its origin was at last revealed. At Great Britain’s Observatory magazine, the editors wryly joked among themselves that Nobel now stood for “No Bell.” Her being a young, female graduate student (only two women have won the physics prize since the first award ceremony in 1901) likely prejudiced the judges.
But Bell Burnell, who went on to a distinguished career as a professor, dean of science, and president of the Royal Astronomical Society, maintained a sanguine attitude about this flagrant oversight. During an after-dinner speech at a relativity conference in 1977, she noted that the final responsibility for the success or failure of a scientific project rests with its supervisor. “I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases, and I do not believe this is one of them. . . . I am not myself upset about it—after all, I am in good company, am I not!”
No—they’re in good company with her. While memories of who won a Nobel Prize dim over time, Bell Burnell will always serve as the main protagonist when recounting the story of the neutron star’s discovery.
CHAPTER EIGHT
Ye Old Black Hole
An eighteenth-century theorist was just
too far ahead of his time
BORN on Christmas day in 1724, the Englishman John Michell was a geologist, astronomer, mathematician, and theorist who regularly hobnobbed with the greats of the Royal Society of London. His companions included such men as Henry Cavendish, Joseph Priestley, and even the Society’s American fellow Benjamin Franklin (during the diplomat’s two long stays in London). The claim could be made, science historian Russell McCormmach has written, that Michell was “the most inventive of the eighteenth-century natural philosophers.” Yet until recently, if he was remembered at all, it was for his suggestion, in 1760, that earthquakes propagate as elastic waves through the Earth’s crust. That earned Michell the title “father of modern seismology.” In addition, a torsion balance he invented was later used by Cavendish to weigh the entire Earth.
Otherwise, Michell has been largely forgotten. That’s because he had the unfortunate habit of burying original insights—such as the inverse-square law of magnetic force—in journal papers that focused on inferior research. Some of his greatest ideas were casually mentioned in brief asides or footnotes. As a consequence, long-lasting fame eluded him.
Michell began his scientific investigations at Queens’ College in Cambridge. Son of an Anglican rector, he entered Queens’ in 1742 at the age of seventeen and after graduation remained there to teach for many years, eventually becoming a rector as well. A contemporary described him as a “short Man, of a black Complexion, and fat. . . . He was esteemed a very ingenious Man, and an excellent Philosopher.”
But by 1763, ready to marry, Michell decided to devote himself to the church. He ultimately settled in the village of Thornhill in West Yorkshire, where he served as a clergyman until his death in 1793 at the age of sixty-eight. Yet, over those years with the Church of England, the reverend continued to indulge his wide-ranging curiosity. He had a nose for interesting questions and was willing to stick his neck out in speculation, though always grounded in his first-rate mathematical skills. One of Michell’s more intriguing conjectures at this time, right when Great Britain was recovering from its war with colonial America, was imagining what today we would call a black hole.
This idea grew out of an earlier prediction that Michell had made. Astronomers in the eighteenth century were starting to see more and more double stars as they scanned the celestial sky with their ever-improving telescopes. The common wisdom of the time declared that such stars were actually at various distances from Earth and closely aligned in the sky by chance alone—that it was just an illusion that they were connected in any way. But, with remarkable insight, Michell argued that nearly all those doubles had to be gravitationally bound together.
He was suggesting that some stars exist in pairs, a completely novel notion. In a groundbreaking paper published in 1767, he worked out the high probability that, given how most other stars were arranged in the sky, the twin stars were physically near one another—“the odds agai
nst the contrary opinion,” he stressed, “being many million millions to one.” (As usual, he displayed the results in a footnote.) In carrying out this calculation, Michell was the first person to add statistics to astronomy’s repertoire of mathematical tools. The paper was “arguably the most innovative and perceptive contribution to stellar astronomy . . . in the eighteenth century,” according to the astronomy historian Michael Hoskin.
At the same time, Michell recognized that double stars would be quite handy for learning lots of good things about the properties of stars—how bright they are, how much they weigh, how vast is their girth. Two stars orbiting each other were the perfect laboratory for testing out Newton’s laws of gravity from afar and arriving at answers. Yet, nearly all astronomers in his day weren’t concerned with such questions. They were too busy discovering new moons or tracking the motions of the planets with exquisite precision. To them, the stars were merely a convenient backdrop for their measurements of the solar system, the arena that most captured their attention.
The British astronomer William Herschel, a friend of Michell’s, was the rare exception to that emphasis, and within a dozen years of Michell’s paper on double stars, he began monitoring and cataloging the stars positioned close together in the sky. Encouraged by Herschel’s growing data bank, Michell decided to extend his ideas on double stars in a paper with the marathonic title “On the Means of discovering the Distance, Magnitude, &c. of the Fixed Stars, in consequence of the Diminution of the Velocity of their Light, in case such a Diminution should be found to take place in any of them, and such other Data should be procured from Observations, as would be farther necessary for that Purpose.” (Whew!) It was in this work that Michell hinted at the possibility of a black hole—or at least his eighteenth-century, Newtonian version of one.
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