“Why,” said the Dodo. “The best way to explain it is to do it.”
—LEWIS CARROLL, ALICE’S ADVENTURES IN WONDERLAND
During my first teaching gig in graduate school, I nearly put out a student’s eye with a pocket supernova. Honestly, the Astro Blaster didn’t seem all that dangerous. It was just a solid rubber bouncy ball with a short plastic stalk affixed to it. Three progressively smaller bouncy balls, each with a hole drilled through it, slid over the stalk so that in the end, you had what looked like a baby’s stacking toy of extremely bouncy balls. The entire apparatus was perhaps 20 centimeters (8 inches) long and weighed no more than an empty coffee mug, but the packaging exclaimed that this four-dollar wonder “works like a real supernova!”
I wasn’t fully convinced that this toy would work like a real supernova, but fantasies of seeing it blast out the Earth’s weight in gold ran through my head. When I got to the unit about exploding stars and the mechanisms behind them, I thought the Astro Blaster might be an amusing demonstration for my introductory astronomy students at the local community college. I took it out, threaded the three progressively smaller balls onto the stalk attached to the largest bouncy ball, and dropped it onto the classroom floor.
In retrospect, staying indoors was a mistake.
Although the whole apparatus was only dropped from the seemingly innocuous height of a meter or so, the top ball shot like a bullet into the ceiling. From there it ricocheted around the room, narrowly missing my now very-much-awake students, and coming to rest under an unoccupied desk.* I sheepishly retrieved the ball and continued the lecture, the mechanism for at least one kind of supernova now abundantly clear: a seemingly small collapse inside a star can provide enough energy to blast the outside completely off.
Unlike the Astro Blaster manufacturer, the universe hasn’t seemed particularly eager to help us on our quest to understand the power source of new dots in the sky. The telescope was invented just a few years after the last known event in our own Galaxy, and the picture in the rest of the cosmos was barely any better. By the early 1930s, only a smattering of telescopically observed transients had been recorded, and of those, precisely one (S Andromedae in 1885) had been hosted by our galactic next-door neighbor. The rest of the roster had occurred, rather inconveniently, in galaxies tens of millions of light-years away.
I say “inconveniently” because understanding something in the cosmos requires more than just spotting it. To get to the heart of these events, astronomers have to study every aspect of their light during the brief time that the objects are visible. Until very recently, distant new dots always got a sizeable head start on astronomers, who couldn’t keep an eye on every galaxy every night. As a result, key information about the early days of events was inevitably lost. Sifting through the spectrum for evidence was no less challenging. Stretching out the light made it that much fainter, that much harder to collect and interpret, particularly if the object studied was 50 million light-years distant. Ideally, astronomers needed to observe where the new dot came from, what it did as the days progressed, and what, if anything, remained after the dust settled.
Still, a handful of these extragalactic events was a good enough start for astronomers Walter Baade and Fritz Zwicky. In 1934, they published the first papers about supernovae, one with the succinct and self-explanatory title “On Super-Novae.” This publication coined the new term, and in it, they outlined the major differences between what they termed “common novae” and “super-novae.”
Common novae are, well, common. Dozens show up in our Galaxy and in other galaxies each year. Their energies are impressive, but not superlative, maxing out around 20,000 times the energy output of the Sun. Supernovae, on the other hand, are much rarer and much more energetic.
With a revised distance in hand, astronomers estimated that S Andromedae, which would become known as SN 1885A, emitted 10 million years of sunlight in under a month, far more than Clerke had imagined. As for how much total light was emitted by this event from the moment it was spotted to the moment it became invisible, Baade and Zwicky were forced to make a number of assumptions. One was that supernova progenitors are “quite ordinary stars” perhaps as much as 50 times the mass of our Sun that, for unknown reasons, suddenly expand. With a set of parameters that gave absolutely no insight into what could drive such a rapid expansion, they arrived at a value approximately equivalent to the energy produced by the Sun over billions of years.
More data were clearly needed. “Unfortunately, at the present time only a few underexposed spectra of super-novae are available, and it has not thus far been possible to interpret them,” concluded Baade and Zwicky.
But that was not the only word the two had on the subject. The companion piece to “On Super-Novae” was the much more audacious “Cosmic Rays from Super-Novae.” On the surface, the second piece concerned itself with the mysterious high-energy particles known as cosmic rays. Without much evidence—but, as it turns out, an amazing amount of prescience—Zwicky and Baade argued that these hyperactive subatomic bits originate in supernovae. Hiding in a section titled “Additional Remarks,” though, was this gem: “With all reserve we advance the view that a super-nova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density.”
A collapse mechanism had been found. The Astro Blaster could now fall.
Admittedly, the mechanism would not have been found had the neutron itself not been found. The 1932 news that English physicist James Chadwick had discovered a neutral particle in the nucleus of the atom was practically still hot off the scientific presses, but theoretically such a beast allowed great masses of material to become very small. More important, they could do so very quickly.
Fritz Zwicky had never intended to become an astronomer. The California Institute of Technology, better known as Caltech, had brought the Swiss physicist to Pasadena with a Rockefeller Foundation grant in 1925, and for the first few years, he largely busied himself with the study of crystal structures. With a larger-than-life personality and a nearly complete lack of a verbal filter—according to John Johnson and others, one of his favorite epithets was “spherical bastards” for those who seemed to him to be “bastards no matter which way you look at them”—Zwicky frequently challenged the old guard. Nobody was safe from his biting criticism. In 1930, he told Nobel Prize–winner and colleague Robert Millikan, who had determined the charge of an electron in a creative 1909 experiment involving tiny droplets of oil, that Millikan had never had an original idea. In response, Millikan somehow managed not to fire Zwicky. Instead, he told him to try astrophysics.
“Supernova Zwicky” was born, although the moniker “Supernova” often referred less to his success at observing supernova than to his explosive personality. He was infuriatingly open with his scorn. In a 1936 publication, he publicly mocked noted astronomer Cecilia Payne-Gaposchkin who, he stated, “failed to make the obvious and necessary distinction” between different regions in a star in her own paper as she grappled with possible spectral clues in supernovae. She had not, in fact, misunderstood.
Regardless of his personal shortcomings, Zwicky knew that he could never expect to get a real handle on supernovae with so few observations. He needed to catch more of these cosmic fireflies and study them in detail, and to do that, he needed to observe as many galaxies as possible as often as possible. By his reckoning, any given galaxy could be expected to host one supernova every millennium. Thus, by tirelessly observing 1,000 galaxies, he could anticipate spotting one supernova per year; by fixating on 10,000, he could anticipate about one per month.
The plan seems simple enough. The problem, though, is that astronomical telescopes historically have sacrificed a sweeping panoramic view for the ability to gather as much light as possible from tiny patches of sky, preferring depth over breadth. This trade-off is fine for most objects most of the time. The quiet and unchanging universe—or, more accurately, the ponderously slowly changing universe—allowed eighteenth- and nineteenth-century astronomers to catalog and map the well-behaved members of the nearby cosmos gradually and with great accuracy. Even some of the less well-behaved objects are cooperative. Henrietta Leavitt’s Cepheid variables are fairly abundant, and unlike supernovae, they endure. The Cepheids that Leavitt studied are still faithfully varying in brightness, and they will continue to do so for generations to come.
What Zwicky was planning was astronomical madness, basically sacrificing endless hours in what might ultimately be a wild goose chase. What was needed to repeatedly observe thousands of galaxies was a side step in telescope technology.
The observatory at Mount Wilson, just northeast of Los Angeles, boasts a telescope that is 2.5 meters (100 inches) in diameter and the length of a truck. Although the diameter of this telescope is huge, the patch of sky it reveals is not. Different optical configurations yield different fields of view, but a monster this size can give researchers a peek at less than a millionth of the sky, or about one-hundredth of a square degree. Even if each patch of sky took only a minute, it would take years to cover it all. Worse yet, the likelihood of anything exciting and short-lived happening within that fraction of a square degree of sky during the minute it would be observed every few years is basically zero. What Zwicky needed was not a larger, more powerful telescope, but one that would capture a great chunk of the sky in one go.
The entire sky, as measured in angular units, is 41,253 square degrees. A closed fist held at arm’s length takes up about 100 square degrees, about enough to cover the Large Magellanic Cloud and all its Cepheid variable stars. The view through birdwatching binoculars is surprisingly limited, revealing 20 or so square degrees, but easily encompassing the entirety of the Moon. The Moon as seen from Earth covers only about 0.2 square degrees, a figure that seems hard to believe. The Moon looks so big. And yet it would take over 200,000 of them to fill up the sky.
The human eye, despite missing out on the occasional meteor, is a marvel. It is continually surveying thousands of square degrees—an enormous swath of the available sights around you—and making note of changes. Just think of all the shiny objects that catch your eye.
Supernovae had caught Zwicky’s eye, and although he had been told that searching for them was tantamount to panning for gold, he persisted. His supernova survey telescope was built atop Palomar Mountain near San Diego in 1936 and was all of 18 inches in diameter—less than half a meter—but it saw several square degrees of sky at a time. In September, he began scanning select patches of sky with high concentrations of known galaxies. Night after night, week after week, photographic plate after photographic plate, he ultimately collected nearly 10,000 “nebular images” in his search for what he would later sometimes call “atom bomb stars.” During the cold winter, the irascible mountaineer and ski enthusiast built a homemade ski jump at the site. The thousands of unchanging galaxies were utterly ignored. He was interested only in finding new dots punctuating the hazy smears.
Within six months he had bagged his first supernova. Spotted on the plate from 16 February 1937, it was on the outskirts of an elongated fuzzy patch known as NGC 4157, a galaxy not too dissimilar to our own. About 100,000 light-years across, NGC 4157 lies nearly 36 million light-years from home. Since Zwicky’s 1937 discovery, this galaxy has hosted two more supernova events, one observed in 1955 and one observed in 2003. As it turns out, a typical galaxy will host a supernova about twice per century, not just once per millennium.* This was good news for Zwicky, as this higher frequency allowed him to single-handedly discover 122 supernovae, the most discovered by any one person.
Once spotted, a new supernova became the subject of intense scrutiny. At the Mount Wilson Observatory, Baade would turn the 100-inch telescope to the newcomer, insert a photographic plate, and measure the brightness of the object. Plates at Mount Wilson added up as the changing intensity of the supernova’s dot was measured. The final product of all this scrutiny is known as a “light curve,” a plot showing how the brightness changed over the weeks and months since the supernova’s discovery.
What Baade and others soon realized is that some supernovae, after fading for a time, seem to level off in brightness for several months, only to fade away rapidly. Others quickly brighten and gradually fade away. No two supernovae are identical twins, but some definitely bear a family resemblance. Intriguingly, when astronomers accounted for the distances to the supernovae, they found that quite a large proportion of them seemed to have the same maximum brightness, give or take. Like Leavitt’s law of Cepheid variable stars, this feature was flagged as a possible way to gauge colossal cosmic distances.
At the time, what light curves revealed about supernovae was not yet certain, but like detectives gathering clues, Zwicky and Baade hoped these data would add something critical to the story. They were right. Decades later, charting a light curve is still a fundamental part of observing supernovae.
By 1937, German scientist Rudolph Minkowski, escaping Nazi Germany, had joined the team. His job was to stretch out the supernova’s light into a wide rainbow and comb through the various bright and dark features to find hints of the physical conditions in these events. By 1940, Minkowski realized that supernovae seem to fall into one of at least two observationally distinct categories. The spectra of some supernovae show telltale stripes at wavelengths associated with the element hydrogen, while such lines of hydrogen are absent in the spectra of others. As more spectra were obtained, astronomers began designating the supernovae with hydrogen-free spectra to be Type I. Those that showed features associated with hydrogen became Type II. These divisions have endured through the decades, although there has been quite a bit of refinement.
What astronomers were doing then—and what many are doing now—is the celestial equivalent of the biological taxonomy studied in high school science classes. Does it have a backbone? If the answer is yes, then it’s a vertebrate. Understanding the evolutionary processes that resulted in the backbone, though, is another matter entirely, one that would never have been achieved without making that first classification.
Why do some supernovae display signs of hydrogen in their spectra, while others don’t? Why do some seem to level off in brightness after a while? Each new discovery seemed to spawn more questions than it answered. Still, now that supernovae were no longer once-in-a-century events, but successfully hunted quarry, the answers promised to lie just over the next hill.