Turn Right At Orion (27 page)

All this time, the first flash of light from the supernova was racing out into space. Traveling at the speed of light (of course), the flash was outrunning the physical debris by 20 to 1, but so far there had not been much. in its path to catch the rays. That changed about a year into the explosion, when the light reached the inner edge of the wind that had been expelled from the star when it was a red supergiant. The supernova now began to light up its surroundings,.

I had anticipated that there would be a year's delay or so between the flash of the explosion and its echo against the matter that surrounded the exploded star. From my own observations as I approached the Large Magellanic Cloud, I knew that the star had changed from red to blue some 3000 or 4000 thousand years earlier. Blue supergiant stars, of course, produce a wind that is, if anything, more powerful than the wind from a red supergiant. But such winds impress via their speed, not via the amount of matter they inject into the region immediately surrounding the star. Thus the light from the supernova could pass straight through the relic of the blue supergiant wind without producing much effect. The red star's wind, however, would
have been slow and dense, and when the intense light of the supernova finally reached the remnant of that wind, it would surely light up like a neon sign.

In 3000 or 4000 years, the matter expelled by the red supergiant, with a typical speed of 20 kilometers per second, would have traveled about a quarter of a light-year. Once the star turned blue, though, its wind must have quickly overtaken the slower wind from behind and hurried it along, so I guessed that the interface would be located somewhat farther out, maybe close to a light-year. The echo would then take nearly a year to get back to my position, so I expected that I would see the sky light up just before the second anniversary of the explosion.

Witnessing the first assault of the supernova on its surroundings would provide a kind of closure to this leg of my journey The initial flash of the supernova's light had been intense with ultraviolet rays, and if the dense gas expelled by the red supergiant surrounded me on all sides, then the whole sky should light up in the exquisite green of twice-ionized oxygen. The contrast with the deep reds of the expanding debris promised to be spectacular, and the symbolism was not lost on me either. A slow wind swept up by a fast wind, the flash of the explosion replacing the steadier light of a newly minted white dwarf . . . the analogy to being inside a planetary nebula was almost too perfect. And like a planetary nebula, this supernova was participating in the grand evolutionary cycle of the Universe. The flash would signify the connections between the death of this star and the lives of stars yet unborn. Instead of being responsible for synthesizing carbon and nitrogen, this star's portfolio included oxygen, neon, magnesium, and silicon, all mixed up with a goodly amount of hydrogen and helium ready for reuse. Only the iron core—the matter that was now to spend eternity as a neutron star—had vanished from circulation, memorializing the exploded star's existence just as white dwarfs constituted lasting monuments to stars of lower mass.

Just shy of 1 ½ years after the explosion, the flash appeared. It did not look as I had expected it to. Instead of lighting up all
over the sky, as it would have done had the red wind formed a spherical container around the blue wind's bubble, the green light appeared as a narrow band, maybe half the width of the Milky Way as viewed from Earth, which lengthened until it circled the entire sky. There was no spherical cavity, that much is clear. Apparently, the nearest parts of the red supergiant's wind had been pinched into a ring, probably the waist of an hourglass shape that fanned out . . . who knew how far? Once again the connections struck me: the Dumbbell Nebula, with its broad waist, about to fade into the interstellar background; the Ring Nebula; the Saturn Nebula, with its mysterious “ears;” and so forth. I thought all the way back to what I had seen in Orion: the star-forming regions with their dual funnels pouring out light and their ubiquitous jets, The same shapes kept appearing and were repeated here, even in the chaos of a star's violent self-destruction. I knew that at this moment—I mean, of course, when the light finally reaches Earth 160,000 years hence—the astronomers of those times would appreciate the deep connections among these phenomena.

The explosion at the edge of the Tarantula Nebula would scar the Large Magellanic Cloud for ages to come. Within 100 years, the shrapnel from the star would have plowed into so much of the surrounding matter that it would have begun to slow down. The surroundings, swept into a thick shell, would absorb what remained of the explosion's energy, carrying on the expansion while the spent debris fell behind. But fingers of the exploded star's matter would continue to poke outward into interstellar space, mixing with and enriching the surrounding gas with the fruits of its nuclear alchemy. Eventually, its own peculiar blend of heavy elements would meld with the dollops of carbon-nitrogen mixture being supplied by planetary nebulae that were as common here as they had been in the Milky Way.

But that would take time. In tens or hundreds of thousands of years, or perhaps a million years, there might still be an ultraviolet or even an X-ray reminder of the star that had exploded as I watched. If no fresh catastrophe overran the neighborhood, shredded filigrees of glowing gas would eventually outline a shell dozens of light-years across, which would gradually slow down and fade until it disappeared into the background. Analysis of the atomic disturbances, X-ray colors as well as ultraviolet and visible, would still bear the imprints of chemicals created
here. Just then my eyes caught the blinding glare from the thousands of hot massive stars that filled the center of the Tarantula Nebula. Each one was destined for a similar explosive demise, and each would propel the surrounding gas into an echo of its own expansion. Chances were that one of these explosions would push aside and smear out my supernova's remnant long before it had run its natural course. I surveyed the onrushing debris and glowing ring wistfully, surprised to feel a proprietary interest in an event so far beyond the scale of any human endeavor. And I was amused to find myself consoled by the thought that the untimely disruption of my star's remnant would only speed up the incorporation of its heavy elements into the fabric of the Large Magellanic Cloud.

Whatever the fate of the supernova's gaseous remnant, the star's imploded core would stand fast as a neutron star, perhaps shining for a time as a pulsar. But was it really there? By now its effect on the debris should have become apparent. I searched in vain for something giving the debris cloud extra oomph, adding some power to it over and above the decaying radioactive cobalt. If the neutron star were sending out a searchlight beam, producing flashes of ordinary light like the Crab Nebula's pulsar, those flashes should have been visible directly. But there was no sign of any pulsar. It was unthinkable that the entire core had exploded. Something was surely left behind, but it seemed to be invisible. Was there some reason why the neutron star had not begun to pulse? Had something robbed the core of its angular momentum, damping down its spin as it collapsed? Or had its magnetic field failed to develop? Had something more interesting happened? Could enough, matter have fallen back onto the core to drive it over the edge, causing it to collapse to a black hole? I briefly considered investigating for myself but decided against it. I was not inclined to enter the nebula any more deeply. Like the debris, my thoughts were turned outward, and I had little inclination to explore the kind of tortured confines near a neutron star—or black hole—that had proved so anxiety-provoking before.

One thing was clear: I had come nowhere close to escaping the complexity of the Milky Way by traveling to its satellites. The Magellanic Clouds were more complex, if anything, because they were
less
self-contained. These galaxies had evolved differently from the Milky Way's disk and from each other. Was it because of their frequent encounters with one another or because of their incessant plunge through the stars and gas that made up the Milky Way's halo? Were the Clouds poor in oxygen and iron because their stars had never manufactured the quantities of heavy elements that larded the Milky Way's disk, or had massive stars once lavished oxygen, silicon, and iron on the Clouds, only to have these elements stripped away along with the gas they permeated? I tried to project the evolution of the Large Cloud into the future, long after the thousands of hot massive stars that filled the Tarantula Nebula had exploded. Would astronomers then say that the Large Cloud had finally caught up to the Milky Way in its chemical richness? Or would much of this oxygen-enriched gas be pulled out of the Large Magellanic Cloud the next time it got too close to the Small Cloud, before it had a chance to form a new generation of stars? Perhaps the gas would be blown out of the Cloud by the force of those concatenated explosions. I wondered whether the outcome of all this stellar activity might be the enrichment, not of the Magellanic Clouds, but of the halo of the hegemonic Milky Way that would soon engulf them. For how long would there be Magellanic Clouds at all?

Having seen the grandeur of a supernova explosion, and having allowed the debris to sweep over me only a few months after the event—having been
inside
a supernova debris cloud—I found further exploration of the Tarantula Nebula and its vicinity hopelessly anticlimactic. I was impatient to move on. I knew that by leaving now, I would miss a spectacle beside which the illumination of the red supergiant's wind would pale. Twenty years after the explosion, the debris shell itself would finally ram the dense ring that formed the waist of the hourglass, and the
ring would sparkle brilliantly in every part of the electromagnetic spectrum.

But a close-up view of this marvel would have to wait for another time and another supernova. There were many pressing questions still to answer. Should I continue to pursue the puzzle of evolution or set it aside in pursuit of other, equally insistent goals? I was bothered that big pieces of the puzzle were missing. For example, this supernova, despite its dramatic nuclear endgame, could not be the major source of cosmic iron. Carbon and nitrogen were ably supplied by the calm outgassing of planetary nebulae; oxygen and many other elements were derived from violent explosions like the one l had just witnessed. But most of the iron had to come from somewhere else. By this time the cobalt had just about finished decaying, and I knew that this supernova would not release much more than the 7 percent of the Sun's mass in iron that had been produced through radioactivity. The much larger quantity of iron that the star had created in its last week or so before exploding had been crushed to neutrons and locked up forever. There had to be yet another route by which matter could be forged into iron in a nuclear furnace and then released for future use.

If I wished to seek out the missing iron, I knew where to look. During my long traversal of the Milky Way's halo, I had noticed, far in the distance, 100 or more flashes at random intervals and seemingly at random locations throughout the halo. This journey, which for me had required only a couple of decades of hibernation, had sampled 160,000 years in the life of the Galaxy. these flashes were appearing every 1000 years or so. I had had the presence of mind to record the signals from some of these brief flares, and now I knew much more about their nature. They, too, were explosions of stars, but explosions of a rather different sort from the one that had produced my supernova. They were brighter and faster, and they showed no signs of the pink hydrogen light that dominated the explosion I had just witnessed close at hand. I remembered my colleagues debating
the provenance of these events during the years before I left Earth. It seemed that they were explosions of stars that had completely lost their hydrogen envelopes. Because they were seen to occur in the Milky Way's halo as well as in the disk, they had to be old stars. People believed that these supernovae were the explosions of white dwarfs.

How, in the calm reaches of the halo, an ancient, dead star could suddenly blow up—that had been a major mystery. It had been supposed that the explosion occurred when so much matter was dumped onto the surface of a white dwarf that the pressure of its electrons could not support the extra weight. In these remote environs, the fresh supply of matter could come only from a companion, star. The dwarf would start to collapse, but this time, nuclear reactions would intervene before gravity got the upper hand. The entire star would explode like a powder keg, incinerating its store of nuclear fuels in a great burst and leaving no cinder behind. This kind of supernova produced the iron in the Universe.

I thought how easy it would be for me to determine, once and for all, how these explosions were triggered. Should I strike out into the Milky Way's halo to pursue this third venue for nuclear alchemy? There was the considerable problem of trying to pinpoint a likely candidate for such an explosion. Positioning myself would be tricky, even given
Rocinante's
maneuverability. But there was a more formidable obstacle: I could not work up the enthusiasm for a further foray through the Milky Way, disk or halo. Even though I had never left the Milky Way's halo, my exploration of the Magellanic Clouds had given me a new perspective on the home Galaxy. I now saw it as though from outside. I dwelt more and more on the relationships of one galaxy to another. Galaxies underwent great cycles of birth and death, all the time evolving into systems more complex, more diverse. But they did not do so in isolation.

I pondered where to go next. Distant galaxies beckoned, but the home Galaxy represented a kind of security. The disk of the Milky Way began to seem more inviting. It filled half the sky
with its mesmerizing spiral pattern, great rows of nebulae, and dark lanes of cloud. It looked like it would contain Everything I might ever want to explore, and I must have lost my nerve momentarily, because the next thing I knew I was heading straight toward the Milky Way's disk. But I did not stop there. In what seemed like a few moments I plunged through the disk, emerged from the other side, and kept on going.