Turn Right At Orion

by Mitchell Begelman

  Still there is no respite. The core is shrinking and getting hotter. Not only are the atomic nuclei slammed together with greater force, but they are also slammed together more frequently, and the nuclear reactions accelerate. Each stage of thermonuclear fusion runs faster than the one before. if it took 1 million years to fuse the helium into carbon and oxygen, then it would take less than 100,000 years for all the remaining carbon to be destroyed and the core to be converted to a mixture of oxygen, neon, and magnesium.

  It was sometime during this frenzied episode of nuclear incineration that the star in the Magellanic Cloud changed color from red to blue. The exact reasons for the change remain obscure, except that, as in Betelgeuse, there was a serious and growing disconnection between the shrinking core and its distended envelope. Just as slight hiccups could cause large portions of a supergiant’s envelope to fly off into space, so could they cause the envelope suddenly to deflate, and this is apparently what had happened to the doomed star in the Large Cloud.

  Oxygen, neon, and magnesium fuse into silicon and sulfur, and thence into iron. The last stage, which takes only a few days, must have occurred as I eased my craft into its orbit about the star, oblivious to the drama inside. The creation of the iron core is a watershed event for the star. Iron is not nuclear fuel. No energy can be released by combining the nuclei of iron atoms with other atomic nuclei. Yet the core, too heavy to support itself, continues to shrink and get hotter. Now gravity steps into the limelight, relegating nuclear power to second place. No longer is it the supporting player that inexorably moves the core from one stage of nuclear burning to the next. From now on, gravity calls the shots, and the nuclear reactions follow.

  As gravity overwhelms all forms of resistance, the core implodes, at first gradually, then at breakneck speed, contracting the last few hundred kilometers in less than a second. The nuclei of iron atoms break apart under the extreme pressures and temperatures. This sphere of iron the size of Earth, formed painstakingly through the elaborate successive stages of nuclear fusion, becomes an ultradense globe of indeterminate composition, a soup of nuclear fragments, less than 100 kilometers across. Reversal of the core’s nuclear alchemy does not stop even with the disassembly of matter into pure hydrogen. The protons and electrons of this primordial element are squeezed together with such vehemence that they merge to form neutrons, the building blocks of the neutron star that is soon to form.

  It was at this point that I received my dose of neutrinos. The particles known as neutrinos—Enrico Fermi’s “little neutral ones”—are famous for being nearly impossible to detect. They hardly interact with anything, and that’s why they were able to pass through the otherwise highly protective shielding of my craft. If I had wanted to build a craft that would protect me from the bath of neutrinos, its skin would have had to be a light-year thick to catch all of them! Yet so many neutrinos flowed out of the imploding core (a billion trillion trillion trillion trillion, one for every proton-electron duo that merged to form a neutron) that 3 million of them managed to come to grief inside my eyeball, snapping me out of sleep. The same thing happened in every other cubic centimeter of my body, but I was lucky. This was not a lethal dose; it wasn’t even harmful. If I had been as close to the star as Earth is to the Sun, instead of 300 times farther away, I would have been done for.

  The neutrinos had a much more devastating effect on the star. The collapsing star’s interior presented such a formidable barrier that even the ultra-elusive neutrinos had a hard time escaping. Most of them eventually made it, but at the cost of pushing so vigorously against the star’s interior that the pressure built up to dangerous levels. The core continued to collapse, releasing neutrinos at an accelerating pace as it shrank the final tens of kilometers down to the city-sized dimensions of a neutron star. But the surrounding shells of partially fused matter and the extensive envelope of hydrogen and helium could not resist the accumulated pressure of all those neutrinos struggling to get out. These portions of the doomed star were blasted away as the dammed up neutrinos finally broke free. It took two hours for the explosion to organize itself and reach the star’s surface, and only when it burst through the surface was I alerted to flee.

  28

  Aftermath

  In the chaotic supernova blast, the shells of material that hadn’t collapsed with the core were stirred vigorously into the exploding envelope. What rushed out at me was composed largely of hydrogen and helium, because most of the star’s envelope had never fused, but it was streaked with intrusions of all the partial and final products of nuclear fusion that had layered the doomed star’s interior with an onion-like structure. Carbon, which had underlain the hydrogen-burning shell, emerged, along with nitrogen and fingers of gas rich in oxygen, neon, and magnesium. Silicon, sulfur, calcium, and a host of other elements were shot through the exploding envelope from below. Most of the iron was gone, having been sucked into the neutron star and destroyed. But hidden below the surface of the expanding blast and traveling along with it were globules consisting of more exotic species, dominated by a radioactive form of nickel. These elements had been forged in the heat of the explosion, and they would have a curiously important effect on the appearance of the blast during the months that followed.

  The blast continued to brighten for about 3 months. By this time I was 1700 times farther from the center of the explosion than Earth is from the Sun, more than 5 times farther than I had been when the explosion went off. Debris still hurtled outward, unimpeded, and would reach this location only 3 months behind me. Should I keep running or allow the debris to overtake my craft? I calculated that after expanding this far, the shrapnel would have thinned out considerably and would present little hazard. It would rock my craft with less than one Earth’s atmosphere of pressure—I had withstood more while traveling to the center of the Milky Way at high Shangri-La factor. True, the debris was full of radioactive material, but Rocinante’s skin would easily protect me from its dangerous emissions. I parked my craft (taking the requisite month and a half to decelerate) and sat back to watch the show.

  The brightening was not due to any extra source of power inside the sphere of debris. When the explosion had started, the interior of the blast had been unimaginably hot and full of radiant energy. But that energy could not escape immediately. It was trapped by the immense amount of material it would have to penetrate. The intense luminosity I was seeing now was the residual heat of the explosion finally making its escape, though in a considerably diluted form.

  Once this heat began to leak out in earnest, the appearance of the explosion changed dramatically. Its opaque “surface” thinned out, and I could see farther and farther into its interior. The featureless disk that the explosion had presented on my sky, which had cooled down to a temperature and color similar to the surface of the Sun, dropped behind the onrushing front of debris. The latter developed a tortured, mottled texture, with dark spaces opening up between writhing, entangled filaments of light. It looked like an exploding ball of snakes. These filaments were not radiating the smooth distribution of colors that had characterized the opaque envelope but, rather, assumed an array of pure hues that I recognized as the signatures of specific atomic disturbances. Different filaments, having different elemental compositions, produced diverse arrays of color, but the gradations were subtle. The dominant color, as in so many other places I had visited, was the pink of hydrogen—not surprisingly, because this was the dominant element of the star’s envelope. Hydrogen’s glow not only outlined sharp striations within the debris globe but also provided a pervasive undercoat in the interstices between filaments. A similar yet distinctive shade of red came from oxygen atoms, confined mainly to certain filaments that must have punched their way through from below. In still other locales, I could detect (with the aid of my instruments) an intense infrared glow associated with calcium. The color contrasts were not so stark as they had been in the Dumbbell Nebula. Here there was no hot star at the center to stir up the atoms with a stea
dy injection of ultraviolet rays. And if the neutron star had become a pulsar, like the one at the center of the Crab Nebula, its emissions were nowhere to be found.

  Though I detected many other atomic signatures, their signals were weak. The breakneck expansion of the debris had apparently chilled it to the point where it was too cold to extract much emission from helium and many other elements I knew to be present. The original heat of the explosion was now mostly gone, and one might have expected the brightness of the exploding debris to fizzle out entirely, at least until it collided with whatever matter surrounded the site of the explosion. But something intervened to keep the supernova shining. After a sharp peak of luminosity and a brief, even sharper plummet, the collective brightness of the debris began to level off and to decline more gradually. Something inside the debris was now providing extra energy, at a very measured rate.

  The culprit was radioactivity. Most of the radioactive nickel that had been created in the moments following the explosion, forged at temperatures of 200 billion degrees or more, had decayed away within the first couple of weeks. Its decay had released some energy, but not enough to compete with the energy of the explosion that had been slowly leaking out at the time. However, the sizable amount of nickel—nearly 7 percent of the Sun’s mass—had not simply vanished, nor had it decayed into something benign. It had simply changed into another radioactive species, a form of cobalt. Radioactive cobalt’s half-life (the time required for half its atoms to disintegrate into stable iron) is 77 days, so there was still plenty of it left after 3 months. Even 6 months after the explosion, the decay of radioactive cobalt, and the gamma rays it emitted, powered the glow from this huge and growing blast.

  Right on schedule, the brunt of the debris overran my craft. The jolt was rougher than I had expected—there were some compacted blobs that hit Rocinante with considerable force—but there was little physical risk and no damage. I was thankful that the debris had thinned out to such a degree that I could see most of the way across the expanding sphere. It would have been unnerving to be caught in the opaque fireball with nothing but a uniform glow on all sides, even if the temperature had cooled down enough to be survivable. Still, the glowing filaments rushing toward and around Rocinante at 15,000 kilometers per second created an eerie sensation, very different from the sensation I had experienced (as a consequence of my motion, in that case) as I traversed the Orion Nebula, back to front.

  I could now appreciate the role that radioactivity played in lighting up the ejected material. The nickel had not been spread uniformly through the debris but had been concentrated into globules within the inner 20 percent, or so, of the expanding sphere. When it decayed into cobalt shortly after the explosion (its half-life is only 6 days) the energy it released was trapped and could not even spread smoothly through the debris cloud, much less escape from it. As a result, the nickel-rich globules became hot compared to their surroundings, and “popped” like popcorn, pushing aside the material that surrounded them. What had started out as compact globules, rich in radioactive nickel, became “holes” full of radioactive cobalt, and the inner part of the debris cloud assumed the texture of Swiss cheese. Now that the cobalt was decaying, these holes were glowing in gamma rays.

  The spectrum of radiation inside this explosion was growing weirder by the day. There were the gamma rays emitted by decaying cobalt, of course, a kind of radiation usually associated with very hot gas. These energetic photons were hitting electrons and bumping them up to high speeds, whereupon the electrons produced X-rays. At the same time, the gamma rays, fast-moving electrons, and X-rays were knocking into the atoms of the debris, creating the atomic disturbances that led to the assortments of colors I have already noted. Yet amid all this activity the gas itself remained rather cool, hovering at only a few thousand degrees. When I was first submerged by the blast, many of the atoms were being driven to produce colors in the visible part of the spectrum. But with time, the level of disturbance declined and the dominant “colors” drifted into the infrared. Some denser pockets were getting down to lower temperatures, and I began to notice indications that molecules were forming. Dust, mostly graphite, was also beginning to condense out of the cooling envelope, just as it had done in the winds from the red supergiants. Eventually, my pristine view across the sphere of debris degraded as I found myself surrounded by clouds of soot.

  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.

  29

  Afterthought

  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.