Turn Right At Orion

by Mitchell Begelman

  The patterns were easier to follow when I looked away from the opaque concentrations of molecular gas. Supergiants were easy to pick out: the rubies, Betelgeuse and Antares, and the diamonds, Deneb and Rigel. I spotted some ordinary red giants—Arcturus, Capella, Aldebaran—and the sharp, colorful disks of a handful of planetary nebulae. Other expanding shells of gas, more ragged than planetaries, marked the sites of violent stellar explosions. Ordinary stars, including our undistinguished Sun, formed a richly textured and multicolored backdrop, filling half the sky from my vantage point.

  The immense pinwheel pattern of the Galaxy roared into view. The disk was amazingly flat and thin here and bore a thick and even peppering of stars, but the superimposed spiral pattern of dark cloud banks and bright rims of star formation gave it a deeply incised look. Toward the outer Galaxy, I could see the spiral arm of Perseus and a few regions of star formation beyond. Somewhere out there the disk petered out in a mush of indistinct hydrogen clouds, thickening and warping out of the plane for reasons not entirely clear. Toward the Galaxy’s center, I made out the Sagittarius arm and beyond it the tightly wrapped bands of atomic and molecular gas pushed aside by the tumbling stellar motions I had encountered on the earliest leg of my journey. The Milky Way’s bulge reared up 20,000 light-years in the distance, encircling the hidden Galactic center and fading gradually into the halo. Studding the bulge were dozens of globular clusters, those marvelously condensed spheres—hundreds of thousands of stars apiece, and only a few light-years across—that were very likely left over from the Galaxy’s primordial distillation.

  I appreciated the contrast this glorious view made with the relative featurelessness of the halo. There were many stars in all directions, but except for the few globular clusters that orbited the Galaxy this far out, they did not congregate in clusters. Nor were they particularly bright. These were all old stars, formed so long ago that even the ones slightly heavier than the Sun had run out of fuel. They shared (and exhibited to a much greater degree) the Magellanic Clouds’ peculiarity that heavy elements were unusually scarce, a fact that made more sense here than in the Clouds because these regions clearly had not benefited from stellar recycling any time within the past few billion years. Bright beacons out here were sparse but welcome sights. There were no supergiants, because these would have burned out long ago, but I noted some yellow-white stars, which must have been fusing helium in their centers, and a few red giants.

  Occasionally, a small, isolated cloud no more than a few light-years across would appear out of nowhere and startle me. Once or twice I passed straight through such a cloud. For the most part, these clouds were of mild temperature and consisted of individual hydrogen atoms with the usual admixture of dust, but a few of them contained dense knots rich in molecules. Were they shards of matter falling into the Galaxy for the first time, or had they been ejected from some explosion in the disk? I could not decide, but I quickly realized that these would not be the last of the clouds I would have to contend with. Like a pilot who climbs through a cloud-deck only to find an inaccessibly high layer of cirrus far above, I perceived that I was heading toward a more ubiquitous layer of atomic hydrogen at about the distance of the Magellanic Clouds.

  26

  Outposts in the Halo

  From my location 100,000 light-years out in the halo, I could see the Clouds’ three-dimensional structure. They were highly elongated, and (in a curious echo of my trip through the bulge of the Milky Way, en route to its center) I noticed that many of their stars shared the kinds of stretched-out and figure-eight orbits that gave these galaxies the appearance of tumbling through space. They were not tumbling in unison; however, they were flying in formation as their orbital trajectories carried them around the Milky Way. The Small Magellanic Cloud, farther distant by maybe 20,000 or 30,000 light-years, was pointed away from the Milky Way’s disk, whereas the Large Cloud presented residents of Earth with a view much closer to side-on. The bright regions of recent star formation, especially in the Large Cloud, were asymmetrically placed with respect to the main bodies of stars, making the clouds look lopsided.

  The vast cloud of cool hydrogen, which I had noticed as I traversed the halo, not only enveloped both Clouds but also spread out in a seemingly interminable stream to either side of them. I had heard about the Magellanic Stream, because it was visible to radio astronomers on Earth. Its significance had puzzled my colleagues for years. Did it consist of matter stripped from the Clouds and left behind as they plowed through the Galaxy’s halo? Or had the gas been teased from both Clouds by their gravitational attractions for one another?

  The trajectories of the Clouds seemed clear enough. Their orbits about the Milky Way had brought them in from a much greater distance—may be 3 times farther out than they were now—and they were skimming along at roughly their minimal separation from the Galaxy’s center before heading back toward the outer halo. Maybe they had made half a dozen passes like this, each one closer than the last as the resistance of the halo’s stars and gas took its toll. It made sense that they should leave behind a smear of liberated gas as their orbits eroded and they spiraled slowly toward their destiny of complete absorption by the Milky Way.

  But the theory that the stream traced out the Clouds’ orbital path had a serious flaw: The “trail” extended in front of the Clouds as well as behind. Friction of the halo against the Clouds’ motion would certainly not draw gas out ahead of the Clouds, but gravity might. Whose gravity, though? The Milky Way’s gravitational field was too gentle to extract so much gas from the Clouds. As galaxies in their own right, the Magellanic Clouds had a strong propensity to hold on to what was theirs. But close encounters between the Large and Small Clouds were assured and must have occurred not too long in the past. These could be much more disturbing to the Clouds’ interiors. Could the stream, then, be the result of devastating tides raised by the Clouds on one another, when they had met fatefully hundreds of millions of years earlier?

  This theory had its problems, too, although it seemed more promising. Try as they might, astronomers had never been able to detect the stars that should have been pulled out of the Clouds along with the gas, and I couldn’t, either. Gravity affects stars and gas equally, and it would be hard to imagine a gravitational interaction that would extrude so much gas from either galaxy without also liberating quite a few stars. My ability to fly around the Magellanic Stream was little help to me in understanding its origins. Fortunately, much less mystery surrounded the origin of the matter that bridged the gap between the Clouds. This gas contained a reassuring complement of stars and seemed certainly to be remnant of one or more close encounters. One of these encounters must have been very recent, because a curved streamer of stars, the very signature of death by gravitational tide, bulged out of the Small Cloud and intruded halfway across the bridge, still intact. The Large Cloud seemed to be pulling the Small Cloud to pieces.

  The peculiar patterns of star formation in the Clouds also seemed to reflect the injuries that these galaxies must have inflicted on each other in the past. Why else would star formation have been dead for nearly the entire history of the Clouds and then suddenly burst into life only a couple of billion years ago? It seemed a likely explanation for the paradoxical conjunction of rapid star formation with a scarcity of heavy elements. The close encounters must have triggered star formation—especially in the Large Cloud, which had held on to more of its molecular gas. I recalled the evidence I had seen for sequential bursts of star formation across the Orion region, where the birth of stars in one location had triggered a subsequent burst of star formation nearby. The mechanism in Orion had been the shock waves, rolling out of the region of massive star formation in one newly formed cluster and overrunning a neighboring molecular cloud, squeezing it and pushing it over the edge of collapse. But a collision between the Clouds: That could trigger an Orion on a massive scale. Here the shocks would carry not merely the momentum of the exploding and wind-producing massive stars but the full imp
etus of two colliding galaxies.

  Once again, I was becoming overwhelmed by details and shaken by the realization of how interconnected these cosmic structures are. My search for a simple, “closed” system (to get away from the complications of the Milky Way) had brought me to a pair of galaxies that were so open to environmental influences that they were literally at the mercy of their surroundings and of each other. Recycling of matter through stars and the inexorable creation of heavy elements was one form of evolution. Here was another, even more dramatic form of evolution—a form that would continuously shift the shapes and contents of entire galaxies and eventually dissolve them into one another. Even a galaxy could not be regarded as a closed system. The effects of all this external buffeting trickled down to the internal structures of the Magellanic Clouds themselves. Had the Small Cloud failed to produce even the paltry complement of oxygen and iron found in the Large Cloud? Perhaps this was because its element-enriched gas had been stripped away before the galaxy had had a chance to recycle it. The Large Cloud had apparently been more effective at holding on to its gas, and one could foresee the day when it might absorb the Small Cloud, before making its final death spiral to join the bulge of the Milky Way. But even the Large Magellanic Cloud had been quiet for many billions of years, until a collision with the Small Cloud had released its pent-up star-forming potential and caused it to burst into glory.

  Dramatic evidence of these influences lay right in front of me, less than 10,000 light-years away. I could not take my eyes from it. Off to one side of the galaxy and amid a scattering of lesser sites of star formation, the Tarantula Nebula shone with the brilliance of several thousand Orions. Orion had its Trapezium of four massive hot stars; the Tarantula had thousands. Weary of generalities, I set my course for the Large Magellanic Cloud, eager to immerse myself in what had to be one of the most glorious environments this side of Andromeda. What I got instead was another lesson in the evolution of stars.

  27

  The Explosion

  Somewhere in the observable universe—by this I mean the Universe within a distance of about 10 billion light-years, within which light can have reached us since the Big Bang—a star explodes every second. But even the observable portion of the Universe is a very big place, and stellar explosions are consequently considered rare events. Thus, when I came to the Large Magellanic Cloud, the last thing on my mind was a life-and-death struggle to outrun the blast from a supernova.

  Even before I approached the Cloud, my attention had been drawn to a very bright star that seemed to be behaving strangely. I had noticed it not long after leaving the Milky Way’s disk, because it was so bright and because it was situated in the middle of a little dark hollow at the edge of the Tarantula Nebula. As I watched, the star changed color from orange-red to a hot blue-white. At this point, my velocity was within 1 part in 10 billion of the speed of light, and events outside my craft appeared speeded up by nearly a factor of 100,000—that is, by my Shangri-La factor. But I was able to calculate that the transition had taken only a few thousand years of Earth time (incredibly fast for a stellar transformation). I should have sensed danger, but I was intrigued. I set a course for this star, thinking that it would be an interesting base for explorations of the Tarantula.

  Pulling up nearby, I confirmed the classification I had hazarded en route. This star, in its present incarnation, was what is known as a “blue supergiant.” The blue coloration reflected the temperature of its surface layer, which was not even 4 times that of the Sun. I estimated it to be about 20,000 degrees, maybe a bit more. I guessed that its mass was about 18 or 20 times that of the Sun, not very different from Betelgeuse. I was well aware that the brightness of a stellar surface went up very steeply with temperature (as did the brightness of any other opaque surface); this fact, and the star’s size (27 times the diameter of the Sun, but only 1/50 the size of Betelgeuse) accounted for its huge luminosity: 100,000 times the Sun’s output. Being so far from home and slightly anxious about having ventured outside the Galactic disk for the first time, I thought it would be pleasant to bask in starlight at just about the level that Earth does in its orbit around the Sun. To compensate for the higher luminosity, this meant staying much farther away. (Rocinante was sufficiently well shielded that I could have parked closer in.) Little did I know that this nostalgic whim would save my life. After circling a bit to get the best view of the Tarantula Nebula, I parked my craft at about 300 times Earth’s distance from the Sun and settled in for a well-deserved nap.

  The “incident” began while I was still asleep. I woke up with a start, thinking that someone had shone a laser beam in my eye. I almost convinced myself that it was stinging but decided that this was my imagination, perhaps the tail end of an unpleasant dream. After a few moments there seemed to be no ill effects, and I went back to sleep.

  No one had shone a laser beam in my eye, of course; this is not a science fiction story. But it wasn’t my imagination, either. What had actually happened was much scarier than a crew member gone mad, loose aboard Rocinante in deep space, wielding a deadly laser. What had happened was that an eruption of energetic particles—electrons, positrons, protons, gamma rays, you name it—had exploded inside my eye. The reason? An energetic neutrino or two had slipped in sideways, through the supposedly impenetrable wall of my craft, through my skull, diagonally across my frontal lobe, and across my eye socket. After all that, when it was about to get away scot-free, it had a head-on collision with a carbon atom in my ocular jelly and stopped dead in its tracks, dumping its energy in situ. And this didn’t happen once or twice, it happened about 3 million times in the space of 10 seconds. Only I didn’t know that at the time.

  Two hours later, all hell broke loose. Without additional warning, the surface of the star exploded. Fortunately, Rocinante’s shielding protected me from the first flash of gamma and X-rays. Then an initial blast of ejected matter rushed past at close to the speed of light. Lucky again: This barrage, which consisted of just the thin outermost layers of the star, it packed little punch and washed over Rocinante without doing much damage. But a much more ominous, dense, tidal wave of debris was rushing toward me at 15,000 kilometers per second, 1/20 the speed of light. I had to get out of there immediately.

  The problem was that it took time to accelerate to a speed high enough to outrun the blast. Despite the sophistication of my propulsion system, I was still subject to all the laws of physics, including the one that said that accelerating at a rate much higher than Earth’s gravitational acceleration would feel exactly like being crushed under an enormous weight. The old test pilots and astronauts had shown that humans could survive 8 or 10g’s for short periods of time, but my craft would not even approach those limits. Because I had not anticipated the need for rapid escape, Rocinante—which normally accelerated at 1g, the acceleration of a falling body on Earth—was limited to a maximum of only twice that . . . if it worked to specifications. If everything worked, it would take 9 days to reach the speed at which the blast was approaching. If I stood still, the blast would overrun my craft in a little over a month. For an instant, I considered playing “chicken”—banking on the 26 days’ leeway I had under the assumption that my technology was flawless. Even if my craft managed only 1g, I Would still have 17 days at my disposal for sightseeing before I had to flee. Then I thought better of it, turned tail, opened up the throttle and hoped for the best.

  Meanwhile, the blast was expanding in my field of view at an alarming rate. Before the explosion, the star had appeared only as big as the Sun did on Earth’s sky. (This star was much larger than the Sun, but I was parked at nearly 10 times the orbital distance of Pluto.) Only 4 hours after the explosion, the expanding blast appeared as big as the Sun. After 2 days, it was more than 10 times bigger. When I finally reached the speed of the exploding material (Everything worked!) I had traveled only a distance equal to 38 times the distance between Earth and the Sun—a shade more than 10 percent of my original distance from the exploding star.
I could have turned off the acceleration at this point and coasted along with the explosion, but the result would have been uncomfortable. The explosion now took up as much space on my sky as the Big Dipper did on Earth’s, and it was emitting as much light as a billion Suns. Under this cosmic broiler, Rocinante’s skin was heating up to 3000 degrees, which meant that it was beginning to vaporize at a furious rate. This was still too close for comfort, I continued to pull away, cutting my acceleration back to 1g only after the temperature began to subside.

  Safe for the time being, I reviewed my thoughts about what had happened inside this star. The interior of a massive star—one that is at least 8 or 9 times as massive as the Sun—never settles down to form an inert core that stays inert for very long. The degeneracy of its electrons is never quite enough to support it against gravity. Every time it uses up one type of nuclear fuel, it continues to shrink, and get hotter, until the next level of fuel ignites.

  I guessed that Betelgeuse had already used up most of the helium in its core by the time of my visit. Even during its helium-burning phase, it would have burned hotter than a less massive star like the one that had produced the Dumbbell Nebula. Three heliums stuck together make a carbon atom, but at such high temperatures the violence of the reactions readily could have added a fourth helium, making oxygen. Thus massive stars like Betelgeuse—and the star that had just exploded—part company with their lighter counterparts as they create a mixture of oxygen and carbon in their cores.