Turn Right At Orion by Mitchell Begelman

nebula didn’t even have an association of place, lying as it does (from Earth’s perspective) near the tip of one of the Bull’s horns, one full zodiacal notch away (clear the other side of Gemini!) from the arthropodous constellation of Cancer. And so far as I could remember, it had nothing resembling what I would call a shell—a shield of luminous or compressed gas, for instance. At best, it might be able to pass for “The Soft-Shell Crab Nebula.” Or maybe the term crab refers to the curmudgeonly attitude with which I was approaching this system. Perhaps it had inexplicably been engendered in Galactic explorers of Victorian times, as well.
    The Crab had to be my next destination, for one reason. Near its center lay a neutron star, the remnant of a star whose explosive demise had been witnessed on Earth in A.D. 1054. In theory, a neutron star resembles black hole as closely as a body can without actually becoming one. Somehow its collapsing precursor knew how to stop gravity in its tracks, to create the tightest possible form of equilibrium. It weighed more than the Sun, but
it was only 20 kilometers across. I needed to see what it could teach me.
    Despite my misgivings about the visit, I took some consolation from the fact that the Crab Nebula, or its precursor, had once known true glory. Before A.D. 1054, no Earthly astronomer could have seen the Crab Nebula (even if telescopes had been invented) because there was no such object in the sky. But for a few weeks during the summer of 1054, this shattered orb outshone any of its compatriots and would have surpassed Venus as the beacon of the predawn horizon. Unbeknownst to our medieval ancestors, a star had already exploded some 6000 years earlier, in the direction of Taurus. Because the explosion had taken place 6000 light-years away, the evidence of this star’s demise did not burst forth upon earthly skies until the evening of the fourth of July 1054, whereupon the Chinese court astronomers duly took note. The Chinese had provided such detailed records that by 1921, astronomers believed they knew the location of this “guest star” and noted its proximity to the well-known nebula. That same year, it was discovered that the Crab Nebula was expanding and apparently had been doing so for nearly a millennium. It didn’t take long for astronomers to put 2 and 2 together.
    I was so fascinated by this rare convergence of historiography with astrophysics that I didn’t notice my entrance into the nebula itself. This surprised me, because I had expected a strong jolt as I crossed from the undisturbed surroundings into the zone occupied by the outward-rushing debris. As I had learned during my very first traversal of the Milky Way, no star lives in a vacuum, and when a star is audacious enough to explode, it must push its surroundings out of the way. At typical explosive speeds (covering 1500 kilometers every second, if not more), the quiet gas that envelops the star cannot anticipate what is about to befall; the blast overruns it without warning. Like the famous horror film in which decent citizens are overcome by the lumbering swarm of zombies and then become zombies themselves, the swept-up atmosphere is shocked into sudden motion and inexorably becomes
part of the blast, plowing in turn into its surroundings and infecting them with unstoppable motion. The signature of such a shock wave is a sharp increase in pressure—this is exactly what a sonic boom is—and, often, the radiance that accompanies flash-heated or disturbed gas. I must have crossed the shock, I thought; the fact that I had felt nothing perplexed me.
    Not that the existence of a shock had actually been demonstrated by my observer-colleagues on Earth. The environment of the Crab Nebula had been regarded as one of its deeper mysteries. Astronomers were always looking for its “shell,” and not merely for the purpose of justifying its name. Optical, radio,