Turn Right At Orion by Mitchell Begelman Page A

and X-ray observations—any of these should have given evidence of the shock—all came up empty. When the nebula’s mass had been toted up in terms of the gas that one could easily “see,” it fell short by at least four times the mass of the Sun. Only stars more massive than a certain threshold were supposed to explode like this, and the four (some said six) missing solar masses constituted the difference between what was seen and what the theorists expected. Some conjectured that the “shell” was not really part of the explosion but, rather, consisted of slowly moving gas that had wafted off the star’s surface during the eons before it blew up, when it was not a hot star but a cool red giant. They contorted their reasoning to find ways in which this effluence could somehow shield itself from disturbance to the point where it was invisible. Others said that the shell was there, and was actually rushing out at speeds several times greater than the expansion speed of the observable nebula, but that it was too faint to see. Why this should be so was anybody’s guess. To me, the most plausible explanation was that the Crab lived in very sparse surroundings, like the interior of one of those hot stellar bubbles, superbubbles, or chimneys I had seen during my trip to the Galaxy’s center. Such bubbles could be blown, over millions of years, by just the sort of star that the Crab once was. Maybe the Crab-star had evacuated its own neighborhood, and now there was little for its debris to run into.

10
    Crab II
    I was confident that I would soon encounter the familiar nebula, but as I continued onward without finding a trace of the exploded star, I became worried. The space around me seemed eerily empty. Debris in any form—if compacted into dense enough clouds—would have been hard to detect (assuming I didn’t run into it!), but not so the luminous features that were easily visible from Earth. As I searched in vain for a landmark, it dawned on me that the reason for this desolation was profound. I was not in the Crab Nebula I remembered from Earth; I was in its ruin. I was disheartened, though I should have known. Ninety thousand years of continuous expansion had sapped the nebula’s vitality. The debris from the star was now well mixed with ordinary interstellar matter, and its explosive energy had spread over a volume thousands of times larger than the historical Crab Nebula.
    I knew there would still be a neutron star to study, if only I could locate it. I looked around—there were hundreds of faint stellar-looking objects that could have been candidates. At this age, the neutron star should have been very faint at all wavelengths except perhaps the radio band, where it might have shown up as a slowly winking pulsar.
    I was debating whether to commence a half-hearted radio search or to give up entirely when I remembered an event that
had made an impression on me as Rocinante approached the location of the Crab. I had spotted a new stellar explosions, off to starboard, and had noted in my journal that it exhibited many features in common with the sort of explosion thought to have produced the Crab. As I moved through the Galaxy, I could tell from its changing displacement against the background of more distant stars that it was less than a thousand light-years away. The remarkable coincidence had pleased me. The chance of a second such, explosion, this close in time and space to the first, was miniscule. Now I viewed it as a godsend. What excellent luck! I could be there in less than a thousand years of Galaxy time and, if it did prove to be another Crab, experience the familiar nebula as though it had been reborn. I set course immediately for the nebula I dubbed Crab II.
    From 30 light-years out. Crab II produced a ghostly effect. It appeared as a vast, dimly shining panel, taking up as much space on my sky as the Big Dipper does on Earth’s. Its total luminescence