Turn Right At Orion by Mitchell Begelman Page B

amounted to only one-hundredth the brightness of the full Moon, spread over an area equal to nearly 1600 full moons. Near its center I caught a glimpse of my destination, the neutron star, appearing as any ordinary star of magnitude zero, just as Arcturus, Vega, or Capella. This neutron star was young and still shone brightly.
    The nebula, like the familiar old Crab, had an oblong shape that appeared more articulated the closer I approached. In addition to the texture created by the luminous filaments, Crab II had a distinctive architecture. Two deep, rounded indentations cut into the nebula, giving it the appearance of having a waist. I could clearly make out that the constriction was three-dimensional, pinching the nebula all around. The filaments girdling the indentation had a very slightly “off” color, compared to the other filaments, and I likened them to whalebone stays corseting a satin-clad figure. The metaphor seemed apt. The expanding nebula was being held back, though not stopped, by the constriction, while in the perpendicular directions, where it was unconstrained, it appeared to expand freely. For the most part the
filaments seemed to form a random network, but there were a few places where it was hard not to visualize a greater degree of organization. At one place in particular, filaments were arranged in such a way that they seemed to describe a tubular conduit. Nothing seemed to be flowing through it, yet it was hard not to attach a dynamical significance to this sharply outlined decoration, if only as a symbol of what I now saw was a delicately squeezed and shaped explosion.
    I was not long to have the luxury of such a global view. Almost without warning, I found myself immersed in the sea of glowing filaments. As I approached the nebula, what had struck me even more than the shape had been the colors. Now I was overwhelmed by the iridescence of the scene. Filaments shone with the familiar rich green of oxygen and the reds of hydrogen, sulfur, and nitrogen, but many more subtle hues could also be discerned. Of course, each time an electron popped from any one orbit in an atom to a lower one, it emitted a very distinctive color, and there were many, many orbits in each type of atom. Also, many of the atoms had lost one or more of their electrons, and each of these needy atoms—ions—had its own assortment of orbits. Thus the array of colors was staggering, and I also noted the ultraviolets of hydrogen, carbon, and helium, the yellow of helium, and the violets of oxygen and neon. The individual colors were all familiar, the stuff of spectroscopy class in grad school. But something seemed odd: The mix was different from what I had come to expect.
    The peculiar combination of hues and their relative intensities—let’s call it the “spectrum” of “lines,” now that we are going to do something quantitative with it—depends, more or less, on two factors. One is how well or roughly the atoms are treated. The other is the mixture of different chemical elements, or “composition.” The atoms in Crab II’s filaments were being disturbed in a couple of ways. Most important, they were being tickled by that bluish glow that I had remarked on earlier. Atoms see light as chopped up into its constituent particles, or photons, which constantly move around at the speed of light
(naturally) and sometimes hit electrons. When that happens, the photon is absorbed and can either knock the electron into a higher orbit, whence it drops back down and emits new photons with very specific colors, or knock the electron clear out of the atom. In the latter case, the precise hues are emitted when the freed electron finds an atom to attach itself to and drops down through a sequence of orbits as it heads for home. The photons that make up blue light do not pack enough punch to jostle most of the electrons out of their preset orbits; as a result, they do little. But it took more