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doubling of stellar velocity. I quickly deduced that gravity was increasing just as Isaac Newton had predicted for a body with a mass two-and-a-half million times that of the Sun, all concentrated in one place. I was clearly sensing the black hole’s gravity. Yet my initial sense of satisfaction faded as I recalled how my colleagues had deduced everything I was finding, from the mass of the black hole to the shape of the star cluster, without leaving the comfort of their observatory 26,000 light-years away. (Funny how one never focuses on one’s advantages in these situations. For example, it never occurred to me to gloat that my colleagues hadn’t witnessed a stellar merger.) In any case, I had no time to wallow in these conflicting emotions. If I had not braked hard and gone into orbit about the Milky Way’s central black hole, I soon would have become part of it!

4
    Ground Zero
    I was surprised that my arrival close to the Milky Way’s central black hole did not strike me more viscerally. There was never a point at which I could feel, between my head and feet, the stretching force due to the stronger gravity closer to the hole. I knew that these bigger black holes are altogether more gentle on visitors than their smaller counterparts, yet I expected some gut feeling to tell me that I was in the presence of an enormous source of attraction. The visuals were hardly as dramatic as I expected, either. There was no grand disk of superheated gas, crackling with energy, swirling into the black hole. All I could see was a diaphanous, bluish luminescence surrounding what I deduced to be the black hole’s location. I could tell where the hole was by observing the distortions of the stars beyond it as their tight rays curved while crossing the hole’s gravitational field. A lens-like distortion of a distant stellar scene, some subtle gradations of the blue glow—was that all there was to see of the Galactic central black hole?
    Yes, I was disappointed, but I should have seen it coming. What made this black hole appear so serene was that it resided in a near vacuum. There was no “donor” star to be plundered for its substance, as I later encountered while visiting the much smaller black hole known as Cygnus X-1. The dense interstellar
clouds were few and far between, and they were so stirred up by all the hot stellar winds and supernovae that they responded little to the black hole’s lure. Quite simply, there was very little matter flowing into the black hole at the center of the Milky Way.
    I ventured into the outer reaches of the blue-glowing corona. The gas there was so tenuous that I felt safe enough immersing my craft in it. I measured the radiation and found that it was not really “blue.” It merely appeared blue when filtered through my visual range. Electromagnetic radiation was being produced across the entire spectrum, from radio waves through microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays, the bulk of it coming out in the infrared and ultraviolet. I tested the temperature of this gas, which was a measure of the energies (and thus the chaotic motions) of its constituent particles. I was still hoping to accumulate more evidence of the effects of gravity pulling the gas inward, thereby causing the particles to move faster. As expected, the temperature seemed to be going up about inversely with distance from the black hole; it reached a billion degrees when I was roughly as far from the black hole as Pluto is from the Sun. But my instruments did not seem to be operating quite as reliably as usual, and I began to record unsettling temperature swings.
    The reason proved to be simple, though bizarre: This gas did not have a temperature, in the sense in which temperature is usually understood. At such low densities and high speeds, it is difficult for particles to share their energies with one another. The roughly equal sharing of energy among all particles