When we dream of other worlds, our dreams may be vivid and real and colorful, but they’re subject to the limitations of our senses. To visualize something, even if in our mind’s eye, we use the visual sense. For most of the history of astronomy we learned about the universe exclusively through visible light. Tens of thousands of years of naked-eye observations were followed by the first night time use of the telescope by Galileo in 1610, followed by a steady march of successively larger telescopes, culminating with the 8-11 meter behemoths of the present day, with dreams of even larger ones to come.1

The historical focus on visible light was natural, since the most powerful human sense evolved to match the peak energy output of our life-giving star. The cool outer envelopes of stars emit most of their radiation in the visible range, and galaxies are just assem­blages of stars. The night sky is visually rich. About 6,000 stars are visible to the naked eye at a dark site, but they are just an infini­tesimal sliver of the stellar plentitude of the universe. We live in a system of 400 billion stars, and the observable universe contains roughly 100 billion stellar systems, for a staggering total of 1023 stars.2 Also, the spectral transitions that are the fingerprints of dif­ferent types of atoms fall mostly in the visible wavelength range. Cosmic chemistry is practiced with telescopes that gather light and disperse it into its constituent wavelengths.

Nevertheless, just over two hundred years ago experiments showed there are wavelengths longer than the reddest waves of light and bluer than the bluest waves of light. A hundred years later, Wilhelm Rontgen performed the first systematic experiments to understand the nature of mysterious radiation that was gener­ated by high-energy particles moving in the vacuum of a sealed glass tube. In 1895, he announced the discovery of “X rays.” The details of the discovery are poorly documented because Rontgen had his lab notebooks burned when he died. X-rays electrified the scientific community and within a year their potential for medical imaging was realized. It was a lot longer before anyone discovered that they could be used for astronomy. Rontgen received the first Nobel Prize ever awarded for Physics in 1901.3

Just as X-rays provided a window into the human body, they now provide a window in space onto worlds barely imagined. X – ray telescopes have revealed the nature of black holes and neutron stars for the first time, they’ve discovered sizzling hot plasma in the centers of galaxy clusters and in the diffuse space between galax­ies, and they’ve let us make great strides in probing extraordinary black holes that lurk in the centers of all galaxies, ranging from a few million up to a few billion times the mass of the Sun. Their high energy relative to visible light means they’re often created by events of unimaginable violence, phenomena unknown to us fifty years ago (figure 10.1).

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