great deal of the star's material rebounds from the implosion to be blown out in a supernova explosion, leaving behind an extremely dense object consisting of neutrons. But if the residual mass after the supernova explosion is greater than a certain critical value, the gravitational contraction of the star does not stop even at the neutron star stage. Instead the star continues to contract indefinitely, pulling the matter tighter and tighter together until the object disappears into a deep gravitational well out of which neither matter nor electromagnetic radiation can escape because of the intense gravitational fields there. Hence the name "black hole."

The binary nature of some of the x-ray objects could be deduced from the doppler shifts in the light from the ordinary companion, the shift being toward the blue as the star moved toward the observer in its orbit, and toward the red as the star moved away. If the stars eclipsed each other the binary nature would show up in a periodical disappearance of the x-rays when the emitter was hidden by the other star and reappearance when the emitter emerged from eclipse.

After careful study astronomers finally concluded that the x-rays were generated by material from the ordinary companion's being pulled into the gravitational well of the degenerate star. If the gravitational attraction were sufficiently strong, then the gas would be accelerated to such velocities that the gas would emit x-rays as particles collided. It seemed that white dwarfs would not provide sufficient gravity to accomplish this, so one was left with the conclusion that the degenerate companion in binary x-ray sources was either a neutron star or a black hole in space. In most cases it appeared that the companion was a neutron star, but the source Cyg X-1, in the constellation Cygnus, could be a black hole. If so, it was the first such object to be detected in the universe.57

The possibility that a black hole had at last been discovered emphasized the fundamental importance to astronomy of the new field of x-ray and gamma-ray astronomy. Gradually scientists had begun to talk about their work as high-energy astronomy, not only because they were working at the high-energy end of the wavelength spectrum, but more significantly because their observations were showing that throughout the universe extremely violent events were rather common, involving enormous quantities of energy and tremendous rates of energy production. And among these energetic events were those occurring during the last stages of a star's evolution, stages in which neutron stars and black holes were created, with intense x-ray emissions. Speculating on the philosophical implications, Giacconi showed excitement:

The existence of a black hole in the X-ray binary Cyg X-1 has profound implications for all of astronomy. Once one such object is shown to exist, then this immediately raises the possibility that many more may be present in all kinds of different astrophysical settings. Super-massive black holes may exist at the center of active galaxies . . . and explain the very large energy

emission from objects such as quasars. Small black holes of masses [very much smaller than the mass of the sun] may have been created at the instant of the primeval explosion. . . . In black holes matter has returned to condition similar to the primordial state from which the Universe was created. The potential scientific and intellectual returns from this research are clearly staggering. 58

Should one then conclude that rocket and satellite astronomy had by the early 1970s generated a scientific revolution in the field of astronomy? The answer may well be yes, although many of the strange concepts that were being dealt with had been considered decades before.59 In any event, it is probably too early to make the case. Certainly these topics, concerning the interplay of energy and matter on a cosmological scale, are fundamental; and if anywhere in the space program one might expect a scientific revolution to emerge, it would be here. But it should also be noted that if any such revolution is to arise, it would almost certainly come from a cooperation between ground-based and space astronomy.

Solar Physics

The sun was a most important target of space science investigations for at least two reasons. First, the sun's radiation supports life on Earth and controls the behavior of the atmosphere. For meteorology it was important to know the sun's spectrum in the visible, infrared, and near-ultraviolet wavelengths. To understand the various physical processes occurring in the upper atmosphere, a detailed knowledge of the solar spectrum in the ultraviolet and x-ray wavelengths was essential. The reader will recall the overriding importance that S. K. Mitra, in his 1947 assessment of major upper-atmospheric problems, gave to learning about the electromagnetic radiations from the sun (pp. 59-60). For this reason many sounding rocket experimenters devoted much of their time to photographing and analyzing the solar spectrum both within and beyond the atmosphere. Some of their work before the creation of NASA was discussed in chapter 6. Finally, with the discovery of the magnetosphere and the solar wind the importance of the particle radiations from the sun for sun-earth relationships became apparent, a topic that was treated in chapter 11. Thus, solar physics was of central importance in the exploration of the solar system.

But the sun was important also to astronomy, to the investigation of the universe. Although an average star, unspectacular in comparison with many of the strange objects that astronomers were uncovering in their probing of the cosmos, nevertheless it is a star, and it is close by. The next star, Proxima Centauri, is 4.3 light-years (400 trillion kilometers) away, while most of the stars in the galaxy are many tens of thousands of lightyears distant. Stars in other galaxies are millions and even billions of lightyears from earth. So the sun afforded the only opportunity for scientists to study stellar physics with a model that could be observed in great detail.

Because of its nearness and its importance, astronomers amassed a great deal of data and theory about the sun in the years before rockets and satellites.60 What they learned came almost entirely from observations in the visible, with only occasional glimpses from mountain tops and balloons at the shorter wavelengths. But, as space observations showed, much of solar activity, particularly that associated with the sunspot cycle, solar flares, and the corona involved the short wavelengths in essential ways. Sounding rocket and satellite measurements were, accordingly, able to round out the picture in important ways.

To understand the significance of these contributions, a brief summary of the principal features of the sun may be helpful.61 The visible disk of the sun is called the photosphere (fig. 66). It is a very thin layer of one to several hundred kilometers thickness, from which comes most of the radiation one sees on Earth. The effective temperature of the solar disk is about 5800 K. Above the photosphere lies what may be called the solar atmosphere; below it, the solar interior.

The sun's energy is generated in the interior, from the nuclear burning of hydrogen to form helium in a central core of about one-fourth the solar radius and one-half the solar mass. Here, at temperatures of 15 X 106 K, some 99 percent of the sun's radiated energy is released. This energy diffuses outward from the core, colliding repeatedly with the hydrogen and helium of the sun, being absorbed and reemitted many times before reaching the surface. In this process the individual photon energies continually decrease, changing the radiation from gamma rays to x-rays to ultraviolet light and finally to visible light as it emerges from the sun's surface.

From the core of the sun to near the surface, energy is thus transported mainly by radiation. But toward the surface, between roughly 0.8 and 0.9 of the solar radius, convection becomes the principal mode of transporting energy toward the surface. The existence of the convection zone is evidenced by the mottled appearance of the photosphere in high-resolution photographs. This mottling, or granulation as it is called, consists of cells of about 1800-km diameter which last about 10 minutes on the average and which are thought to be associated with turbulent convection just beneath the photosphere. A larger scale system of surface motion-20 times the size of the granulation cells, called supergranulation-is believed to be much more deeply rooted in the convection zone.

Analogous to Earth's upper atmosphere is the chromosphere, or upper atmosphere, of the sun. The chromosphere overlies the photosphere and is about 2500 km thick. While the density drop across the photosphere is less than an order of magnitude, density in the chromosphere decreases by four orders of magnitude from the top of the photosphere to the top of the chromosphere.

Not long ago, photospheric and chromospheric temperatures were extremely puzzling to astronomers, dropping from about 6600 K at the base

Figure 66. Idealized structure of the sun (not to scale). There is a complex interplay among the different regions of the sun. Edward G. Gibson, The Quiet Sun, NASA SP-303 (1973), p. 11, fig. 2-3.