massive. Some of the gas in the disk, heated to temperatures between 1 million and 1 billion degrees celsius, would radiate x-rays. Such x-ray emission will be studied by AXAF. Another characteristic feature of such high energies could be the production of electrons and their antiparticles, positrons. Once produced, particles and antiparticles annihilate each other in a burst of gamma rays. The Gamma Ray Observatory, to be launched by NASA in 1991, and other proposed gamma-ray detectors in space, will search for such gamma rays. Ground-based telescopes may have found gamma rays of extremely high energy coming from astrophysical objects: pulsars, x-ray binary stars, and black hole candidates. The gamma rays are detected by light or particles produced in showers when the gamma rays enter the earth's atmosphere. Experimental physicists are working on better ways to detect this radiation, and theorists are studying novel astrophysical mechanisms to explain its existence.

The mysterious “x-ray background” that has puzzled astronomers for 25 years may also yield its secrets to new x-ray satellites. They should be able to indicate definitely what part of radiation comes from hot gas, what part from distant quasars, and what part, if any, from still more exotic objects.

The dramatic and energetic behavior of active galaxies and quasars raises other questions. Some of the observed gaseous “jets” emanating at great speed from these objects are extremely narrow and well collimated. What produces and controls such columns of matter? Recently, some progress has been made on these questions. The jets radiate x-rays, visible light, and radio waves. Images made with large visible-light telescopes, radio telescopes, and the Einstein Observatory x-ray telescope of the 1980s have revealed exquisite details of the structures and blobs in the gaseous jets (Plate 2.13 and Plate 2.14). In particular, radio observations with very high angular resolution show that new concentrations of gas enter into the jet every year or so. New radio interferometers should be able to investigate jets and the central sources at much higher resolution.

Continued theoretical work in the coming decade will also be crucial to understanding energetic jets. In the 1980s, some of the features of jets were reproduced in large computer simulations (Plate 2.15). In such simulations, the scientist programs the computer with the basic laws of physics describing how gas, radiation, and magnetic fields behave, sets up some initial configuration of matter and radiation, and then lets the computer calculate how the system evolves in time. Comparison to observation then guides refinements of the theory and suggests new observations.

The Birth of Galaxies

If we peer out far enough into space, we should see back to the epoch of galaxy formation. What should a young galaxy look like? Astronomers are not sure. Galaxies were probably formed about 10 billion to 20 billion years ago,



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