lites that were monitoring the skies for possible thermonuclear test explosions. At its peak, the energy flux observed from a single burst can be greater than that from all of the nighttime stars and galaxies in the universe! The apparent brightness of the bursts led many astronomers to conclude that they had to be in our galaxy, but during the 1990s the Compton Gamma Ray Observatory found them to be equally distributed over the whole sky and therefore almost certainly extragalactic. The Italian-Dutch BeppoSAX satellite permitted more accurate localization of a few of the bursts, leading to the discovery of theoretically predicted afterglows at other wavelengths (Figure 2.10). Observational monitoring of these afterglows confirmed that the bursts originate from the far reaches of the universe. While the precise origin of the bursts remains a mystery, it is believed that they are most likely associated with the formation of compact stellar objects such as neutron stars and black holes (Figure 2.11). With GLAST, EXIST, and MIDEX missions such as Swift, it will be possible to find gamma-ray bursts that are fainter than those previously visible and to locate them more quickly for prompt follow-up observations at other wavelengths. Because they are so luminous, bursts associated with the first generation of star formation may be detectable.

GALAXIES

On very large scales, galaxies are the building blocks of the universe, as fundamental to astrophysics as ecosystems are to the environment. They come in a variety of types, ranging from disk galaxies like the Milky Way to elliptical and irregular systems. While visible primarily through the light from the stars they contain, galaxies are actually far more complex than a simple grouping of stars. Most of their matter is “dark” in that it is not visible at the sensitivity limits of today’s telescopes. Many galaxies, including our own, harbor supermassive black holes in their nuclei, and these will almost certainly have an important role in galactic evolution. Finally, in most galaxies there is a significant amount of gas and dust between the stars, out of which new stars continue to form.

FORMATION AND EVOLUTION OF GALAXIES

During the past decade astronomers were for the first time able to study galaxies so distant that their light was emitted when the universe was only a small fraction of its present age. From the work of Edwin



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