EVOLUTION OF GALAXIESThe formation and evolution of galaxies is a major problem for students of extragalactic systems. Galaxies
are not seen forming, and human lifetimes are far too short for astronomers to watch galaxies evolve. Most galaxies formed several billion years ago, and most changes in a galaxy occur on time scales of millions of years or more. Thus indirect methods must be used to discover how galaxies are born and change.The broad outline of galaxy formation can be divined from basic physical principles, combined with circumstantial evidence provided by current characteristics of galaxies. The beginning of a galaxy's life is measured from the time it attains a gravitational identity separate from other nearby clumps of matter. This can occur either before or after its first stars form. Much theoretical modeling has gone into attempts to find out how a galaxy such as those now seen might have formed. Did small clouds of gas appear first, then stars within them, and did these objects then gradually coalesce into a large galaxy? Or did a large galaxy begin as a large cloud of gas that collapsed into a rotating disk, leaving some stars out in a halo, with most stars forming a little later in the disk into which the gas collapsed? This choice is still being investigated.Mergers and other interactions between galaxies are being recognized as greatly important in the histories of galaxies. When the universe was smaller and the distances between galaxies were much less, galaxies were continuously being influenced by close encounters or outright collisions. Such events are seen even now. (A 1997 event that is apparently the most immense cosmic explosion yet knownÑfor 40 seconds it outshone the rest of the universeÑmay have been in the nature of such a collision; some astronomers suggest that it represented a black hole colliding with a neutron star or another black hole.) Most of the peculiar galaxies that have been recorded owe their strange shapes and peculiar dynamics to encounters or mergers. Therefore theories of galaxy evolution have to take into account the fact that galaxies are not isolated systems, and that at some time in the long history of a galaxy it has probably had at least one highly disturbing tidal encounter with another galaxy. In fact a merger can be seen happening right here in our own galaxy. A dwarf elliptical, first recognized by Michael Irwin and his colleagues at Cambridge University in 1994, is falling into our Galaxy, having ventured too close on its travels through the Local Group. Much of the galaxy, called the Sagittarius dwarf, has been drawn out into a long tidal stream that melts into the central area of the Milky Way in the direction of the constellation Sagittarius.Just as an archaeologist can dig down into the earth to reconstruct the history of an ancient city, so can the astronomer reach down into the depths of a galaxy to reconstruct the history of its star formation. It is possible for astronomers to measure the ages of star groups by measuring the colors and luminosities of the stars and by comparing the measurements with theoretical models of stellar evolution, which are well supported by detailed cluster studies. By such dating it is possible to determine the star-formation history of different kinds of galaxies. Some galaxies, especially those classified as "star-burst galaxies," are at present forming stars at a rate far greater than average, indicating that they are in a rare state of high activity. On the other hand, elliptical galaxies are in a low state; most of their stars were formed long ago. For some elliptical galaxies star formation lasted only a few hundred million years, with virtually nothing happening for the remainder of their 15-billion-year lifetimes. For other ellipticals there is evidence that two or more intervals of active star formation occurred, but no star formation has taken place for the last 3 or 4 billion years. Astronomers still do not know what tiggers star formation in every case. In some galaxies, it is clear that encounters have been responsible, but it is not known how general this effect is or what other circumstances can engender star-forming events in galaxies.