The stellar epoch in the evolution of the Galaxy

We consider the astrophysical evolution of the Galaxy over large time scale s, from early stages tan age of similar to 10(8) yrs) to the end of traditi onal stellar evolution (similar to 10(11) yrs). Despite the fact that the b asic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteris tics of stars (their total luminosity, color, mass function, and chemical c omposition) are rather substantial. The interaction of the Galaxy with othe r stellar systems becomes an important factor in its evolution 100-1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as fellows. The Gal axy forms as the result of the contraction (collapse) of a protogalactic cl oud. The beginning of the Milky Way's life-the relaxation period, which las ts about 1-2 Gyr-is characterized by active star formation and final struct urization. The luminosity and colors of the Galaxy are correlated to the st ar formation rate (SFR). The young Galaxy intensely radiates high-energy ph otons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady-state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reaching 3-5M./yr at the present epoch and decreasing to 0.03M./yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Lat er, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M. (i.e., those able to lose matter and renew the suppl y of interstellar gas) comes to an; end. The Galaxy enters a period of "dyi ng", and becomes fainter and redder. The variation of its chemical composit ion is manifested most appreciably in a dramatic enrichment of the interste llar gas in iron. The final "stellar epoch" in the life of the Galaxy is co mpleted similar to 10(13) yrs after its formation, when the evolution of th e least massive stars comes to an end. By this time, the supplies of inters tellar and intergalactic gas are exhausted, the remaining stars become dark , compact remnants, there is no further formation of new stars, and the Gal actic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission. (C) 2000 MAIK "Nauka/Interp eriodoca".