4. WHAT WE HAVE LEARNED ABOUT THE MILKY WAY

From the discussions of the previous sections we can extract some important conclusions on the formation and evolution of the Milky Way, derived from chemical abundances. In particular:

• The inner halo formed on a timescale of 1-2 Gyr at maximum, the outer halo formed on longer timescales perhaps from accretion of satellites or gas.

• The disk at the solar ring formed on a timescale not shorter than 7 Gyr.

• The whole disk formed inside out with timescales of the order of 2 Gyr or less in the inner regions and 10 Gyr or more in the outermost regions.

• The abundance gradients arise naturally from the assumption of the inside-out formation of the disk. A threshold density for the star formation helps in steepening the gradients in the outer disk regions.

• The bulge is very old and formed very quickly on a timescale smaller than even the inner halo and not larger than 0.5 Gyr.

• The IMF seems to be different in the bulge and the disk, being flatter in the bulge, although more abundance data are necessary before drawing firm conclusions.