Gamma-Ray lines from the galaxy: the GRO revolution

The Compton Gamma-Ray Observatory (GRO) functioned flawlessly for more than8 years. Its accomplishments, both in novelty and breadth, have virtuallyrevolutionized the field of gamma-ray astrophysics. Since the review ofits entire palette of results would be outside the scope of the presentreview, we will limit ourselves to the sub-field of Galactic gamma-raylines; furthermore, we will have to neglect the minor topic of very-highenergy gamma-ray lines (pion decay, axion annihilation, etc.).This review is divided into three main parts, plus an introductionpresenting GRO and a summary in the form of a table showing the originalobjectives, GRO's accomplishments, and the remaining goals in eachsub-field. The first main part deals with the knowledge gained from longand extensive observations of the Galactic electron-positron annihilationline radiation; the second major section treats the Galactic lines fromradioactive isotope decay; the third major section discusses advances(and recent failures) in the very interesting field of Galactic nuclearde-excitation lines. In each case, an effort will be made not only topresent the current knowledge in the field, but to particularly highlightthe contribution made by GRO to our knowledge.