SAO/NASA Astrophysics Data System (ADS)Commonly called the Astrophysics Data System, this is a digital portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant. All abstracts and articles in the ADS are copyrighted by the publisher, and their use is free for personal use only. It is comprised of three bibliographic databases containing more than 9.1 million records: Astronomy and Astrophysics, Physics, and arXiv e-prints.