Harvard–Smithsonian Center for Astrophysics
The Astrophysics Data System (ADS) is an online database of over eight million astronomy and physics papers from both peer reviewed and non-peer reviewed sources. Abstracts are available free online for almost all articles, and full scanned articles are available in Graphics Interchange Format (GIF) and Portable Document Format (PDF) for older articles. It was developed by the National Aeronautics and Space Administration (NASA), and is managed by the Harvard–Smithsonian Center for Astrophysics.