The SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA cooperative agreement. The ADS maintains more than 8.7 million bibliographic records and full-text scans of astronomical literature.
The NASA/IPAC Extragalactic Database (NED) is built around a master list of extragalactic objects for which cross-identifications of names have been established, accurate positions and redshifts entered to the extent possible, and some basic data collected. Bibliographic references relevant to individual objects have been compiled, and abstracts of extragalactic interest are kept on line. Detailed and referenced photometry, position, and redshift data, have been taken from large compilations and from the literature. NED also includes images from Two Micron All-Sky Survey (2MASS), from the literature, and from the Digitized Sky Survey (DSS). NED's data and references are being continually updated, with revised versions being put on-line every 2-3 months.