1 Introduction

1.1 The CDS

The Centre de Données astronomiques de Strasbourg (CDS) defines, develops, and maintains services to help the astronomers find the information they need from the very rapidly increasing wealth of astronomical information, and particularly of on-line information.

CDS is operated at the Strasbourg astronomical Observatory, under an agreement between French Institut National des Sciences de l'Univers (INSU) and Université Louis Pasteur, Strasbourg (ULP). CDS personnel created and implemented the SIMBAD data bank and maintain its data and software system.

A detailed description of the CDS on-line services can be found, e.g., in Egret et al. ([1995]) and in Genova et al. ([1996], [1998], [2000]), or at the CDS web site. Questions or comments about the CDS services can be sent to the hot line question@simbad.u-strasbg.fr.

1.2 SIMBAD

The SIMBAD database contains identifications, "basic data'', bibliographical references, and selected observational measurements for more than 2.7 million astronomical objects (November 1999). Data and information published in SIMBAD come from selected catalogues and tables and from the whole astronomical literature.

The specificity of the SIMBAD database is to organize the information per astronomical object, thus offering a unique perspective on astronomical data. This is done through a careful cross-identification of objects from catalogues, lists, and journal articles. The ability to gather together any sort of published observational data related to stars or galaxies has made SIMBAD a key tool used worldwide for all kinds of astronomical studies.

SIMBAD is the acronym for Set of Identifications, Measurements and Bibliography for Astronomical Data. The main access point to SIMBAD is the WWW home page; there is a mirror copy at SAO, Harvard.

1.3 Historical background

Building a reference database for stars - and, later, for extragalactic objects and all astronomical objects outside the Solar System - has been the first goal of the CDS: SIMBAD is the result of an on-going effort which started soon after the creation of CDS in 1972. SIMBAD was created by merging the catalog of Stellar Identifications (CSI, Ochsenbein et al. [1981]) and the Bibliographic Star Index (Ochsenbein [1982]) as they existed until 1979. The resulting data base (at that time, about 400 000 objects, mainly stars) was then expanded by the addition of source data from the catalogs and tables, and by new literature references. The database was extended to galaxies and other non-stellar objects in 1983 (Dubois et al. [1983]). For details about the early developments of SIMBAD see Egret ([1983]). The first on-line interactive version of SIMBAD was released in 1981, and operated at the Strasbourg Cronenbourg computer center until December 1984, when it was moved to Université Paris-Sud at Orsay, and operated there until June 30, 1990. The database is now hosted on a Unix server, at the Strasbourg Observatory. The original command line interface has been complemented by an interactive X-Window interface ( XSIMBAD) in 1994, and by a World-Wide Web interface in 1996. There is also a client/server mode, providing quick responses to simple queries, essentially for the name resolution in archives and information systems (see Sect. 5).

For descriptions of earlier stages of the database, see Heck & Egret ([1986]), and Egret et al. ([1991]).