Astrophys Space Sci (2006) 303:133–145 DOI 10.1007/s10509-006-9057-4 ORIGINAL ARTICLE Fundamental Problems in Astrophysics Ana I. G ´ omez de Castro · Willem Wamsteker · Martin Barstow · Noah Brosch · Norbert Kappelmann · Wolfram Kollatschny · Domitilla de Martino · Isabella Pagano · Alain Lecavelier des ´ Etangs · David Ehenreich · Dieter Reimers · Rosa Gonz ´ alez Delgado · Francisco Najarro · Jeff Linsky Received: 21 February 2006 / Accepted: 14 March 2006 C  Springer Science + Business Media B.V. 2006 Abstract Progress of modern astrophysics requires the ac- cess to the electromagnetic spectrum in the broadest energy range. The Ultraviolet is a fundamental energy domain since it is one of the most powerful tool to study plasmas at tem- peratures in the 3,000–300,000 K range as well as electronic transitions of the most abundant molecules in the Universe. Moreover, the UV radiation field is a powerful astrochemical and photoionizing agent. The objective of this review is to describe the crucial issues that require access to the UV range. A summary has been added to the end with a more classic view of UV needs by astronomical object type; this approach is followed at length in the rest of the contributions of this issue. A. I. G. de Castro () Instituto de Astronom´ ıa y Geodesia (CSIC-UCM), Universidad Complutense de Madrid, Madrid, E-28040, Spain W. Wamsteker (†) INTA-LAEFF, Apartado 50.727, E-28080 Madrid, Spain M. Barstow Dept of Physics and Astronomy, University of Leicester University Road, Leicester LE1 7RH UK N. Brosch The Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel N. Kappelmann Institut f ¨ ur Astronomie und Astrophysik T ¨ ubingen (IAAT), Universit T¨ ubingen, Germany W. Kollatschny Institut f ¨ ur Astrophysik, Universit¨ at G ¨ ottingen, Friedrich-Hund-Platz 1, D-37077 G ¨ ottingen, Germany D. de Martino INAF-Osservatorio Astronomico di Capodimonte Napoli, Via Moiariello 16, I-80131, Italy I. Pagano INAF-Catania Astrophysical Observatory, via Santa Sofia 78, 95125 Catania, Italy A. L. des ´ Etangs · D. Ehenreich Hamburger Sternwarte, Universitt Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany D. Reimers Institut d’Astrophysique de Paris,UMR7095 CNRS, Universit´ e Pierre & Marie Curie, 98 bis boulevard Arago, F-75014 Paris, France R. G. Delgado Instituto de Astrof´ ısica de Andaluc´ ıa (CSIC), Apdo. 3004, 18080 Granada, Spain F. Najarro Instituto de Astrof´ ısica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, E-28006 Madrid J. Linsky JILA/University of Colorado and NIST/Boulder, CO 80309-0440 USA Keywords UV astronomy 1. Introduction Access to the UV range is fundamental for the progress of astrophysics since UV spectroscopy is the most powerful tool to study plasmas at temperatures in the 3,000-300,000 K range. Also, the electronic transitions of the most abundant molecules in the Universe (H 2 , CO, OH, CS, CO + 2 ,C 2 ...) are in this range. Moreover, the UV radiation field is a powerful astrochemical and photoionizing agent. The impact of UV instruments in modern astronomy can be clearly traced through the considerable success of the Springer