Over the last century, our general view on the structure and composition of the Universe has changed dramatically, not least because of the advent of powerful observatories that give access to the electromagnetic spectrum beyond the optical regime. Among the various spectral ranges, the ultraviolet (UV) range stands out in modern astrophysics because of it extraordinary relevance to study the abundance and distribution of atoms and molecules in large range of astrophysical environments by way of spectroscopic methods. Most of the electronic transitions in atoms and molecules are located in the UV between 90 and 300 nanometers, resulting in a very high information density of spectral features that can be used to explore planets, stars, gaseous nebulae, galaxies, active galactic nuclei, intergalactic matter, and cosmological scales at very high precision.
The extraordinary importance of previous UV spectroscopic missions, such as Copernicus, IUE, ORFEUS, FUSE, and the various UV instruments installed on the Hubble Space Telescope for our understanding of the Universe and its components is evident. UV observations require space-based observatories, as the earth’s atmosphere is opaque for radiation with wavelengths <300 nm. However, despite the importance of the UV range for modern astrophysics, there are no plans at ESA or NASA for future UV spectroscopic observatories after HST (with its powerful UV spectrograph COS) will have ceased operating around 2020. Its successor, JWST, is designed to primarily observe highly redshifted objects in the early Universe at infrared wavelengths and thus has no UV capabilities. In the following, the importance of UV observations for our understanding of the Universe is sketched for various important astrophysical environments.
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