Ultraviolet (UV) Astronomy

Ultraviolet (UV) astronomy is the branch of Observational Astronomy dealing with the electromagnetic radiation emitted by celestial bodies in the UV wavelength range, a portion of the spectrum simultaneously shielded by our own atmosphere and beyond the sensory limits of our sight.

The technological development inherited from the World War  II,  especially  in  the  field  of  long-range  ballistic weapons, made it possible to take rocket-borne payloads above the atmosphere starting  in  the  late 1940s.  In particular,  the  first,  unguided  experiments  flown  by  the Naval  Research  Laboratory  (NRL)  on  board  captured

German V2 rockets returned unique information on the intensity and spectral distribution of the UV emission of the Sun. The rate of progress of UV space astronomy was quite fast.

Actually, the electromagnetic ‘window’ accessible to ground-based observers is quite limited; being virtually confined to the wavelengths the human eye is responsive to. In particular, the shielding effect of the Earth’s atmosphere on the radiation coming from space becomes very high at wavelengths shortward of 320 nm, the adopted limit of the UV region of the spectrum. This phenomenon, mainly due to the absorption of oxygen and ozone, affects the entire UV and x-ray spectral regions, thus preventing  astronomers  from  recording  not  only  high energy phenomena giving origin to x- or γ-rays, but also common processes  involving intermediate energies and producing mainly UV  radiation.  This  obstacle  could  be  overcome only when—starting in the 1960s—it became possible, by means  of  rockets  and  orbiting  vehicles,  to  carry  astronomical telescopes above the bulk of the  atmosphere.

The wide term ‘UV’ often refers to the wavelength interval starting from the atmospheric cut-off (~ ~320 nm) down to the ‘Lyman break’, i.e. the limit of the hydrogen Lyman line series at ~ ~90 nm. The advantage of accessing the UV range was manifold. First, as already pointed out, the majority of radiation  emitted  by  stars whose  photospheric  temperature exceeds  10 000 K  falls  in  the UV  region. The reason is that stars’ energy distribution crudely follows Wien’s law for an ideal radiator:

λmax T = constant

(Where λmax is the wavelength of maximum emission and T the temperature), thus showing the emission peak at shorter wavelengths as the temperature increases. As

a  consequence,  one  has  to  access  the  UV  to  properly record  the  energy distribution  of  the hottest  stars  as  a function of wavelength, as well as their total energy output.

Secondarily,  UV  observations  represent  a  superb research tool to investigate both physics and chemistry of astronomical  bodies  owing  to  the  occurrence,  in  this wavelength  range,  of  the  so-called  resonance  spectral transitions, i.e. the most intense, ground-state transitions for most common atoms, ions and molecules. Finally, when moving to UV, one can carry out deep surveys at significantly reduced levels of sky back-ground. Actually, space observations ensure a sky 40× darker than at any wavelength from the ground at λ~ ~200 nm.  This is obviously valuable when observing faint, extended sources such as distant galaxies.

Questions to Ponder

• Mention some important achievements of UV astronomy.
• Why certain objects are better observed in UV rather than the visible range of wavelengths?