Telescopes I've Used - IUE

IUE (International Ultraviolet Explorer), geosynchronous Earth orbit

IUE This engineering mockup of IUE was photographed at the National Air and Space Museum, in Washington, D.C. The telescope points left, with the antennas at the back around the nozzle of the apogee motor.

"The little satellite that could" was as international as its name suggests, involving NASA, ESA, and the UK Science and Engineering Research Council (now PPARC, for acronym aficionados). It was launched on Jan 26 1978 for a 3-5 year mission, and just kept going and going past gyroscope failures and onboard crises. IUE was finally turned off, while still functional but going downhill, on 30 Sept 1996. It carried a 45-cm telescope (which looks awfully small in the replica at the GSFC visitor center), feeding high- and low-dispersion spectrographs for the 1150-3300 Angstrom range. Two TV detectors with UV converters could be used with each, for short- and longer-wavelength UV radiation. The picture shows the fixed but multi-angle solar arrays used to reduce moving parts (which also explains why, horror of horrors, the telescope was focussed using heaters on the primary mirror). IUE was in a geosynchronous (but inclined and elliptical, therefore not geostationary) orbit, so that it could be continuously controlled from GSFC, and most of the time from the ESA tracking station at Villafranca, Spain, from which 1/3 of operational shifts were normally run. At mission's end, IUE had taken 104,470 spectra of 9600 objects, from the planets to high-redshift quasars.

IUE console at GSFC IUE acquisition of NGC 828 IUE introduced a generation of astronomers to space instrumentation and operations, in a relatively gentle way because the real-time 24-hour operations of IUE felt not unlike traditional ground-based observing. Target acquisitions used a photomultiplier scanning the field of biew, giving a (slow) picture of the field of view to be sure you pointed IUE at the right place. The data from IUE made the importance of data archives in astrophysics clear, and was probably the first archive to reach such a level that reuse of old data was generating more papers and results than the satellite itself was in the mission's later years. In both operational and archiving aspects, the IUE experience echoed through the checkout and data-handling plans adopted for HST.

Some of the memorable science contributions from IUE included tracing the velocity and occurrence of strong stellar winds, an important factor in how massive stars evolve; showing that the old stellar populations in elliptical include an important role for surprisingly hot stars; and a crucial role in mapping the surroundings of Seyfert nuclei and quasars through intensive and uninterrupted spectroscopic campaigns to tell which spectral lines responded when to changes in the central source.

I was involved in four programs with IUE, starting when it was only four years into the mission (I gather than IUE badge number 1015 was sort of a badge of honor later on when the numbers passed 2000). With Bob Goodrich, I looked at UV spectra of some galactic nuclei that showed evidence of weak quasar-like activity, putative missing links between ordinary and active galaxies. Of three we looked at, one showed evidence of an appropriate UV source of nonstellar radiation, one showed hot stars, and one had enough dust not to be detected at all. And that's more or less how the statistics have continued into the HST era, though the spectra that it can deliver are dramaticaly more enlightening these days. At about the same time, I used IUE to study a star-forming region in the barred spiral galaxy NGC 5430, that was known to be extrardinarily rich in Wolf-Rayet stars. These stars mark a short-lived phase in the lives of massive stars, when their winds are so powerful that they shape the stars' spectra. This knot turned out to outshine the whole surrounding galaxy in the UV, and be remarkably free of dust. It must be a brief, and perhaps recurring, event, whose location at one end of the galaxy's bar suggests how the gas was collected to produce these stars so quickly.

Rogier Windhorst and I spent several IUE observing seasons looking at nearby radio galaxies, as points of comparison for the objects that were then being found at high redshifts. The points here were that optical observations at high redshift show us the radiation that was emitted in the UV, and that the fainter distant radio galaxies were more likely to correspond to nearby objects than the extremely rare powerful radio galaxies. We were able to show that many of them have Lyman alpha in emission, like their higher-redshift cousins, and that they practically all have stellar populations with hot components like many bright and metal-rich elliptical galaxies.

In IUE's final years, I observed several galaxy nuclei which showed evidence of simultaneously hosting a central "monster", perhaps a massive black hole, and a burst of star formation, whose existence might be related. This was done with the remote-observing setup, allowing me to send a student to my intro class one day to announce that Dr. Keel would be there as soon as he finished pointing a satellite. The most interesting result from these observations was in combination with some INT and KPNO spectra, from which I could show that the tiny nucleus of NGC 4569 not only underwent a starburst in the past, but that the visible light is dominated by A-type supergiants and only fairly deep in the ultraviolet do we see the typical population of young stars. Only thing I can think of is that the burst was remarkably quick and we're seeing it in a transient phase.

Telescope Collection

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