Astronomical Applications Department, U.S. Naval Observatory thesis Page 29
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that the UV-bright stars in M33 are evolved B supergiants indicating that the
optically brightest stars are not necessarily the ionizing sources. Since the FUV is
able to isolate the massive stellar component over timescales similar to the kine-
matic ages of the HI holes, a comparison between the HI and FUV morphologies
o ers fertile ground for nding a progenitor population. Instead of searching for a
requisite number of lower-mass neighbors, a search for the progenitor stars them-
selves can be made by comparing the FUV and HI emission. For these reasons, a
comparison between the relative emission morphologies of the FUV ux and the
HI density is another useful tool in studying star formation processes in irregular
galaxies.
1.4 Observing in the Ultraviolet
Despite the wealth of information provided by UV images, they are seldom
used in studies because spatially-resolved UV images are rare. Since observations
in the UV must be made above above the atmosphere, the developments in UV
astronomy have been closely coupled with those in the space program.
1.4.1 History
The history of observing in the UV commences approximately two decades ago
with the International Ultraviolet Explorer IUE satellite. The small aperture
spectroscopy provided by this instrument comprises the bulk of UV studies to
date. The Hubble Space Telescope Faint Object Spectrograph HST FOS now
continues to provide UV spectroscopy in place of the retired IUE satellite after
its nearly 19 years of service. Ultraviolet imaging developed slowly following the
rst UV image of another galaxy, the LMC, was taken from the the lunar surface
during the Apollo 16 mission in 1972 Page & Carruthers 1981. Low-resolution
