Ultraviolet
line spectrum measurements (
spectroscopy) are used to discern the chemical composition, densities, and temperatures of the
interstellar medium, and the temperature and composition of hot young stars. UV observations can also provide essential information about the
evolution of galaxies. They can be used to discern the presence of a hot
white dwarf or
main sequence companion in orbit around a cooler star. The ultraviolet
universe looks quite different from the familiar
stars and
galaxies seen in
visible light. Most stars are actually relatively cool objects emitting much of their electromagnetic radiation in the visible or near-
infrared part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their
evolution. In the Earth's sky seen in ultraviolet light, most stars would fade in prominence. Some very young massive stars and some very old stars and galaxies, growing hotter and producing higher-energy radiation near their birth or death, would be visible. Clouds of gas and dust would block the vision in many directions along the
Milky Way. Space-based solar observatories such as
SDO and
SOHO use ultraviolet telescopes (called
AIA and
EIT, respectively) to view activity on the Sun and its
corona. Weather satellites such as the
GOES-R series also carry
telescopes for observing the Sun in ultraviolet. The
Hubble Space Telescope and
FUSE have been the most recent major
space telescopes to view the near and far UV
spectrum of the sky, though other UV instruments have flown on smaller observatories such as
GALEX, as well as
sounding rockets and the
Space Shuttle. Pioneers in ultraviolet astronomy include
George Robert Carruthers,
Robert Wilson, and
Charles Stuart Bowyer. - in
high-energy X-ray and ultraviolet light (released 5 January 2016). ==Ultraviolet space telescopes==