Swirling spiral arms of galaxy M33 can be seen in visible light, but the true extent of these spiral arms are revealed in ultraviolet light. Just as a dog can hear a whistle just outside the range of human hearing, bugs can see light just outside the range our eyes can see. A bug zapper emits this ultraviolet light to attract insects. Johann Ritter conducted an experiment in 1801 to find out what, if any, electromagnetic waves are beyond violet. Ritter knew that photographic paper would turn black more rapidly in blue light than in red light. So he tried exposing the paper beyond the violet end of the visible spectrum. Sure enough, the paper turned black proving the existence of light beyond violet, ultraviolet rays These ultraviolet rays, or UV radiation, vary in wavelength from 400 nanometers to 10 nanometers and can be subdivided into 3 regions: UV-A, UV-B and UV-C. Visible light from the Sun passes through the atmosphere and reaches the Earth’s surface. UV-A, long wave ultraviolet, is the closest to visible light. Most UV-A also reaches the surface. But shorter wavelengths, called UV-B, are the harmful rays that cause sunburn. Fortunately, about 95% of these harmful UV-B rays are absorbed by ozone in the Earth’s atmosphere. UV-C rays are the shortest and most harmful and are almost completely absorbed by our atmosphere. The Ozone Monitoring Instrument aboard NASA’s Aura satellite detects ultraviolet radiation to help scientists study and monitor the chemistry of our atmosphere, including UV absorbing ozone. While atmospheric protection from harmful UV radiation is good for humans… it complicates the study of naturally produced UV rays in the Universe, by scientists here on the Earth’s surface. Young hot stars shine most of their light beyond the visible light spectrum at ultraviolet wavelengths. Scientists need telescopes in orbit above the Earth’s UV absorbing atmosphere to find and study these UV-bright regions of star formations in distant galaxies. New young stars in the spiral arms of galaxy M81 can be seen in this Galaxy Evolution Explorer, GALEX, image from NASA. Chemical substances, both atoms and molecules, interact with UV light making this region particularly interesting to scientists. An ultraviolet instrument aboard Cassini has detected hydrogen, oxygen, water ice, and methane in the Saturn system. UV data have also revealed details of Saturn’s aurorae. Scientists also use UV waves shining from distant stars to view permanently shadowed regions of lunar craters. The Lyman-Alpha Mapping Project, or LAMP, instrument aboard NASA’s Lunar Reconnaissance Orbiter can use this faint star-shine to look for possible water ice on the moon. Ultraviolet rays may be harmful to humans, but they are essential to studying the health of our planet’s protective atmosphere and give us valuable clues to the formation and composition of distant celestial objects.