ultraviolet radiation

The sunburn you get on a cloudy day is caused by light you never saw: ultraviolet. Ultraviolet light lies just beyond the violet end of the visible spectrum, with wavelengths a little shorter than violet light, roughly from 400 nanometers down to about 10 nanometers. Each ultraviolet photon carries more energy than a visible one — enough to damage skin and break molecular bonds — which is why this band marks the start of what we loosely call high-energy light.

In astronomy, ultraviolet is the signature of heat and youth. Hot, massive, blue stars radiate a large share of their light in the ultraviolet, so a survey in this band lights up exactly the youngest, most energetic stars and the star-forming regions around them, while ignoring older, cooler, redder ones. Ultraviolet photons are also energetic enough to tear electrons off atoms (ionize them), so they carve out and light up glowing clouds of ionized gas around newborn stars. The hot gas in the outer layers of stars and the leftover atmospheres of dying stars also shine here.

There is a catch that shapes the whole field: Earth's atmosphere — especially the ozone layer — absorbs almost all ultraviolet light, which is fortunate for life but means ultraviolet astronomy can only be done from space, using satellites above the atmosphere. The same energetic photons that let ultraviolet diagnose hot stars also make this band a powerful probe of the gas between galaxies, because ultraviolet light from distant quasars is absorbed by intervening hydrogen along the way.