The sun isn’t the only light source behind that summer tan

Tiny amount of ultraviolet light comes from the Big Bang and other stars, galaxies

Milky Way Galaxy

CATCHING SOME GALACTIC RAYS Galaxies, such as those in this image from the Paranal Observatory in Chile, are responsible for less than one-billionth of 1 percent of your suntan. The bright blue-white points of light in this picture are stars in the Milky Way; most other objects are galaxies.

ESO/UltraVISTA team

Your summer suntan is almost entirely locally sourced. But a smidgen of that healthy glow hails not from the sun but from the ultraviolet light of nearby stars and other galaxies: less than one-billionth of 1 percent. Even photons lingering from the Big Bang contribute some: roughly 0.001 percent.

Simon Driver, an astronomer at the University of Western Australia in Crawley, and colleagues calculated these numbers, but not because they’re interested in tanning. They were trying to decipher the extragalactic background light, or EBL, a diffuse glow that fills the universe (SN: 9/7/13, p. 22). Using galaxy observations from multiple telescopes, they assessed the number of EBL photons, from infrared to ultraviolet, that reach Earth. About half originated with the formation of galaxy cores and supermassive black holes during roughly the first 4 billion years of cosmic history, the researchers report in the Aug. 20 Astrophysical Journal. The growth of disks of stars in galaxies since that time accounts for the other half. 

T. Tibbitts

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