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Not All of Your Suntan Comes From the Sun

Not all of that golden glow after a day at the beach comes from the sun. Radiation from sources well beyond our solar system, even outside our galaxy, travels for upwards of billions of years to arrive just in time to meet you on the sand.

The sun is still largely responsible for your tan, however. Overwhelmingly so, in fact. But a full ten trillionths of your summer look can be credited to photons from black holes, other galaxies and other sources, finds a study to be published in The Astrophysical Journal that analyzed data from a small squadron of space telescopes.

Any time we're outside, day or night, 10 billion photons per second traveling from intergalactic space land on our skin, found an international team of researchers from International Centre for Radio Astronomy Research, Arizona State University and Cardiff University. Although some of that energy can be potentially damaging, cosmic dust acts as a kind of natural sunscreen, blocking about half of the energy coming from ultraviolet light of galaxies.

Harmless or not, that dose of radiation we receive from beyond the Milky Way is of course miniscule compared to what sources a little closer to home. The sun sends our way some 1 sextillion (or 1,000,000,000,000,000,000,000) photons per second. And that's just what we get directly from the sun.

Solar radiation scattered in the sky is the next largest source of our summer rays, sending over some 300 quintillions photons per second. Even photons that passed Earth find their way back to the beach, reflected off of dust in the solar system and landing on our shores at a rate of 100 trillion photons per second. Even photons left over from the Big Bang show up for beach day, the researchers found, at a clip of 10 quadrillion photons per second. These photons, however, won't make you any more bronze. So, all of this begs the question, of course: Why exactly were a team of astronomers from three continents so interested in the sources of our tans?

Well, that wasn't what they were trying to find out exactly. The team analyzed data collected from NASA's Galaxy Evolution Explorer and Wide-field Infrared Survey Explorer telescopes, the Spitzer and Hubble space telescopes, the European Space Agency's Herschel space observatory and Australia's Galaxy And Mass Assembly survey. The idea is to measure light hitting Earth from outside the galaxy across a broad spectrum, and the latest effort is part of ongoing research to understand the evolution of energy in the universe.

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