Photographer: Adhemar Duro#144041/Getty Images

No Wonder You’re Hot. You’re Being Blasted by a Trillion Suns

Here’s how it works, and how energy gets around the universe.

An Australian astrophysicist and nine colleagues have answered a question of galactic import to beachgoers: How much of my sunburn can I reasonably blame on stars outside our galaxy? 

The good news: Uncovered vacationers are getting hit 10 billion times a second by photons that originated far outside the Milky Way.

The bad news: That's only 10 trillionths of what the Sun is bringing to the fight.

The measurements, published in Astrophysical Journal, are part of a larger effort to understand how matter and energy get around the universe and why the cosmos is structured the way it is. The study was led by Simon Driver of the International Centre for Radio Astronomy Research at the University of Western Australia.

Translating a serious astrophysics investigation into a confectionery tidbit—10 billion photons!—is a nice touch of out-of-this-world cleverness. The effort behind the study was anything but trivial.

The work is an estimate of how much we're hit by light that originated outside our galaxy in stars active as long as billions of years ago. It's difficult because the only way to estimate the "extragalactic background light" is from deep inside the Milky Way. ​It's a little like standing under a bright streetlight and trying to measure how much light all the other streetlights are putting out. ​

This light signal is nothing less than "a product of the dominant astrophysical processes, which have taken place over the past 13 billion years, in terms of energy distribution." That is, how the matter smoldering in stars shoots off light.

The research relied on a phenomenal array of data sets from high-energy light in the ultraviolet spectrum to the low-energy infrared. The data came from an all-star roster of astronomical sluggers: the Hubble, Spitzer, and Herschel space observatories and several major ground telescopes.

Much of the light changes on its journey, protecting us, a little tiny bit, from skin-searing UV light​. Dust particles in distant galaxies knock ultraviolet photons down to visible-light or infrared strength. 

"The galaxies themselves provide us with a natural suntan lotion with an SPF of about two," Rogier Windhorst of Arizona State University said in a statement. 

A lucky few live on the beach. Landlubbers might travel a great distance to it. The schlep should feel shorter considering that the smallest travelers—extragalactic photons—needed three times the age of the Earth to get there.

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