Alien Sunlight From Far Beyond The Galaxy Is Actually Causing Your Suntan

Alien sunlight, mmm.

11/08/2016 10:32

As we top up our tan on the beach alien sunlight is helping give us that bronzed look, a new study found. 

Your body will be bombarded by about sextillion photons of light per second - but ten trillionths comes from beyond our galaxy. 

Most of these photons, or small packets of energy, originate from the Sun. 

But a very small fraction have travelled across the Universe for billions of years before ending their existence when they collide with your skin. 

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Astronomers for the first time have accurately measured the light hitting the Earth from outside our galaxy over a very broad wavelength range - from a fraction of a micron (damaging) to millimetres (harmless). 

But radiation from outside the galaxy is so small there is no immediate need for alarm. 

International Centre for Radio Astronomy Research (ICRAR) astrophysicist Professor Simon Driver said we are constantly bombarded by about 10 billion photons per second from intergalactic space when we’re outside, day and night. 

He said: “Most of the photons of light hitting us originate from the Sun, whether directly, scattered by the sky, or reflected off dust in the Solar System. 

“However, we’re also bathed in radiation from beyond our galaxy, called the extra-galactic background light. 

“These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into supermassive black holes.” 

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Lead author Professor Driver, based at the University of Western Australia, measured this ambient radiation from the Universe, from a wide range of wavelengths by combining deep images from a flotilla of space telescopes. 

While 10 billion photons a second might sound like a lot we would have to bask in it for trillions of years before it caused any long-lasting damage. 

Professor Rogier Windhorst, from Arizona State University, added the Universe also comes with its own inbuilt protection as about half the energy coming from the ultraviolet light of galaxies is converted into a less damaging wavelength by dust grains. 

He said: “The galaxies themselves provide us with a natural suntan lotion with 
an SPF of about two.” 

The study is part of ICRAR’s ongoing work to understand the evolution 
of energy, mass and structure in the Universe. 

The research programme examines how we went from the smooth distribution 
of atoms in the early Universe to the emergence of the Periodic Table and the multitude of stars, galaxies and galaxy clusters we see today. 

Prof Driver said: “The processes which shape and shuffle mass generate vast quantities of energy, dwarfed only by the vastness of space 

“The precise physics as to how this energy is released is still not fully understood and work continues to build numerical models capable of explaining the energy that we’ve now measured.” 


  • Sun 1,000,000,000,000,000,000,000 photons per square metre per second 
  • Reflected off the sky 300,000,000,000,000,000,000 photons per square metre per second 
  • Leftover from the Big Bang 10,000,000,000,000,000 photons per square metre per second 
  • Reflected off dust in the Solar System 100,000,000,000,000 photons per square metre per second 
  • Extra-galactic background light 10,000,000,000 photons per square metre per second 

The research was published in The Astrophysical Journal 

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