Ten trillionths of your suntan comes from beyond our Milky Way Galaxy, according to a team of astronomers led by Prof. Simon Driver from the University of Western Australia and the International Centre for Radio Astronomy Research.
This is an infographic explaining the fact that 10 trillionths of your suntan comes from beyond our Milky Way Galaxy. Image credit: Dan Hutton / ICRAR.
In a study published in the Astrophysical Journal (arXiv.org preprint), Prof. Driver and co-authors measured the light hitting our planet from outside the Milky Way over a very broad wavelength range.
“We are constantly bombarded by about 10 billion photons per second from intergalactic space when we’re outside, day and night,” Prof. Driver 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 extragalactic background light. These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into supermassive black holes.”
For their research, Prof. Driver and his colleagues used several ground- and space-based telescopes, including NASA’s Hubble and Spitzer space telescopes, ESA’s Herschel space observatory and Australia’s Galaxy and Mass Assembly Survey.
They looked at photons whose wavelengths vary from a fraction of a micron (damaging) to millimeters (harmless).
“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,” Prof. Driver said.
“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,” added co-authors Prof. Rogier Windhorst, from Arizona State University.
“The galaxies themselves provide us with a natural suntan lotion with an SPF of about two.”
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Simon P. Driver et al. 2016. Measurements of extragalactic background light from the far-UV to the far-IR from deep ground- and space-based galaxy counts. ApJ 827, 108; doi: 10.3847/0004-637X/827/2/108
