Study finds how much of your tan comes from outside the galaxy

Suntans and summertime go together like peanut butter and jelly, as the more time people spend outdoors swimming, playing sports, lying on the beach or cutting the grass, the greater amount of time they spend being exposed to rays originating from our galaxy’s central star, right?
Not exactly, according to researchers at the International Centre for Radio Astronomy Research (ICRAR). While it is true that most of the photons of light that bombards our skin in the summer do originate from the sun, a small fraction of them have origins beyond our solar system, experts from the Centre reported in a new study published this week in The Astrophysical Journal.
A team led by ICRAR astrophysicist Professor Simon Driver analyzed the photons that make up the light hitting the Earth and found that radiation originating from beyond our galaxy constitutes approximately one ten-trillionth of the average suntan. These photons (or tiny packets of energy) that comprise those waves range in wavelength from harmless to damaging, they 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,” Driver explained in a statement. “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.”

Despite intergalactic origins, no cause for concern over extrasolar photons

Fortunately, the nearly 10 billion photons per second of extrasolar radiation that we are exposed to constitutes such a small fraction of the particles bombarding our skin on a regular basis during the summertime, the study authors emphasize that there is no need to be alarmed.
According to Professor Driver, while 10 billion photos per second may seem like a high amount, an individual would have to be exposed to it for several trillion years before they caused any real damage. Furthermore, co-author Rogier Windhorst from Arizona State University noted that the universe itself offers some protection against some of the incoming intergalactic radiation.
“The galaxies themselves provide us with a natural suntan lotion with an SPF of about two,” he explained. As the ultraviolet light of galaxies makes it way towards the Earth, nearly half of it is converted into a less damaging wavelength by dust grains, he explained. This helps to limit how much harm it can cause to those of us innocently going about our business during the summer.
The analysis was conducted using the Spitzer and Hubble space telescopes along as well as other ground- and space-based NASA and ESA observatories, and was part of ICRAR’s ongoing work to better understand the evolution of mass and energy in the universe, the Centre noted. Driver’s team said that, using these instruments, they were able to collect the most accurate measurements of the extra-galactic background light to date.
“The processes which shape and shuffle mass generate vast quantities of energy, dwarfed only by the vastness of space,” the professor added. “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.”
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