An international team of scientists has
accurately measured how much of the light striking, or tanning your
body comes from outside of our galaxy. Depending on your current
levels of self-esteem, those photons, many of which traveled for
billions of years to reach Earth, are either incredibly lucky, or you
may have just ruined a very, very long trip.
As you wander around soaking up the summer sunshine, your body will be struck by roughly a sextillion light particles, or photons each second. The researchers ascertained the various sources of photons with wavelengths varying from a fraction, to millimetres of a micron by analyzing deep images harvested by a veritable cornucopia of space telescopes, including the Hubble, Spitzer, Herschel and WISE observatories.
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The work formed part of an overarching effort aiming to better understand how the universe went from a mass of atoms in the aftermath of the Big Bang, to the complex and highly structured universe we exist in today.
So here's a breakdown of the origins of photons privileged enough to end their existence by striking you. Prepare to get lost in a sea of 1s and 0s. The obvious and most bountiful source of tan comes directly from our Sun, which provides an astonishing 1,000,000,000,000,000,000,000 photons per square metre per second.
We get a further 300,000,000,000,000,000,000 photons per square metre per second reflected onto us from our atmosphere. 10,000,000,000,000,000 extra photons per square metre per second leftover from the Big Bang (as if creating the cosmos was not enough), and another 100,000,000,000,000 photons per square metre per second reflected from random pieces of dust scattered throughout the solar system.
Finally, we receive a mere 10,000,000,000 photons per square metre per second from extra-galactic background sources, cast out by distant stars and matter being consumed by voracious black holes. So, when it comes down to it, photons originating outside of our galaxy constitute a measly trillionth of your tan. In fact, you would need to be exposed to this level of radiation for trillions of years for it to do any kind of damage.
So, next time you're showing off your hard earned tan, don't give all of the credit to the Sun, because the wider cosmos had a (very, very) small part to play as well.