Not just
the Sun, but the light travelling from galaxies across the universe for
billions of years is also responsible for our suntans. We are constantly bombarded
by about 10 billion photons per second from intergalactic space when we are
outside, day and night. When we lie on the beach, our bodies are bombarded by
about sextillion photons of light per second. 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. Astronomers measured the light
hitting Earth from outside our galaxy over a very broad wavelength range. The study
looked at photons whose wavelengths vary from a fraction of a micron (damaging)
to millimeters (harmless).
Radiation
from outside the galaxy constitutes only ten trillionths of your suntan, so
there is no immediate need for alarm. 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 form
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 super massive black holes.
