(Every day until Christmas, I’ll be posting a science-related image.)

Day 13

The Sun is the ultimate source of most of the energy on Earth: the various chemical reactions that power life are due to sunlight. However, there’s an aspect of the Sun that doesn’t take the form of light at all. The nuclear reactions in the Sun’s core fuse hydrogen into helium, producing light…and neutrinos, extremely low-mass particles that interact only weakly with other forms of matter. Photons ping-pong around the interior of the Sun, keeping the plasma hot through collisions; they take an average of 100,000 years to travel from the core to the surface. Neutrinos, on the other hand, stream out from the core, and only rarely hit anything on their way out. In other words, by “seeing” neutrinos from the Sun, we’re getting a view of the core as it is a few minutes ago, while the light from the Sun reflects the core’s activity 100,000 years ago.

The image above is of the Sun in neutrinos, collected by the Super-Kamiokande neutrino observatory in Japan. The detector (often called SuperK) is located deep underground in a mine, to limit the number of extraneous particles entering it. Neutrinos aren’t bothered by puny matters like miles of rock, which stop wimpier particles like electrons and protons. In fact, this image was at night, when the Sun was on the opposite side of the planet, so the neutrinos forming the picture passed all the way through Earth.

Astronomy is not limited to what our eyes can see—and is not even limited by light. The invisible is made visible through science.