The Long Climb from the Sun's Core

You may have noticed that the Sun has two overwhelmingly obvious characteristics: it's bright, and it's hot. These two things are related. The source of the Sun's heat wasn't understood until the middle of the 20^th century, when nuclear fusion was first being mathematically analyzed. Although even today we do not completely understand what is happening inside the Sun, we have a pretty good grasp of it. Basically, the nuclei of hydrogen atoms are compressed together so hard that they fuse to form helium atoms (the actual process is quite a bit more complicated, but fusion to helium is the end result). This releases a tiny bit of energy. At least, tiny when you only do it once. But the Sun converts millions of tons of hydrogen into helium in its core every second, and so a lot of energy is released. This energy is in the form of photons, or light.

These photons have to work their way out from the core of the Sun to the surface. That's a distance of 700,000 kilometers, or almost twice the distance from the Earth to the Moon, so you might expect it takes a while. You might not expect just how long it does take. The center of the Sun is extremely dense, and a photon can only travel a tiny distance before running into another hydrogen nucleus. It gets absorbed by that nucleus and the re-emitted in a random direction. If that direction is back towards the center of the Sun, the photon has lost ground! It will get re-absorbed, and then re-emitted, over and over, trillions of times. The path it follows is called a "random walk" (or sometimes a "drunkard's walk"). Eventually it does make its way to the surface, but it takes a long time: the average photon may bounce around inside the Sun for 40,000 years!^* So the light you see from the Sun is really very old. The photons were first emitted long before our civilization began!

* In this Snack, I originally said it takes a million years for a photon to get out. Since then, I have found more recent papers that show that it takes far less, perhaps even as little as 17,000 years. 40,000 is a number that seems to me to be the best supported, but who knows? Maybe future papers will refine the time even more.

I have a page that picks up the travels of the photon where this one leaves off. Ever wondered just why the sky is blue...?

Would you like to know more about the Sun? Bill Arnett's "The Nine Planets" page is the definitive guide to our Solar System.