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The Long Climb from the Sun's Core
Week of July 21, 1997It has been said that our Solar System really consists of onlythree things: the Sun, Jupiter, and assorted rubble. With that inmind, we'll start our snacking tour of the Solar System with thepart of it that gives it its name: the Sun.
You may have noticed that the Sun has two overwhelmingly obviouscharacteristics: it's bright, and it's hot. These two things arerelated. The source of the Sun's heat wasn't understood untilthe middle of the 20th century, when nuclear fusionwas first being mathematically analyzed. Although even today wedo not completely understand what is happening inside the Sun, wehave a pretty good grasp of it. Basically, the nuclei of hydrogenatoms are compressed together so hard that they fuse to formhelium atoms (the actual process is quite a bit more complicated,but fusion to helium is the end result). This releases a tiny bitof energy. At least, tiny when you only do it once. But the Sunconverts millions of tons of hydrogen into helium in its coreevery second, and so alot 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 Sunto the surface. That's a distance of 700,000 kilometers, oralmost twice the distance from the Earth to the Moon, so youmight expect it takes a while. You mightnot expect just howlong it does take. The center of the Sunis extremely dense, and a photon can only travel a tiny distancebefore running into another hydrogen nucleus. It gets absorbedby that nucleus and the re-emitted in a random direction. If thatdirection is back towards the center of the Sun, the photon has lostground! It will get re-absorbed, and then re-emitted, over and over,trillions of times. The path it follows is called a "random walk" (orsometimes a "drunkard's walk"). Eventually it does make its way to thesurface, but it takes a long time: the average photon may bouncearound inside the Sun for 40,000 years!* So thelight you see from the Sun is really very old. The photons were firstemitted long before our civilization began!
*In this Snack, I originally said it takes a million yearsfor a photon to get out. Since then, I have found more recent papersthat show that it takes far less, perhaps even as little as 17,000years. 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.
Would you like to know more about the Sun?Bill Arnett's "The Nine Planets" page is the definitive guide to our Solar System.
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