学物理的都不吭声么？
这个链接是wiki参考的。够不够简单？都不看参考文献的么？
http://www.badastronomy.com/bitesize/solar_system/sun.htmlYou 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 20th 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!
那个时间是从聚变反应放出的高能射线为起始时间的。讨论的是那个伽马射线或者whatever出来以后的历程，为什么会经历漫长时间才能最终逃出内核。
这个和光子传播速度有一毛关系？92