You may have noticed something unusual when looking at photos of the sun. Perhaps you've even projected the sun's image onto a screen using a good telescope, in which case you've seen this phenomenon. It's hard to miss. It's called limb darkening, and it means that when you see the sun, its outer rim, all the way around, seems to be dark. The interior of our sun, where nuclear fusion takes place, is at a temperature of about 15 million degrees Celsius. This is the hottest part of the sun. The outer layer of the sun, the photosphere, is by contrast much cooler. This thin outer layer's temperature is a mere 5,000 degrees Celsius. This is still very hot ... it's brighter and hotter than a cutting torch. This outer layer is the part of the sun you see. When you look at the middle of the sun, you are looking directly 'down' at the photosphere, through a small fraction of the sun's radius. When you look at the edge of the sun, you are seeing the photosphere from an angle ... along a path that goes through much more of this upper layer of gas. This tends to dim the light seen from the edge, in comparison to the light from the centre, making it seem slightly darker. The Sun | Main Page