The Darkening Stars

Solar Dynamics Observatory Limb darkening observed during the transit of Venus. The Sun is so intensely bright we can’t safely look at it directly. We need a filter to block much of its light. And when we do, we can start to see some interesting features. Sunspots are one of these features, and the ebb and flow of their numbers has been studied for centuries. But a more subtle feature is that the edge of the Sun appears less bright than its central region. It’s an effect known as limb darkening. The limb darkening of the Sun was first measured by Pierre Bouguer in the 1700s. It is an effect of the Sun’s atmosphere. When you look at the center of the Sun you see light that passed straight through the solar atmosphere. Not much of the Sun’s light is absorbed or scattered. When you look at the edge of the Sun, that light had to take a shallow path through much more of the atmosphere to reach you. More of the light is scattered or absorbed, so less light reaches you. Thus, the edge…