First Look Inside a Star – The Atmosphere
As astronomers, we soon learn that the blindingly brilliant visible face of the Sun is called the photosphere, and we could be forgiven for thinking that this is some kind of precisely defined surface, like the surface of a planet. However, all stars including the Sun are entirely gaseous, and the photosphere consists of what amounts to the inner part of a star’s surface layers – in effect, its atmosphere. It is a layer of gas of finite depth that is relatively dense, and so it is the principal source of the continuum part of a star’s spectrum. However, photons that are emitted within the photosphere and which escape from its topmost layers do so as a result of having traveled along a range of paths of both varying distance and optical depth. This has interesting consequences for the overall appearance of the Sun’s photosphere and maybe that of other stars, too.