What are the sun's photosphere, chromosphere and corona?
The photosphere, chromosphere and corona are three layers of the Sun's atmosphere.
The photosphere is the Sun's outer shell from which light is radiated. It has a surface temperature of about 6,000K.
The chromosphere is above the photosphere. I is a low density layer which is about 2,000km thick. It is called chromosphere because it has a red to pink colour. It is normally only visible during a solar eclipse.
The corona is plasma surrounding the Sun which extends hundreds of kilometers from the surface of the Sun. It is much hotter than the surface of the Sun with temperatures reaching 3,000,000K. It is clearly visible during a solar eclipse or using a coronagraph. The shape of the corona varies with solar cycles.
This is a photograph of the corona taken during the 22 July 2009 solar eclipse.