fotosfeer

Fotosfeer, or photosphere, is the visible surface of a star. It is the deepest layer from which photons escape and is the part of the star that shapes its visible appearance. For the Sun, the fotosfeer is the lowest layer of the solar atmosphere and is the part we see when we look at the Sun.

In the Sun, the fotosfeer has a temperature of about 5700 kelvin. Its spectrum shows many absorption lines created by atoms in the gas, superimposed on a spectrum that is close to that of a blackbody. The layer is relatively thin—roughly a hundred kilometers thick—while photons escape to space from depths where the optical depth is ~1.

Convection in the photosphere creates granulation: bright, cellular patterns about 1,000 kilometers across, lasting several minutes. Sunspots are cooler, darker regions caused by strong magnetic fields that suppress convection, and they appear on the fotosfeer as blemishes on the solar disk.

In stars, the photosphere is the visible surface and its temperature and spectral lines define the star's spectral type. The chemical composition of the fotosfeer includes hydrogen and helium with heavier elements present in smaller amounts, influencing line strengths.

The photosphere is distinct from higher layers of a star's atmosphere—the chromosphere and corona—above it. Observationally, the fotosfeer is the source of most visible light from a star and determines its apparent color and brightness; limb darkening occurs because the outer parts of the disk come from cooler, higher layers.