<P> The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface . In this layer, the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation . Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface . Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise . As a result, an orderly motion of the mass develops into thermal cells that carry the majority of the heat outward to the Sun's photosphere above . Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues . At the photosphere, the temperature has dropped to 5,700 K and the density to only 0.2 g / m (about 1 / 6,000 the density of air at sea level). </P> <P> The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales . Turbulent convection in this outer part of the solar interior sustains "small - scale" dynamo action over the near - surface volume of the Sun . The Sun's thermal columns are Bénard cells and take the shape of hexagonal prisms . </P> <P> The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light . Above the photosphere visible sunlight is free to propagate into space, and almost all of its energy escapes the Sun entirely . The change in opacity is due to the decreasing amount of H ions, which absorb visible light easily . Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions . The photosphere is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth . Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening . The spectrum of sunlight has approximately the spectrum of a black - body radiating at about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere . The photosphere has a particle density of ~ 10 m (about 0.37% of the particle number per volume of Earth's atmosphere at sea level). The photosphere is not fully ionized--the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form . </P> <P> During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth . In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbed helium, after the Greek Sun god Helios . Twenty - five years later, helium was isolated on Earth . </P>

What is the name of the surface of the sun