The highly luminous surface of the Sun is called the PHOTOSPHERE. The photosphere is the sharp disc as observed with the eye or a small telescope. Larger telescopes used under excellent observing conditions show that the photosphere is not uniformly bright, but has a mottled texture termed GRANULATION. (Graduations in this structure exist, but the smallest granules consist of bright patches of light, about 1 000 km across, with a dark border. The pattern is changing continuously as granules dissolve away and arc replaced by new ones, so that the appearance changes completely in only a few minutes. Measurements of the Doppler shifts present in light from the photosphere have shown that the bright centre of a granule is moving upwards, whereas the boundary is cooler, descending gas. Almost certainly the solar granulation is associated with the highest convective tier in the Sun. Doppler-shift measuring techniques have also revealed large-scale motion of the photosphere. Within supergranular cells 30000km in diameter the gas moves predominantly horizontally from the centre to the edge of the cell. Disturbances deep in the convection zone may also be the cause of the rhythmic rising and falling of the photosphere on a cycle time of five minutes.

A remarkable property of the photosphere is that its edge, or UMB, appears sharp to the naked eye, rather than merging gradually into the blackness of space, which is how we might expect an incandescent ball of gas to appear. This indicates that the layer from which most of the light is coming is shallow in comparison to the solar radius. The reason for this is as follows: as photons move through the convection zone, the temperature, pressure and density fall steadily. At visible wavelengths the negative hydrogen ion, which is a hydrogen atom that has temporarily captured a second electron, is a major contributor to the emission and absorption of radiation. Above the convection zone the density of this ion decreases much more rapidly than the total density of the solar atmosphere because it is extremely sensitive to changes in the temperature. One consequence of this is that most of the radiation that we can see is emitted in a layer only 500km thick. Below this the Sun is opaque and above it is completely transparent.

The photospheric disc appears slightly less bright at the edge when observed visually or photographically this phenomenon is termed LIMB-DARKENING. It arises because a line of sight to the centre of the visible disc penetrates the solar atmo¬sphere vertically and enables us to view slightly deeper, and there¬fore hotter, more luminous layers, as illustrated in figure 8.7a. Towards the edge of the disc, the line of sight passes obliquely through a greater thickness of cooler and partially opaque atmo¬sphere. Consequently we see only to a slightly higher level; where the temperature is lower the material is less .luminous, and there-tore the limb appears darker. The magnitude of the effect is dependent on wavelength: it is most noticeable in blue light.

There are several ways of deriving a temperature for the Sun’s visible surface. The BLACK-BODY TEMPERATURE is 6000 K. This is obtained by matching the spectrum of the continuum radiation from the Sun to theoretical curves derived from radiation laws.

The EFFECTIVE TEMPERATURE is related to the Sun’s surface luminosity, which is (5.44 x 107 watts nr2. According to the Stefan-Boltzmann law an object radiating at 5800K would match this value. We see that the temperature obtained from the shape of the spectrum is greater than the temperature derived from the luminosity. This is because the radiation we receive is coming from a 500-km layer in the solar atmosphere, and the temperature varies in this layer. At the centre of the disc most of the energy is coming from a zone with a temperature of 6500 K, whereas at the limb lower values prevail. For this reason it is not possible to define a unique temperature for the Sun’s surface.

At the temperature of 6200K all substances are entirely gaseous, tungsten is the most refractory of all known elements, melting at 3643 K and boiling at 6200 K.

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