In earlier lessons we had asserted that one facet which makes scientific discover possible is the possession of an intuitive feeling for nature, particularly for its quantitative aspects. The Sun, being the closest star to us, has fashioned and refined that intuitive feeling that astronomers possess for the nature of stars when they approach their study. Because other stars are immensely far away, astronomers, extrapolating and feeling their way forward by analogy with the Sun, have pursued a study of stellar surface features even when they could not directly observe their surfaces. Hence, the Sun has been a vital bridge to the world of stars and the variety of phenomena that must be occurring in their outer layers. Even today in the largest telescopes, stars are still basically just points of light. However, over the last twenty years innovative technology has made it possible to study the outer layers of other stars at a level of detail that is still crude compared to what we can achieve for the Sun, but is, nevertheless, a dramatic improvement over earlier achievements. Let us begin this chapter by surveying the Sun's surface layers and then extending that discussion to what we have learned about other stars. The table below contains some of the important attributes of the Sun as a star to keep in mind throughout our discussion.
General Properties of the Sun Limb Darkening
The solar photosphere is a transition layer from the invisible interior to the external environment surrounding the Sun. Through this 500-km thick layer temperature declines outward by several thousand Kelvins. The average temperature, or surface temperature as it is known, is about 6000 K as derived from the observed luminosity by using the blackbody radiation laws. The reason that the solar photosphere approximates a blackbody is because it quickly becomes opaque as we probe deeper into it. Photospheres of other stars also approximate blackbodies to a greater or lesser extent than the Sun. However, when we look closely at the solar photosphere, we find a far more detailed structure than one might presume from something as bland as the blackbody radiation laws.
The Photospheric Spectrum
The photospheric spectrum is an absorption spectrum containing some 25,000 absorption lines.
Photospheric Granules
Even in photographs of the solar photosphere taken in white light a number of features are evident. We have already noted that the photosphere darkens toward the limb; near the limb we can also see bright patches called faculae. And in high-resolution white-light photographs, one can see that the entire disk is covered at all times by small, bright features separated by dark lanes called granules.
Remarkably clear pictures of the solar surface in narrow wavelength ranges have been made by a telescope mounted in the Skylab station. Fine details on the surface are not blurred by the Earth's atmosphere at the altitude that the station was orbiting. Such high-resolution photographic studies reveal a potpourri of bright granules with dark intergranular lanes; these give the surface a honeycombed appearance. Time sequences of photographs show granules forming, disappearing, and re-forming in cycles lasting several minutes. At any given time the whole photosphere is broken up into better than 4 million granules, each occupying roughly 1 million km2 of the surface. Obviously the photosphere is not a uniform layer of gas; the temperature of the photosphere must vary not only in depth but also laterally across the face of the Sun.
The granules are cells of gas with characteristic diameters of 1000 km and lifetimes of several minutes. From the bright center of the granule to the darker intergranular region, the brightness variation corresponds to a temperature difference of about 200 K. Photospheric granules are a form of convection resulting from the upwelling of unstable convective elements from the hydrogen convection zone below the photosphere. As evidence of this convective exchange, spectral lines from the bright centers are Doppler-shifted toward the blue (coming toward us) and those from the dark intergranular regions are shifted toward the red (moving away from us). These Doppler shifts indicate that the bright centers are hot rising gas moving at a few tenths of a kilometer per second that radiates its excess energy and then forms the cool sinking gas in the dark intergranular lanes.
Oscillatory Motions
In 1960 vertical oscillatory motions were detected in and above the solar granulation which possess a period of almost exactly 5 minutes with velocities of about 0.5 km/s. Thus the layers above the hydrogen convection zone are moving up and down with respect to the mean position of the photosphere and low chromosphere. The typical excursion is on the order of 50 to 100 km. The motion seems to be organized over a few thousand kilometers and has been reported to cover areas as large as 50,000 km, with roughly two-thirds of the solar surface experiencing oscillations at any given moment. In 1984, the Sun's closest stellar neighbor, Alpha Centauri, was also shown to be also undergoing 5-minute oscillations. It now appears that the 5-minute oscillation is but one extreme in a range of oscillations, with a 160-minute oscillation as the other extreme. Thus the Sun quivers much like a bowl of gelatin; the consequences of these oscillations will be taken up later.