Combustion, meteoritic impact, and gravitational contraction cannot account for the huge amount of energy that the Sun has produced during the 4.6 billion years that it has been luminous.
The Sun's energy is produced by nuclear fusion through a sequence of reactions known as the proton-proton chain. The Sun's core contains enough hydrogen for hydrogen fusion to power the Sun for a total of about 10 billion years.
In the deep interior of the Sun, energy is carried outward by the flow of radiation. In the outer 30% of the Sun's radius, most of the energy is transported by convection. About 170,000 years is required to transport energy from the center of the Sun to the surface.
Unlike photons, solar neutrinos pass directly through the Sun's gases. By detecting neutrinos, therefore, astronomers have been able to study the core of the Sun. Solar neutrinos are detected at only a fraction of the expected rate. The explanation probably requires that neutrinos have mass.
The visible layer of the Sun is the photosphere. The sharp edge and limb darkening of the photosphere tell us that density increases swiftly inward in the photosphere and that temperature also increases inward.
The photosphere is pebbled by granules, which are rising convective columns of gas. An even larger convective pattern, supergranulation, is also present in the photosphere.
The Sun's surface vibrates in a complex pattern of oscillations. By studying these oscillations, astronomers have been able to learn about the way that temperature, composition, and rotation rate vary with depth.
Sunspots are dark because they are relatively cool. The low temperatures of sunspots are attributable to large magnetic fields, which inhibit convection and reduce the amount of energy reaching the solar surface. By watching sunspots move across the Sun's disk, it was discovered that the Sun's rotation rate decreases as distance from the Sun's equator increases.
The chromosphere is a tenuous layer that lies above the photosphere. The chromosphere consists mainly of spicules, which are rapidly rising jets of gas.
The outermost layer of the Sun is the corona, within which the temperature of the gas reaches millions of degrees. The corona is thought to be heated by magnetic fields. Above active regions, the corona is confined by magnetic fields and is hot, dense, and bright. In coronal holes, magnetic fields are unable to trap coronal gas, which accelerates away from the Sun.
Prominences are clouds of relatively dense, cool gas that reach upward into the corona. Although prominences can last for months, they often erupt, sending a blast of gas outward through the corona. Flares are sudden releases of energy stored in magnetic fields. Flares result in very hot regions of gas that emit high-energy radiation. Large numbers of energetic ions and electrons are also produced in flares.
The solar wind is a tenuous, hot gas that flows from the Sun into interplanetary space. At about 100 AU from the Sun, the solar wind is thought to merge with the interplanetary gas.
Although there have been long periods of time when sunspots were rarely seen on the Sun, the number of spots usually rises and falls in an 11- year cycle. It has been proposed that the sunspot cycle and the magnetic properties of sunspots can be explained by a model in which magnetic field lines beneath the photosphere are stretched and twisted by the differential rotation of the Sun.
