The Sun is a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its centre. Its interior regions consist of the core, where nuclear reactions generate energy, the radiation zone,, where the energy travels outward in the form of electromagnetic radiation, and the convection zone, where the Sun's matter is in constant convective motion.
From Earth, we see a sharp solar disk. It is actually the photosphere – the thin region at the Sun's surface from which essentially all visible light is emitted. Above the photosphere lies the chromosphere, which is separated from the solar corona by a thin transition zone in which temperature increases from a few thousand to around a few million kelvins.
The Sun's luminosity is dependent on the total amount of energy radiated from the solar surface per second. It is determined by measuring the amount of solar radiation reach each square metre at Earth's distance from the Sun and multiplying that amount by the area of an imaginary sphere of radius 1 A.U.
The effect of the solar convection zone can be seen on the surface in the form of granulation of the photosphere. As hotter (and therefore brighter) gas rises and cooler (dimmer) gas sinks, a characteristic "mottled" appearance results. Lower levels in the convection zone also leave their mark in the form of larger transient patterns called supergranulation.
Most of the absorption lines seen in the solar spectrum are produced in the upper photosphere and the chromosphere. Studies of these allow the scientists to determine the Sun's composition and the temperature structure of the solar atmosphere.
At about 10-15 solar radii, the gas in the corona is hot enough to escape the Sun's gravity, and the corona begins to flow outward as the solar wind. Most of the solar wind flows from low-density regions of the corona called coronal holes.
Sunspots are Earth-sized regions on the solar surface that are a little cooler than the surrounding photosphere. That is why they appear darker than the photosphere. But they are still very hot, although when compared to the normal photosphere, they are cooler. They are also regions of intense magnetism. They appear to move across the solar disk as the Sun rotates and usually survives for 1-2 months.
Both the numbers and locations of sunspots vary in an 11-year sunspot cycle. At solar minimum, only a few spots are typically seen, and they lie far from the solar equator. At solar maximum, the number of spots is much greater, and they generally lie much closer to the equator. The sunspot cycle is quite irregular. Its length varies from 7 to 15 years. There have been times in the past when no sunspots were seen for long periods. The overall direction of the solar magnetic field reverses from one sunspot cycle to the next. The 22-year cycle that results when the direction of the field is taken into account is called the solar cycle.
Solar activity tends to be concentrated in active regions associated with sunspot groups. Prominences are loop or sheetlike structures produced when hot gas ejected by activity on the solar surface interact with the Sun's magnetic field. The more intense flares are violent surface explosions that blast particles and radiation into space.
The Sun generates energy by "burning" hydrogen into helium in its core by the process of nuclear fusion. When four protons are converted into a helium nucleus in the proton-proton chain, some mass is lost. The law of conservation of mass and energy requires that this mass appear as energy, eventually resulting in the light we see. Some particles produced during the solar fusion process are the positron, or antielectron, which quickly annihilates with electrons in the Sun's core to generate gamma rays, the deuteron, an isotope of hydrogen consisting of a proton and a neutron, and the neutrino, a near-massless particle that escapes from the Sun without any further interactions once it is created in the core.
The Sun is our source of life. Without it, our Earth would be a barren, lifeless planet. By studying the Sun more closely, we can obtain a better understanding of the universe as a whole.