The Sun is the star at the center of our solar system, and is by far the largest member with a thousand times more mass than all the planets, moons, asteroids and comets put together. The Sun itself is a very typical G-type, main sequence, Population I star. The average distance between the Sun and the Earth is 1 AU or 149,598,000 km. The diameter of the Sun is about 1,392,000 km, or about 109 times the diameter of the Earth. The temperature at the surface of the Sun is about 5800 K, or 5,527 °C and the temperature at the center is about 1.55 x 107 K. By mass, about 74% of the Sun is hydrogen, 25% is helium and 1% is other elements. It is orbiting the center of the Milky Way Galaxy every 220 million years at a speed of 220 km/s. Light from the Sun takes 8.32 minutes to reach the Earth.
From the center of the Sun to about 0.25 of the solar radius is the core. The core of the Sun is extrememly hot and dense and is fusing hydrogen to helium at a rate of 600 million metric tons per second. The density at the center of the Sun is 160,000 kg/m3 and by 0.3 of the solar radius, the pressure has dropped to 13,000 kg/m3. For comparison, the density of air on Earth at sea-level is approximately 1.2 kg/m3. 94% of the mass of the Sun is located within the inner 0.5 of a solar radius. Outside of the core, the density and temperature are too low for thermonuclear reactions to take place. Out to about 0.7 of the solar radius, energy flows by radiative diffusion. The radiative zone is transparent enough that photons can travel moderate distances before being absorbed or scattered. Beyond 0.7 of a solar radius, the temperature is low enough that hydrogen nucleii can join with electrons to form atoms, and these atoms are very effective at absorbing photons. From here to nearly the surface of the sun is the convective zone where hot atoms and molecules rise to the surface where they cool and sink, causing convection currents.
The Sun has an atmosphere composed of three main layers. The lowest one is the photosphere and essentially all of the visible light we see from the Sun comes from this thin layer of gas. The Sun's density at the photosphere is very low and we can see about 400 km into the photosphere. When atronomers observe the photosphere with telescopes they see a blotchy pattern of light and dark regions. This effect is called granulation. Each granule is about 1000km across and is caused by the convection of gas. Hot gas rises in the middle of a granule, and this area appears brighter, then the gas cools and sinks on the edges of the granule and these areas are darker. Granules only last several minutes. On a larger scale are supergranules which are superimposed on the granules but are about 35,000 km across and last about 24 hours. The surface of the photosphere is about 5800 K and has a density of about 10-4 kg/m3. This is about 0.01% the density of air at sea level.
A view of an irregular-shaped sunspot and granules on the Sun's surface, seen on August 22, 2003. (Swedish 1-m Solar Telescope (SST) operated by the Royal Swedish Academy of Sciences, Oddbjorn Engvold, Jun Elin Wiik, Luc Rouppe van der Voort, Oslo)
The next layer out is the chromosphere, which is one ten-thousandth as dense as the photosphere. The chromosphere is about 1600 km thick and is only visible when the Sun is eclipsed by the moon or when astronomers use instruments that block out the disk of the Sun. The temperature of the chromosphere increases with altitude, so the temperature at the top of the photosphere is approximately 4400 K, at the top of the chromoshpere it is nearly 25,000 K! This seems counter-intuative, but can be explained by the magnetic fields generated by the Sun, which interact with the charged particles in the chromosphere and cause them to accelerate, which makes them hotter. Astronomers do not yet fully understand the mechanisms that cause this.
The Sun has a very complex magnetic field, which astronomers are actively studying. The magnetic field causes many of the features on the sun including sunspots, prominences, flares and coronal mass ejections.
Sunspots are irregularly shaped dark regions in the photosphere of the Sun such as the one in the image above. They have an inner dark area called the umbra, and a brighter edge region called the penumbra. Although these terms are the same as the ones used to describe eclipses, sunspots are not shadows. The are regions of cooler temperature on the photosphere. The umbra of a sunspot typically has a temperature of around 4300 K while the penumbra is typically 5000 K. This is quite a bit cooler than the photosphere's average temperature of 5800 K.
Sunspot groups usually last about two months, and Galileo was the first to use a telescope and filter to observe the movement of sunspots to determine that the Sun rotates about once every four weeks. The number and position of sunspots varies with time over an 11 year cycle. At the beginning of each cycle, sunspots typically form near latitudes of 30 north or south. As the cycle continues, the spots usually form closer to the equator.
At the beginning of each cycle there are very few sunspots, called a sunspot minimum. These occurred in 1976, 1986, 1996 and 2008. A sunspot maximum occurred in 1979, 1989 and 2000. Sunspots are caused by concentrations in the Sun's magnetic field, which deflect the charged gas of the photosphere, creating areas that are cooler than the rest of the surface. In recorded history there have been periods of relatively low sunspot activity for longer than the typical cycle. from about 1645 to 1715 virtually no sunspots were seen, and Europe experienced a very cold period referred to as the Little Ice Age. In the eleventh and twelfth centuries there was a period of increased sunspot activity and the climate on Earth was temporarily warmer.
Prominences are giant arching columns of gas in the corona that often form just before a sunspot appears below them in the photosphere. They are a result of the interactions of the gas in the corona with the magnetic fields of the Sun. Particularly energetic promineces may break free from the magnetic fields of the Sun and burst into space.
Flares are violent eruptions on the Sun that occur in complex sunspot groups. Within a few minutes, the temperature in the region may rise as much as 5,000,000 K and large quantities of particles and radiation are blasted into space. The energy of a solar flare can be as much as the energy that would be released by 100 trillion nuclear bombs!
Coronal mass ejections are larger scale versions of solar flares caused by major disturbances in the Sun's magnetic field. In a coronal mass ejection, over a billion tons of high temperature gas is ejected from the corona at very high speed. They occur every few months and if an ejection is pointed towards the Earth, it can disrupt satellites, interfere with communication equipment on Earth, and pose health hazards to astronauts in orbit.
Image credit: SOHO Consortium, ESA, NASA