The Sun is a star. Among stars, it is very average in size, energy output, age, and composition. But as a member of the solar system, the Sun dominates everything around it. Light and heat from the Sun supports almost all life on Earth, and drives the Earth's weather and climate.
The Sun is by far the largest object in the solar system, with an equatorial diameter of 864,000 miles (1,391,000 km). It is 109 times larger than Earth, and almost ten times the diameter of Jupiter. The Sun contains 99.86% of all matter in the Solar System, and is nearly 333,000 times more massive than the Earth.
Observing the Sun
The Sun displays observable features that can be seen in small telescopes. However, one should NEVER look at the Sun through a telescope without using proper solar observing equipment. The result will most likely be permanent blindness! The Sun is best observed by projecting its image from the eyepiece onto a flat screen, or by placing a mylar filter specifically designed for solar observation over the front of the telescope tube.
Either of these methods will show sunspots. Sunspots are dark regions on the Sun's surface that rotate with the Sun. They can be used to track the Sun's rotation period: 25.38 days at the equator, and slower near the poles (the Sun does not rotate as a rigid sphere). Sunspots are dark because they are cooler than the surrounding areas.
At the edge of the Sun's disk, we can often see plumes of gas streaming out from the surface. These are called prominences, and they are sometimes visible to the naked eye during a solar eclipse.
Composition and Structure
The Sun is an enormous ball of gas, composed mostly of hydrogen (92.1%) and helium (7.8%). Its temperature and density vary greatly with depth from the surface. Although the Sun's average density is less than the Earth's, the Sun's core is 25 times denser than the Earth, and 150 times denser than water. The Sun's surface temperature is about 10,000°F (5500°C); but at its core, the temperature reaches 27 million degrees F (15 million degrees C).
The Sun's mass is held together by gravity, which produces the immense pressure and temperature at its core. There, these are great enough to sustain a thermonuclear fusion reaction which converts hydrogen atoms into helium. The energy produced by this reaction powers the Sun, and generates essentially all of the heat and light that we receive on Earth.
Energy generated in the Sun's core will be absorbed and re-emitted countless times by solar atoms before it reaches the surface. A single photon of light energy takes anywhere from 10,000 to 170,000 years to escape the Sun. After leaving the core, it first will pass through the radiative zone. Here, solar material is dense enough to transfer the intense heat of the core outward by thermal radiation.
When temperature drops below 2 million degrees Celsius (3.5 million degrees Fahrenheit), solar material is not dense enough to transfer energy via radiation, so convection or "boiling" occurs. In the convective zone, columns of hot material rise to the surface of the Sun; once the material cools off at the surface, it plunges back downward to the base of the convective zone, to receive more heat from the top of the radiative zone.
The Sun's visible surface, the photosphere, is a 300-mile (500-kilometer) thick region, from which most of the Sun's radiation escapes outward. The sunlight we observe on Earth reaches us about eight minutes after it leaves the photosphere. Above the photosphere is the 2000-3000 km thick chromosphere, so named because of its reddish color (chromos means "color"). The chromosphere can only be seen easily during a total solar eclipse, and its red color comes from "H-alpha" light emission by hydrogen atoms.
Above the photosphere and chromosphere lies the tenuous corona. Visible light from the corona is usually too faint to be seen against the brighter photosphere, but during total solar eclipses - when the Moon covers the photosphere - the corona forms a beautiful white halo. In the corona, the temperature increases with altitude, reaching as high as 3.5 million degrees F (2 million degrees). The source of coronal heating has been a scientific mystery for more than 50 years.
Origin and Evolution
The Sun, and the planets which orbit around it, are believed to have formed about 4.6 billion years ago, from the collapse of a giant molecular cloud inside a nebula like M 42 in Orion or M 16 in Sagittarius. When temperature and pressure at the center of the collapsing cloud were sufficient to sustain nuclear fusion, the Sun was born - and the remaining hydrogen, helium, and other light elements in the nebula were blown into interstellar space.
Heavier substances, which were only trace elements in the nebula - like oxygen, carbon, silicon, and iron - formed particles that accreted with each other over time, and eventually became the planets. Farther from the Sun, the planets were able to retain some of the hydrogen and helium from the proto-solar nebula, and became the "gas giants" Jupiter, Saturn, Uranus, and Neptune. Closer to the sun, higher temperatures made this impossible; this resulted in the small, rocky, "terrestrial" planets Mercury, Venus, Earth, and Mars.
Today, the Sun is a type G2 V main-sequence star; this means that the fusion reaction which is converting hydrogen into helium at its core is stable. The Sun contains enough hydrogen fuel to continue life as a main-sequence star for another five or six billion years.
When the hydrogen in the Sun's core is consumed, the core will contract and heat up. When the core temperature reaches about 100 million degrees C, helium fusion will begin; this will produce carbon. The Sun's outer layers will expand and cool, and the Sun will become a red giant star.
As a red giant, the Sun will have a maximum radius 250 times its current value. This is larger than the Earth's present orbit. Even if the Earth escapes being swallowed into Sun, our oceans will be boiled away, and our atmosphere will escape into space.
Following the red giant phase, intense thermal pulsations will cause the Sun to eject its outer layers, forming a planetary nebula. After the outer layers are blown off into space, what will remain is the intensely hot core, which will slowly cool and fade as a white dwarf star over many billions of years. This evolutionary path is typical of medium-mass stars like our Sun.