Basic Data

• Equatorial Radius: 695,500 km. (109 Earth)
• Mass: 1.99 x 10³⁰ kg. (332,946 Earth)
• Surface Gravity: 274.0 m/s² (27.94 Earth)
• Surface Temperature: 5500° C

Wikipedia - The Sun

About the Sun

No tour of the solar system would be complete without its main member. Making up 98.6% of its total mass, the Sun provides heat and light to all of the planets and other objects orbiting it. Yet the Sun is merely an average star in our galaxy of over two hundred billion stars. Only its nearness to us makes it important.

The Sun is composed mostly of hydrogen and helium, and can be divided into distinct regions. The innermost region, the core, extends outward from the center to about 20% of the Sun's total radius. The density of core material is around 150 times that of water, under tremendous pressure and at a temperature of over 13.6 million degrees C. It is in the core that nuclear fusion converts hydrogen into helium, thus providing the energy that the Sun ultimately radiates.

Outward from the core to a point around 70% of the total solar radius is the radiative zone, where energy is transferred by radiation. From the edge of the radiative zone to the solar surface lies the convective zone, where huge cells of gas rise from the depths carrying energy outward, and similar cells sink back, depleted of their heat. These convective cells account for the granular appearance of the solar surface.

Above the "surface" is the solar atmosphere. Tenuous outer layers—a high vacuum by Earth standards—actually extend out to the outer solar system. The so-called "solar wind" is made up of material ejected from the Sun and "blown" outward.

The Sun has a strong magnetic field. Because the Sun is not a solid body, and because different parts rotate at different rates, the magnetic field becomes very twisted. Often, huge loops of magnetic flux "pop" out into space, leaving localized spots of high field strength in pairs on the surface. These spots are cooler than the rest of the surface and appear darker, and are called "sun spots". Sections of the magnetic field can also recombine, releasing tremendous amounts of energy into the solar corona (a part of the Sun's outer atmosphere). This results in a solar flare, which can release large amounts of radiation. When this radiation hits the Earth, it can result in increased aurora activity and even damage to electrical power grids. It can also be a hazard to astronauts in orbit.

As a source of energy for eternity, the Sun is not actually reliable. As it fuses hydrogen into helium in its core, the hydrogen is used up. Eventually, there will come a time when the amount of hydrogen available for fusion runs out. At that point, the core will begin to collapse, which will actually release even more energy, causing the rest of the sun to expand tremendously into a red giant. Ultimately, the red giant will disperse due to thermal pulsations in the remnant core, until nothing but the core is left. This core, now called a white dwarf star, will continue to gradually cool until someday it gradually fades into a cinder. However, this will take many billions of years to happen.

Observation from Earth

In a way, the title of this section almost sounds ridiculous. The Sun is the biggest single fact up in the sky, provides all the heat and light that make our world habitable, defines day and night, winter and summer, etc. But it was only in recent centuries that man began to seriously study our local star scientifically.

Many ancient cultures worshipped the Sun. Many others associated a god with it. In a way, this is understandable, since if the Sun were to vanish, life would quickly follow. Nevertheless, it was the ancient Hebrews who got it right when they chose to worship the Creator, rather than the creation. For all its immensity and power, the Sun is not God.

The Sun has been studied extensively from Earth throughout the history of modern astronomy. Unlike standard telescopes, where instruments (including human eyeballs) peer directly through the optics, solar telescopes often project an image of the Sun for study. And when instruments are directly attached, heavy filters are used to cut the light levels down to avoid damage. Never look directly at the Sun. Eye damage can result.

Exploration of the Sun

Since the dawn of the space age, many satellites have been launched to observe the Sun. Most of these have operated from Earth orbit, although a number have been sent into various orbits around the Sun itself. Because of the extreme heat, no probe can actually enter or attempt to "land" on the Sun directly. One of the most interesting solar probes was Ulysses, which was first sent out to Jupiter in order to be slingshot into a polar orbit around the Sun.