The Sun is a star, one of more than 100 billion stars in our galaxy.
The Sun is also the largest object in our solar system.
Diameter: 1,390,000 km
Mass: 1.989e30 kg
Temperature: 5,800 K at the surface, 15,600,000 K at the core
Distance from Earth: 149,600,000 km (92.9 million miles)
Composition: 75% hydrogen, 25% helium (slowly changes over time as hydrogen is converted to helium)
God of the Sun. Greeks Called him Helios and to the Romans he was Sol. Though they have different names they are virtually the same. Helios with his golden chariot rises at dawn from the ocean in the east, rides through the sky, and disappears at sunset into the ocean in the west.
The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. The photosphere has cooler regions on it known as sunspots, only 3800 K. Above the photosphere is a small region known as the chromosphere, and above that is the corona. The corona extends millions of kilometers into space but is visible only during eclipses. Temperatures in the corona are over 1,000,000 K.
Conditions at the Sun's core are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. The Sun's energy output is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light.
The outer layers of the Sun show differential rotation. At the equator the surface rotates once every 25 days, near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun has a gas body, not a solid body like the Earth. The differential rotation extends down into the interior of the Sun, but at the core the Sun rotates as a solid body.
The activity on the Sun produces particles that are thrown out into space. This stream of particles, called the solar wind, consists primarily of protons and electrons and spreads throughout the solar system at about 450 km/sec. The solar wind can have a wide range of effects on the Earth, such as power line surges, radio interference, and the beautiful aurora borealis. The solar wind also has large effects on the tails of comets and the trajectories of spacecraft.
The Sun's output is not entirely constant. Nor is the amount of sunspot activity. The Sun is about 4.5 billion years old and will continue on for about another 5 billion years. Since its birth it has used up about half of the hydrogen in its core, and eventually it will run out of hydrogen fuel. Since it's formation, the Sun's output has increased by about 40%.