Sun

The Sun is one of more than 100 billion stars in our galaxy.

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the solar system (Jupiter contains most of the rest).

At present, the Sun is about 75% hydrogen and 25% helium by mass. Everything else ("metals") amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The outer layers of the Sun rotate at different rates.  At the equator, the surface rotates once every 25.4 days.  Near the poles the surface rotates once every 36 days.

Conditions at the Sun's core are extreme. The temperature is 28,000,000° F and the core's gases are compressed to a density 150 times that of water.

The Sun outputs 386 billion billion megawatts of energy produced by nuclear fusion reactions.

The surface of the Sun, called the photosphere, is at a temperature of about 9,980° F. Sunspots are "cool" regions (only 6380° F) caused by complicated interactions with the Sun's magnetic field and can be very large, as much as 31,000 miles in diameter.

A small region known as the chromosphere lies above the photosphere.

The region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during eclipses.    Temperatures in the corona are more than 1,800,000° F.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind, which travels throughout the solar system at about 280 km/sec.

The solar wind and the much higher energy particles ejected by solar flares can dramatically affect the Earth with changes ranging from power line surges to radio interference to the beautiful aurora borealis.

The solar wind affects the tails of comets and the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's edge.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so, but eventually it will run out of hydrogen fuel. It will then expand into a red giant, resulting in the total destruction of the Earth.