A typical star, the Sun has a diameter of approximately 865,000 miles (nearly 10 times larger than the diameter of Jupiter) and is composed primarily of hydrogen. The Sun's core is an astonishing 27,000,000 degrees Farenheit, while the pressure is about 100 billion times the atmospheric pressure here on Earth. Under these conditions, hydrogen atoms (with one proton in the nucleus) come so close together that they fuse into helium atoms (with two protons in the nucleus).

In its beginning, the Sun was approximately 75% hydrogen, 24% helium, and 1% heavier atoms. Right now, about half the amount of hydrogen originally in the core of the Sun has been fused into helium. This took roughly 4.5 billion years to accomplish.

When the hydrogen is exhausted, the Sun's temperature at the surface will begin to cool and the outer layers will expand outward to near the orbit of Mars. The Sun at this point will be a "red giant" and 10,000 times brighter than its present luminosity. After the red giant phase, the Sun will shrink to a white dwarf star (about the size of the Earth) and slowly cool for several billion more years.

The Sun is the closest star to Earth and is the center of our solar system. The energy from the Sun provides light, heats our world, and makes life possible. The Sun is also an active star that displays sunspots, solar flares, erupting prominences, and coronal mass ejections. These phenomena impact our near-Earth space environment and determine our "space weather."

1. http://www.crh.noaa.gov/fsd/astro/sunspots.htm - National Weather Service Forecast Office
2. http://astrogeology.usgs.gov/assets/wallpaper/sun.jpg - U.S. Dept. Of The Interior