Did you know...
...that lightning is about 3 times hotter than the surface of the Sun? The surface temperature of the Sun is around 5,810° K, while lightning is approximately 16,900° K.
...that if you were to drive a car at 100 km/hr, 24 hours a day, you would reach the Sun in about three years?
...that it would take 91,365 years to reach the nearest star (Proxima Centauri, 4.23 lt-yr) at 50,000 km/hr?
...that if the Sun blows up right now, we'd have eight minutes before our lives are snuffed out?
...that the 11-year sunspot cycle is actually part of a larger 22-year cycle in which the entire magnetic field of the Sun may reverse itself?
...that the Sun is moving at 19 km/hr toward the constellation Hercules?
...that if the Sun were the size of the dot over the letter "i", the nearest star would be a dot 16 km away?
...that the fusion of one pound of hydrogen inside the Sun releases as much energy as burning 10,000 tons of coal?
...that the Sun's mass decreasses by 4 million tons every second due to the conversion of hydrogen to helium by thermonuclear fusion?
...that only one two-billionth of the Sun's energy actually reaches us?
The Sun is the center of the solar system. This basic truth seems obvious to us now, but that has not always been the case. The Greeks had a theory for what the universe must look like, called the "Ptolemic system." However, in this model everything was centered around the Earth, not the Sun. This theory went unchallenged for more than one thousand years! In 1543, Polish astronomer Nicolaus Copernicus published his own theory which said that the Sun was the center of the universe and that everything circled it. There were other theories for how the universe worked, including a Tychonic system which said that the other planets revolved around the Earth, but the Earth revolved around the Sun. However, Copernicus was closest to being right. The actual orbits are ellipses (ovals), as Yohannis Kepler proved, but Copernicus's view was a major step forward from old ways of thinking.
The Sun is an ordinary G2 star. Even so, it seems gigantic by our standards. The mass of all the planets, moons, asteroids, and comets combined is insignificant by comparison. The sun makes up approximately 98.77% of the solar system's total mass. It is also very active; violent events like solar flares are common. Solar flares are huge releases of energy in active areas of the Sun. Atomic particles are ejected and all sorts of radiation shoots out. Magnetic field fluctuations have a big impact on these solar flares.
The Sun's extreme heat (of up to 15 million degrees Celsius!) can be traced back to its core. Nuclear reactions are constantly occurring there, releasing huge quantities of energy which eventually travel to the surface. Right now, hydrogen atoms in the Sun's core are fusing together to create helium. Currently three quarters of the Sun is hydrogen and one quarter is helium, but this ratio changes slowly time passes.
The Sun's visible surface is not solid like Earth's, but is rather a shallow shell of gas about 400 km thick. This surface, or "photosphere," is the part of the Sun which emits light. It's not as hot as the inner parts, only around 6000 degrees Celsius. Cooler regions on the photosphere appear darker than their surroundings and are called sunspots.
Outside the Sun's photosphere is a region called the chromosphere, which is relatively cool. The chomosphere takes an unusual shape which, under normal conditions, can't be seen. However, during a solar eclipse, the photosphere's light is blocked and the chromosphere is visible for a brief moment. At the chromosphere's edge lies the beginning of a region known as the corona. The corona is even thinner than the chromosphere and is much larger. It extends outward several times the Sun's radius in length. The corona is very hot, about 1 to 5 million degrees, but it is not very dense so the actual amount of heat stored in it is not as great as the photosphere. Stretching out much further is the Sun's magnetic field, or magnetosphere. This field exerts its forces out far past the orbit of Pluto.
Since the Sun isn't solid, it exhibits differential rotation. That is, the different parts rotate at different angular speeds. The equator rotates the fastest, completing a rotation in 26 days. The areas surrounding the poles, on the other hand, can take as long as 36 days.
The Sun won't be around forever. Once the amount of hydrogen in its core drops below a certain level, the Sun will expand into a red giant. Unfortunately, Earth will be destroyed by this process. Please don't be alarmed. We still have roughly five billion years before we'll have to worry about it.
The Sun - Diagram
The Sun in X-Rays - Info about X-Ray emissions
The Sun: Course Notes - Lots of info
* Photo credit - NASA