The sun is a star - an average-sized star that is about 1.39 million kilometers across. All of the stars you see in the night sky are actually suns and lots of them have planets orbiting them. Don't be fooled, though, our sun is still BIG. It makes up about 99.8% of the mass of our solar system and 1,300,000 planets the size of Earth would fit inside it! The reason our sun is so much brighter than any of the other stars is because it is so close - only 150 million kilometers away. The next nearest star is several trillion kilometers away. Every square centimeter of the sun's surface gives off as much light as a 6000 watt light bulb! Traveling at 300,000 km/second it only takes light about 8 minutes to reach Earth. The sun is made entirely of gas - mostly hydrogen and helium (together making up 98% of the sun's mass) but it contains over 60 elements all together. The sun rotates once every 27.5 days or so, but what is strange is that it's equator spins faster than the upper and lower regions!

The sun doesn't appear to have clear boundaries, but it is generally accepted that there is an inner layer, the core, surrounded by a radiative and convection zone, and then three atmospheric layers called the photosphere, chromosphere, and corona. Here are short descriptions of each area and the solar features that might be found in them.

Core: The core is HOT! Temperatures near the center of the core must be 15 million degrees Celsius. With these temperatures and pressures of a billion pounds per square inch nuclear fusion can take place. In this fusion reaction, four atoms of hydrogen fuse to form one atom of helium and energy. This energy leaves the inner core and travels through the Radiative Zone which begins about 175,000 Km from the center of the sun.

Radiative & Convection Zone: The radiative zone is where the intense heat produced in the inner core is transferred away from it's source. It extends for about 300,000 Km before giving way to the convection zone. In this zone convection currents begin to develop creating convection cells where hot gas rises. Cooler gas falls and begins to heat up. The result of these convection cells are what scientists call "granulation" which describes the contrast of the red and orange areas on the visible surface.

Photosphere: The photosphere is what we call the "bright, visible surface of the sun". It is actually one of three layers of the atmosphere. It is about 550 Km thick and is often referred to as the "surface of the sun". It is where sunspots occur. The temperature of this layer is approximately 6,000 degrees Celsius but cooler areas exist and, thus, do not glow as bright (and actually appear black). We call these cooler areas sunspots and they can be as small as 16 Km across or several times larger than Earth! These cooler areas are the result of magnetic storms and occur in cycles of 11 years. In other words, every 11 years the number of sunspots peak, with low sunspot numbers in between the peaks. The last peak occurred in 2001 so when would you expect the next peak to occur. How about the one after that?

Chromosphere: The reddish layer of the sun's atmosphere, the chromosphere, rests above the photosphere and extends for several thousand kilometers. Temperatures average 27,000 degrees Celsius. On occasion heated gas explodes into space forming twisted loops. These loops of heated gas are called Prominences. Sometimes they fall back into the sun, other times they break free and drift into space. Another solar storm is called a solar flare. These are bursts of light from super hot spots on the sun's surface. They look like bright yellow/white "spots" on the surface in the picture above.

Corona: The outer-most layer of the sun (and the least dense) is the corona. Temperatures in the corona reach 1,700,000 degrees Celsius. The cool thing about this layer is that it can only be observed during a total solar eclipse or with special equipment. When it is visible, it appears as a ghostly, glowing region that lacks regular shape. It is far less bright than the other regions of the sun so we cannot see it when viewing the sun with solar filters. Solar wind (a continuous stream of high-energy particles) leaves the sun from the corona and travels in all directions at incredible speeds (350 - 700 km/sec or 217 - 434 miles/sec).


An illustration of the structure of the Sun (3):

1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona

7. Sunspot
8. Granules
9. Prominence
10. Flares (not shown - appear as bright hot spots)

The sun produces an amazing amount of energy. Every second the energy equivalent to the explosion of 92 billion one-megaton nuclear bombs is released (1). Every square meter of the Earth's surface collects a constant 1,400 Watts of energy! Imagine if we could efficiently harness that. So how does the sun produce so much energy? The answer - Nuclear Fusion. Every second near the center of the sun about 700 million tons of hydrogen is turned into helium, resulting in 5 million tons of energy being transformed into energy. The equation for nuclear fusion is: 4H yields He + energy.

Questions to Consider:
1. Why is the sun round and the size that it is? The sun is made of gas. Gas has mass and gravity can act upon it AND because it is gas it has no rigid shape. So, the intense heat produced from the nuclear fusion reactions in the core cause the gas to expand away from the core. Gravity, however, tugs at the expanding gas and keeps it from escaping completely. The round shape and size of the sun are the result of these two opposing forces.

2. Of the information presented above, which things are observations and which are inferences? In other words, which things can be observed and tested directly and which things are our best guess based on the observations and tests we can make?

Cool Sun Websites:
Space Weather

1. Maran, Stephen P. Astronomy for Dummies. Wiley Publishing, Inc. 2005
2. Maton, Anthea et. al. Exploring Earth Science. Prentice Hall. 1997
3. Structure of the Sun image from:
4. Image of the sun from