Ⅳ. The Sun

The Sun

The sun is the fixed star that is closest in distance from the earth, and is the most important celestial body available for the study of stars. The appearance of the sun differs depending on the wavelength used in the observation. By observing the visible rays, we can clearly identify the photosphere emitting bright light and observe the sunspots on the surface. When we observe using X-rays, the photosphere appears relatively dark while the corona, composed of extremely hot gas, or the areas near the sunspots with high activity appear very bright. The corona cannot be seen by the visible rays under ordinary circumstances due to the bright light emitted from the photosphere, but during a total solar eclipse, the corona can be seen shining brightly.

The mass of the sun is approximately 2x1033 g, amounting to around 300,000 times the mass of the earth. The sun’s radius is 700,000 km, around 109 times the radius of the earth, and its average density is 1.4g/cm3. The surface temperature is approximately 5800K, the luminosity is around 4x1033 erg/sec, and the absolute magnitude is 4.8. Around 75% of the sun’s mass consists of hydrogen, around 24% consists of helium, and around 1% consists of elements that are heavier than helium. The central temperature of the sun is estimated to be approximately 150 million degrees. Although we are unable to directly gauge the age of the sun, we have estimated it to be around 4.5 billion years old based an analysis of a meteorite that is considered to be the oldest object in the solar system.

The Structure of the Sun

The interior of the sun is composed of the core that generates the energy, the radiation layer that conveys this energy outwards, and the convection layer. The sun’s surface consists of the atmospheric layer composed of the photosphere, the chromosphere, and the corona. The visible rays that our eyes can observe are emitted from the photosphere with structure including sunspots and granulations. The granulations are created by convective phenomena on the inside of the photosphere, while the sunspots appear dark because of their low temperature with the existence of a strong magnetic field. There are variations in the time period when the sunspot population reaches its peak on a cycle of 11 years. The chromatic layer is the region that appears pink during a total solar eclipse, and this is where we observe phenomena such as spicules, prominence and flares. The border region between the chromatic layer and the corona is referred to as the transition region, where a mere increase of around 300 km is accompanied by a temperature increase of 1 million degrees and where can be found the cause of various phenomena that occur on the sun or of those phenomena that we refer to as solar activities. The corona remains in high temperature of around 1 million degrees, and appears silvery white during a total solar eclipse. This is the region where the solar wind emitted from the coronal hole is dispersed. It has not yet been accurately explained how the corona is able to maintain such a high temperature.

The Energy Source of the Sun

The sun emits an enormous quantity of energy amounting to approximately 4x1033 erg per second. ?This energy is generated by the fusion of hydrogen near the sun’s core. The fusion of 1 g of hydrogen transforming into helium results in converting approximately 0.007g of the mass in to energy. This is the means by which the sun is able to stably emit energy for around 10 billion years. This generated energy passes through the radiation layer in the core region and the convection layer in the outer region during its movement to the surface and is then emitted into outer space. The earth maintains its current temperature by receiving the light emitted by the sun.

Solar Wind

Solar wind refers to the rarefied gas that is emitted from the sun’s surface and flies out into the empty space between planets. The emission of solar wind is particularly increased during the time period when the population of sunspots is on the rise, and in some cases the solar wind affects the magnetosphere surrounding’ the earth’s atmosphere, resulting in disruptions of communications here on earth.


  1. Given that the difference the values of the radial velocity observed respectively at the eastern and western edges of the sun is 3.8 km, determine the rotation period of the sun.
  2. The value of the solar constant observed from the earth is 1.4×106km/cm2sec. What would be the value of the solar constant observed from Saturn?
  3. Name the activities that we can observe occurring in the photosphere, the chromatic layer and the corona that constitute the atmospheric layer of the sun.
  4. Why does do the sunspots on the sun appear dark? Also, what can be learned based on the movements of the sunspots?
  5. What is the reason that the size of the sun appears to vary during the course of the year?
  6. The surface temperature of the sun is approximately 6000K. Use Wien’s law to determine the wavelength at which the maximum energy is emitted. What is the color of this radiant ray?
  7. What is the source of the sun’s energy? If we assume that the sun is made of coal and the radiation energy of the sun is generated by the combustion of coal, what would be the life expectancy of the sun? Explain in terms of comparison to the earth’s age, which is 4.6 billion years.