The Sun--The Closest Star

Size
The Sun is by far the biggest thing in the solar system. From its angular size of about 0.5° and its distance of almost 150 million kilometers, its diameter is determined to be 1,392,000 kilometers. This is equal to 109 Earth diameters and almost 10 times the size of the largest planet, Jupiter. All of the planets orbit the Sun because of its enormous gravity. It has about 333,000 times the Earth's mass and is over 1,000 times as massive as Jupiter. It has so much mass that it is able to produce its own light. This feature is what distinguishes stars from planets.
Composition
What is the Sun made of? Spectroscopy shows that hydrogen makes up about 94% of the solar material, helium makes up about 6% of the Sun, and all the other elements make up just 0.13% (with oxygen, carbon, and nitrogen the three most abundant ``metals''---they make up 0.11%). In astronomy, any atom heavier than helium is called a ``metal'' atom. The Sun also has traces of neon, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, potassium, and iron. The percentages quoted here are by the relative number of atoms. If you use the percentage by mass, you find that hydrogen makes up 78.5% of the Sun's mass, helium 19.7%, oxygen 0.86%, carbon 0.4%, iron 0.14%, and the other elements are 0.54%.
The Sun's Interior
Here are the parts of the Sun starting from the center and moving outward.
Core
The core is the innermost 10% of the Sun's mass. It is where the energy from nuclear fusion is generated. Because of the enormous amount of gravity compression from all of the layers above it, the core is very hot and dense. Nuclear fusion requires extremely high temperatures and densities. The Sun's core is about 16 million K and has a density around 160 times the density of water. This is over 20 times denser than the dense metal iron which has a density of ``only'' 7 times that of water. However, the Sun's interior is still gaseous all the way to the very center because of the extreme temperatures. There is no molten rock like that found in the interior of the Earth.
Radiative Zone


The radiative zone is where the energy is transported from the superhot interior to the colder outer layers by photons. Technically, this also includes the core. The radiative zone includes the inner approximately 85% of the Sun's radius.
Convective Zone
Energy in the outer 15% of the Sun's radius is transported by the bulk motions of gas in a process called convection. At cooler temperatures, more ions are able to block the outward flow of photon radiation more effectively, so nature kicks in convection to help the transport of energy from the very hot interior to the cold space. This part of the Sun just below the surface is called the convection zone.

The deepest layer of the Sun you can see is the photosphere. The word ``photosphere'' means ``light sphere''. It is called the ``surface'' of the Sun because at the top of it, the photons are finally able to escape to space. The photosphere is about 500 kilometers thick. Remember that the Sun is totally gaseous, so the surface is not something you could land or float on. It is a dense enough gas that you cannot see through it. It emits a continuous spectrum. Several methods of measuring the temperature all determine that the Sun's photosphere has a temperature of about 5840 K.
Measuring the Sun's Temperature
One method, called Wien's law, uses the wavelength of the peak emission, wavelengthpeak, in the Sun's continuous spectrum. The temperature in Kelvin = 2.9 × 106 nanometers/wavelengthpeak.

Another method uses the flux of energy reaching the Earth and the inverse square law. Recall from the Stellar Properties chapter that the flux is the amount of energy passing through a unit area (e.g., 1 meter2) every second. From the Inverse Square Law of Light Brightness, you find that the solar flux at the Earth's distance = the Sun's surface flux × (Sun's radius/Earth's distance)2 = 1380 Watts/meter2. Since the Sun's photosphere is approximately a thermal radiator, the flux of energy at its surface = sigma × (the Sun's surface temperature)4, where sigma is the Stefan-Boltzmann constant. Rearranging the equation, the photosphere's temperature = [(solar flux at Earth)/sigma) × (Earth distance/Sun's radius)2]1/4.

These two methods give a rough temperature for the Sun of about 5800 K. The upper layers of the photosphere are cooler and less dense than the deeper layters, so you see absorption lines in the solar spectrum. Which element absorption lines are present and their strength depends sensitively on the temperature. You can use the absorption line strengths as an accurate temperature probe to measure a temperature of about 5840 K.

alileo discovered that the Sun's surface is sprinkled with small dark regions called sunspots. Sunspots are cooler regions on the photosphere. Since they are 1000--1500 K cooler than the rest of the photosphere, they do not emit as much light and appear darker. They can last a few days to a few months. Galileo used the longer-lasting sunspots to map the rotation patterns of the Sun. Because the Sun is gaseous, not all parts of it rotate at the the same rate. The solar equator rotates once every 25 days, while regions at 30° above and below the equator take 26.5 days to rotate and regions at 60° from the equator take up to 30 days to rotate