Sun's Atmosphere

Sun Facts Average distance from Earth = 93 million miles (~150 million km) Radius = 418,000 miles (696,000 km) Mass = 1.99 x 1030 kg (330,000 Earth masses) Makeup (by mass) = 74 percent hydrogen, 25 percent helium, 1 percent other elements Average temperature = 5,800 degrees Kelvin (surface), 15.5 million degrees Kelvin (core) Average density = 1.41 grams per cm3 Rotational period = 25 days (center) to 35 days (poles) Magnitude = -26.8 (apparent), +4.8 (absolute) Apparent magnitude refers to a star's brightness in the sky from our vantage point on Earth. Absolute magnitude is the star's true brightness if all of the stars were the same distance from Earth. The lower the number, the brighter the star. Distance from center of Milky Way = 25,000 light-years Orbital speed and period = 138 miles per second (230 kilometers per second) and 200 million years

Above the surface of the sun is its atmosphere, which consists of three parts as shown in the lower half of Figure 1:

• Photosphere
• Chromosphere
• Corona - extremely hot outermost layer extending outward from the chromosphere several million miles or kilometers

We will see that all of the major features of the sun can be explained by the nuclear reactions that make its energy, the magnetic fields that are caused by the movements of the gas, and the immense gravity.

Photosphere
The photosphere is the lowest region of the sun's atmosphere and is the region that can be seen from Earth. It is 180-240 miles or 300-400 km wide and has an average temperature of 5,800 degrees Kelvin. It appears bubbly or granulated, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath and each granulation can be 600 miles (1,000 km) wide. As we pass up through the photosphere, the temperature drops and the gases, because they are cooler, do not emit as much light energy. Therefore, the outer edge of the photosphere looks dark, an effect called limb darkening that accounts for the clear crisp edge of the sun's surface.

Chromosphere
The chromosphere lies above the photosphere to about 1,200 miles or 2,000 km. The temperature rises across the chromosphere from 4,500 degrees Kelvin to about 10,000 degrees Kelvin. The chromosphere is thought to be heated by convection within the underlying photosphere. As gases churn in the photosphere, they produce shock waves that heat the surrounding gas and send it piercing through the chromosphere in millions of tiny spikes of hot gas called spicules. Each spicule rises to approximately 3,000 miles or 5,000 km above the photosphere and lasts only a few minutes. Spicules may also follow along magnetic field lines of the sun, which are made by the movements of gases inside the sun.

Corona
The corona is the final layer of the sun and extends several million miles or kilometers outward from the photosphere. It can be seen best during a solar eclipse and in X-ray images of the sun. The temperature of the corona averages 2 million degrees Kelvin; although no one is sure why the corona is so hot, it is thought to be caused by the sun's magnetism. The corona has bright areas (hot) and dark areas called coronal holes. Coronal holes are relatively cool and are thought to be areas where particles of the solar wind escape.