The Sun

The sun is the only star of the Solar System. It is a yellow star from the main sequence, being classified by the astronomers as a star of G2 degree, and having 4.6 million years old. The sun is constituted basically of hydrogen (about 75%) and helium. Its diameter is of 1,400,000 km, being located about 150 million quilometers away from the earth. Due to the large distance between the earth and the sun, every electromagnetic radiation coming from the sun lasts about eight minutes until achieve ehe earth.

Internal Composition of the Sun

The sun is considered as a large thermonuclear reactor burning away in the sky. It's supposed that it is formed by three main regions and three outer layers. The core is located 500,000 km below the visible surface, with a temperature of 15 million degrees, and a pressures up to 250 billion than on earth. In the core, hydrogen is converted into helium by nuclear fusion. The radiation core surrounds the core; here the energy is transferred outwards in the form of gamma and X-rays. Due to the great pressure this energy is continuously absorbed and reabsorbed. The convection zone begins at 200,000 km below the sun's surface. The energy of the previous zone heats the gas in this zone, causing its expansion and dispersion outwards. The temperature decreases from 2,000,000 degrees at its innermost part to about 5,700 degree next to the photosphere.

Composition of the Solar Atmosphere

The solar atmosphere is divided into three regions: photosphere, cromosphere, and corone, being described below.

Photosphere

The photosphere is a shallow region of about 500 km, having a temperature of about 6,000 degrees. It has a granular appearance caused by "cells"l; of convection. Sunspots appear in this region, and have temperature relatively low. The sunspot number increases and decreases according a eleven year cycle, called solar cycle. A typical sunspot has a darker centre which is called the umbra, surrounded by a lighter region called the penumbra. Sunspots can also appear in groups.

Cromosphere

The cromosphere is located above the photosphere and it is supposed that has about 3,000 km deep, with a temperature of 4,300 degrees. This region can only be viewed, as a red ring around the sun, just before and after a total eclypse. Also in this region irregular bright areas called plage appear (plage is the French world for beach). Sunspots are always accompanied by surrounding plage areas; however, plage areas can occur without sunspots. Solar flares always erupt out from plage regions, often causing shortwave fadeouts and geomagnetic storms.

Corona

The corona is the outer layer of the sun's atmosphere, composed by a hot gaseous halo, visible during a solar eclipse, and achieves the earth via the solar wind. Its higher temperature close to the sun is about 2,000,000 degrees. During the declining phase of the solar cycle, holes appear in the corona. Because of this, material from within the sun expands at points where the solar gravity is unable to constrain it. This is the nebulous boundary between the corona and the solar wind, that is made up of charged and highly conductive material or plasma. It has an average speed of around 400 quilometers per second, but this varies enormously between 250 and 900 quilometers per second. The charged particles and the magnetic field in the solar wind interact with the terrestrial magnetic field, forming a region called magnetosphere, with a form similar to a comet.

References:

• Solar Activity 1997: article published in WRTH - World Radio TV Handbook, Volume 51, 1997.

Related links:

• The Sun: description of the sun and its properties.
• The Sun: web page containing lots of graphic contents about the sun: high-resolution photos, animations, etc.
• Sun: material related to the Sun, containing both text and various graphics.