Sun :-
The Sun is the star at the center of the Solar System.. It is a nearly perfect sphere of hot plasma with internal convective motion that generates a magnetic field via adynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, i.e. 109 times that of Earth, and Its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.
Structure and Energy Production:-
- Core - the innermost 20 - 25% of the Sun's radius, where temperature (energies) and pressure are sufficient for nuclear fusion to occur. Hydrogen fuses into helium (which cannot currently be fused at this point in the Sun's life). The fusion process releases energy, and the helium gradually accumulates to form an inner core of helium within the core itself.
- Radiative zone - Convectioncannot occur until much nearer the surface of the Sun. Therefore, between about 20-25% of the radius, and 70% of the radius, there is a "radiative zone" in which energy transfer occurs by means of radiation (photons) rather than by convection.
- Tachocline - the boundary region between the radiative and convective zones.
- Convective zone - Between about 70% of the Sun's radius and a point close to the visible surface, the Sun is cool and diffuse enough for convection to occur, and this becomes the primary means of outward heat transfer, similar to weather cells which form in the earth's atmosphere.
- Photosphere - the deepest part of the Sun which we can directly observe with visible light. Because the Sun is a gaseous object, it does not have a clearly-defined surface; its visible parts are usually divided into a 'photosphere' and 'atmosphere'.
- Atmosphere - a gaseous 'halo' surrounding the Sun, comprising the chromosphere, solar transition region, corona and heliosphear. These can be seen when the main part of the Sun is hidden, for example, during a solar eclipse.
Some important facts about Sun :-
Solor Mission :-
The first satellites designed to observe the Sun were NASA's Pioneers5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long time, transmitting data until May 1983.
In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona.
