The Sun is a self-luminous ball of gas held together by its own gravity and powered by thermonuclear fusion in its core. Our Sun is a typical star among the various stars in the Galaxy, average in mass, size and temperature. It is a ``dwarf'' star (compared to supergiant stars, see AST122 next term) with a radius of 109 Earth radii and a mass of 3.3x105 Earth masses.
The Sun's lifetime is about 10 billion years, meaning that after this time the hydrogen in its core will be depleted. The Sun will then evolve into a red giant, consuming Mercury, Venus and the Earth in its expanded envelope. The Sun is currently 5 billion years old.
The most outstanding characteristic of the Sun is the fact that it emits huge quantities of electro-magnetic radiation of all wavelengths. The total output of the Sun is 3.99x1033 ergs/sec. Only 1.8x1024 ergs/sec strikes the Earth (since it is small in angular size), which is called the solar constant, but the amount of energy reaching the Earth in 30 mins is more than the power generated by all of human civilization. This energy is what powers the atmosphere and our oceans (storms, wind, currents, rainfall, etc.).
The energy emitted by the Sun is divided into 40% visible light, 50% IR, 9% UV and 1% x-ray, radio, etc. The light we see is emitted from the ``surface'' of the Sun, the photosphere. The Sun below the photosphere is opaque and hidden.
The Sun is divided into six regions based on the physical characteristics of these regions. The boundaries are not sharp.
The radii and temperatures of these regions are the following:
The Sun rotates differentially since it is not a solid. The solar equator completes one rotation in 25 days. The poles complete one rotation in 36 days.
region radius temperature ------------------------------------------------- fusion core 0.3 solar radii 15x10^6 K radiation shell 0.3-0.6 solar radii 6x10^6 K convection shell 0.6-1.0 solar radii 1x10^6 K photosphere 100 km 6000 K chromosphere 2000 km 30,000 K corona 10^6 km 1x10^6 K
Stars form from clouds of gas and collapse under self-gravity. The collapse is stopped by internal pressure in the core of the star. During the collapse, the potential energy of infalling hydrogen atoms is converted to kinetic energy, heating the core. As the temperature goes up, the pressure goes up to stop the collapse. The heat from the collapse is sufficient for the Sun to shine, but only for a timescale of 15 million years (called the Kelvin-Helmholtz time). Since the Sun is 5 billion years old, then it must be producing its own energy rather than shining on leftover energy from formation (like Jupiter).
The structure of the Sun is determined by 5 relations or physical concepts:
Energy generation is the heart of the solar process. Normally, particles with like charges (positive-positive or negative-negative) repel each other, this is called electrostatic repulsion. But at temperatures above 15x106 K, the motions of protons are high enough to overcome the electrostatic forces and the nuclei can ``fuse''. Nuclear reactions involve many elementary particles that make up all of matter (this is called the Standard Model). The primary output from nuclear reactions are photons in the form of gamma-rays, but a large number of other particles are important as well.
There are several tests to a solar model produced from the about relationships: