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 next Chapter) with a radius of 695,000 km (109 Earth radii) and a mass of 1.99x1030 kg (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 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.

The most outstanding characteristic of the Sun is the fact that it emits huge quantities of electromagnetic radiation of all wavelengths. The total output of the Sun, called the solar constant, is 3.99x1033 ergs/sec. Only 1.8x1024 ergs/sec strikes the Earth (since it is small in angular size), 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.


Solar Structure:

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:

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


Sun's Interior:

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:

  1. hydrostatic equilibrium - the fact that pressure balances the self-gravity

  2. thermal equilibrium - the amount of energy generated equals the amount radiated away

  3. opacity - the resistance of the solar envelope to the flow of photons (how fast the energy is released)

  4. energy transport - how energy is transported from the core to the photosphere (convection or radiation)

    There are three ways to transfer energy; conduction, convection and radiation. Conduction, the collisional transfer of energy between atoms, only occurs between solids (such as a hot pan and your hand), so is not found in the Sun. Convection is the motion of heated material, such as bubbles in boiling water. Radiation is the transfer of energy by electromagnetic waves (light). Only convection and radiation transfer are important in the Sun and the opacity determines which method is used. When the temperature is high and all the atoms are stripped of their electrons, the opacity is low and radiation transfer is dominant.

    When the temperature drops, such as in the outer layers of the solar interior, the protons and electrons recombine to form atoms and the opacity goes up. High opacity slows the transfer of energy by radiation, so bubbles form. These bubbles are hot and low in density, thus starting a convective flow.

  5. energy production - in the case of stars, energy is produced by thermonuclear fusion (see below)

These 5 relationships, described as mathematical formula, show how energy is generated, how that energy effects the structure of the Sun and how that energy is transported to the surface to make the Sun shine.


Thermonuclear Fusion:

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. 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.

This fusion reaction in the Sun is called the proton-proton chain (the same process that powers H-bombs).

All the gamma-rays in the core are scattered many, many times. Each scattering exchanges energy so that the photons convert into visible, UV, IR and radio photons, as well as high energy ones, producing a thermal spectrum.


Photosphere:

The photosphere is the effective ``surface'' of the Sun since it is the point where the photons break free of scattering and zip into outer space. However, the photosphere is not a thin surface, but rather has a thickness of about 100-500 km. Within that distance, the temperature drops from 6000 K at the bottom to 4000 K at the top. Lower temperature means less luminosity from the Black Body curve, so the edge of the Sun's disk is darker than the center, this is called "limb darkening".

There are several features seen within the photosphere:

  1. granules - small (1000 km wide), bright regions that change brightest on the order of several minutes = tops of convection cells
  2. faculae - large, bright regions associated to the sunspots
  3. sunspots - dark regions that travel in pairs (north and south magnetic poles)

Sunspots pairs are due to magnetic flux tubes on the surface of the Sun. The flux tubes carry energy away causing the surface to be cooler (1800 degrees cooler) than the surrounding material, thus their darker appearance. The sunspots always travel with a north and south pole, oriented along the equator. The number of sunspots per year varies with an 11 year cycle and the peaks are associated with times of high solar activity (many flares and solar storms).


Chromosphere:

The chromosphere is a pinkish atmosphere above the Sun's photosphere. It emits an emission spectrum to indicate it is a very hot gas (20,000 K). The most complex and transient solar phenomena occur in the chromosphere including:

  1. solar arcs
  2. solar prominences
  3. solar flares
Solar flares, arcs and prominences are linked to sunspot activity. Gas is trapped in the flux lines created by the sunspot pairs and lifted off the photosphere into the chromosphere.


Corona:

The corona of the Sun is a large, white halo of glowing gas visible during a total eclipse. The corona gas is extremely hot (temperatures on order of a million degrees) and is the source of the solar wind.

The solar wind is a constant stream of solar particles moving at faster than the escape velocity of the Sun's gravitational field. This wind, together with the solar flares and prominences, also creates a large flux of high energy particles which reach the Earth as magnetic storms and cause a sharp increase aurora activity.