Sun

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.

Solar Structure:

The Sun is divided into six regions based on the physical characteristics of these regions. The boundaries are not sharp.

fusion core - region of energy generation
radiation shell - the region where energy transport is by radiation flow
convection shell - the region where energy transport is by convection cells
photosphere - the surface where photons are emitted
chromosphere - the atmosphere of the Sun
corona - the superhot region where the solar wind originates

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

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.

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:

hydrostatic equilibrium - the fact that pressure balances the self-gravity
thermal equilibrium - the amount of energy generated equals the amount radiated away
opacity - the resistance of the solar envelope to the flow of photons (how fast the energy is released)
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.
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 (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.

This fusion reaction in the Sun is called the proton-proton chain (the same process that powers H-bombs). It has the following four stages:

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.

There are several tests to a solar model produced from the about relationships:

Solar oscillations - the Sun is not in perfect balance (hydrostatic equilibrium) but oscillates with periods from 5 to 160 minutes. The details are similar to seismic waves and are used to investigate the density changes in the core.
Solar neutrinos - since the interact weakly with matter, solar neutrinos created during the proton-proton chain reactions are a direct look into the current reaction rates. Large underground neutrino detectors, such as the Super Kamiokande in Japan, are currently detecting less than a 1/3 of the number of neutrinos then what is predicted by the equations.

Solar Oscillations:

Although direct study of its interior is impossible, insights into the conditions - temperature, composition and motions of gas - within the Sun may be gained by observing oscillating waves, rhythmic inward and outward motions of its visible surface. The study of these solar oscillations is called helioseismology. In many ways, it resembles the study of seismic waves generated by earthquakes to learn about the Earth's interior.

The complex pattern of periodic throbbing motions appears on the surface due to acoustic (sound) waves that are trapped inside the Sun. Although they cannot be observed with the naked eye, the tiny motions can be detected as subtle shifts in the wavelength of the spectral absorption lines. The most intense of these are low frequency waves that oscillate on a time scale of about 5 minutes, coinciding with velocities of 0.5 km/s. However, the overall pattern is extremely complex the result of millions of oscillations, both large and small - that simultaneously resonate with periods ranging from a few minutes to one hour. Motions as slow as a few millimeters per second have been detected, but they may also be remarkably long-lived, persisting for up to one year.

Above is a computer representation of one of nearly ten million modes of sound wave oscillations of the Sun, showing receding regions in red and approaching regions in blue. It turns out that the entire Sun is ringing like a bell, with global oscillations that may continue for weeks. Each of the 10 million sound waves reverberates around the interior before it reaches the surface. Waves of different frequencies descend to different depths. On their return journey, they are influenced by changes in temperature, density and composition, just like seismic waves inside Earth. The lower-pitched waves, with a frequency of about 3 MHz (a 5 minute period), have been used to probe the solar interior and even to make images of the far side of the Sun, when they give advance warning of flares and active regions before they appear around the limb and start to impact Earth.

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 km. Within that 100 km, the temperature drops from 6000 K at the bottom to 4000 K at the top. Lower temperature means less luminosity from Planck's 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:

faculae - large, bright regions. Faculae occur where strong magnetic fields greatly reduce the local density of the gas. The low density makes it nearly transparent, so the lower levels of granules are more easily visible. At these deeper layers, the gas is hotter and radiates more strongly, explaining the brightening.
granules - small (1000 km wide), bright regions that change brightest on the order of several minutes = tops of convection cells
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 varys with an 11 year cycle and the peaks are associated with times of high solar activity (many flares and solar storms).

During the 11 year cycle, sunspot pairs are created at high solar latitudes and move towards the equator during the cycle. This effect is seen in the Maunder butterfly diagram. The typical lifetime of a sunspot is one or two solar rotations.

The origin of sunspots and the 11 year cycle are related. The solar magnetic field is unlike the magnetic fields of planets in that it is a surface magnetic field, instead of extending into space it is confined to the photosphere. Magnetic flux tubes can only be created when the surface field lines distort and overlap until a loop pops off the photosphere. The endpoints of the loop become the north and south poles of the sunspot. The process begins with a quiet Sun (low activity) and magnetic field lines that are smooth and lined up north/south on the Sun's surface. The differential rotation of the Sun causes the magnetic lines to wrap up.

After the solar cycle peaks, the energy is released from the magnetic fields and the field lines relax to their original, smooth north/south orientation. Then, cycle begins again. Total time for the whole process is 11 years.

A sequence of snapshots of the changing solar magnetic field (left) and the soft X-ray corona (right) every year from 1991 to 2000 almost an entire solar cycle. Obtained one year apart between one solar maximum (lower right) and the next, they show the evolution of coronal structure due to changes in the magnetic fields at its base. Note the few magnetic features and lack of X-ray bright loops in the middle, at solar minimum. The regions where the magnetic fields are strongest, i.e. in the active regions, coincide with the brightest coronal X-ray emissions. The strongest magnetic fields are shown in dark blue and white, where white is upward pointing and dark blue points toward the Sun.

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:

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. Over a period of a few hours, the magnetic fields collapse hurling the hot gas outward (much like a breaking rubber band). These event also create a large flux of high energy particles which reach the Earth as magnetic storms and cause a sharp increase aurora activity.

Review of the Sun's surface at many wavelengths

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. They escape through windows in the solar corona called coronal holes, regions where the magnetic fields are weak and the charged solar wind particles are not trapped in magnetic bottles.

X-ray and the above UV picture of the corona show that the hot gas is connected to the magnetic features in the photosphere. Those low level structures extending into long streamers in the outer corona and heat the corona to its million degree temperatures.