School Education

1. 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 3.3x105Earth 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 3.99x1033ergs/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

2. 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.