Chapter 16 The Sun
16.1 Physical Properties of the Sun
Radius: 700,000km
Mass: 2.0 ×1030 kg
Density: 1400kg/m³
Rotation: Differential; period about a month
Surface temperature: 5800 K
Apparent surface of Sun is photosphere.
The sharp edge: thin photosphere
Corona: hot upper atmosphere, normally too faint to be seen, but visible during eclipse.
Interior Structure: Core, Radiation Zone, Convection Zone, Photosphere, Chromosphere, Transition Zone
Core:
where nuclear fusion takes place and generates the Sun’s enormous energy output
Radiation zone:
solar energy transported out by radiation, rather than convection
Convection zone:
constant convection motion
Photosphere:
Chromosphere:
lower atmosphere
Transition zone:
Temperature rises dramatically
Corona:
Solar Wind:
Luminosity— total energy radiated by the Sun— can be calculated from the fraction of that energy that reaches Earth.
Solar constant— amount of Sun’s energy reaching Earth is 1400 W/m²
Total luminosity is about 4 × 1026 W (watts)— the equivalent of 10billion 1-megaton nuclear bombs per second
16.2 The Sun Interior
Energy transport:
The radiation zone is relatively transparent; the cooler convection zone is opaque
Mathematical Models, consistent with observation and physical principles, provide information about the Sun’s interior
In equilibrium, inward gravitational force must be balanced by outward pressure
Discovery 16-1: SOHO: Eavesdropping on the Sun
SOHO: Solar and Heliospheric Observatory
Orbits at Earth’s L1 point, magnetosphere
Multiple instruments measure magnetic field, corona, vibrations, and ultraviolet emissions
16.3 The Sun’s Atmosphere
Spectral analysis can tell us what elements are present, but only in the chromosphere and photosphere of the sun.
Composition of the Sun:
By percentage of number of atoms
Hydrogen 91.2%
Helium 8.7%
Oxygen, Carbon. Nitrogen, Silicon, Magnesium, Neon, Iron, Sulfur
Small solar storms in chromosphere emit spicules
Corona is much hotter than layers below it— must have a heat source, probably electromagnetic interactions
16.4 Solar Magnetism
Sunspots: dark areas
Appear dark because slightly cooler than surroundings
Typical size: approximately the size of the Earth
Sunspots come and go, typically in a few days.
Sunspots are linked by pairs of magnetic field lines
Magnetic field in a typical sunspot is about 1,000 times greater than the neighboring region.
Magnetic field is observed through Zeeman Effect
Zeeman Effect: broadening or splitting of spectral lines by magnetic field
Discovery 16-2: Solar-Terrestrial Relations
Does Earth feel effects of 22-year solar cycle directly?
Possible correlations:
Periods of climate dryness
Increased atmospheric circulation
storm system deepen, extend over wider range of latitude, carry more moisture
Long-term climatic influence
maunder minimum, Little Ice Age
Cause not understood as energy output doesn’t vary much (0.2-0.3%)
One definite correlation: Geomagnetic disturbance
Solar flares and coronal mass ejections ionize atmosphere, disrupting electronics and endangering astronauts
16.5 The Active Sun
Solar flare is a large explosion on Sun’s surface, emitting a similar amount of energy to a prominence, but in seconds or minutes rather than days or weeks
Coronal mass ejection occurs when a large “bubble” detaches from the Sun and escapes into space
Solar winds escapes Sun mostly through coronal holes, which can be seen in X-ray images
16.6 The Heart of the Sun
Given the Sun’s mass and energy production, we find that, on average, every kilogram of the sun produces about 0.2 milliwatts of energy
This is not much—gerbils could do better—but it continues through the 10-billion-year lifetime of the Sun
We find that the total lifetime energy output is about 3× 1013 J/kg
This is a lot, and it is produced steadily, not explosively
Nuclear Fusion is the energy source for the Sun
In general, nuclear fusion works like this
nucleus 1 + nucleus 2+ nucleus 3+ energy
But where does the energy come from?
It comes from the mass; if you add up the masses of the initial nuclei, you will find that it is more than the mass of the final nucleus
The relationship between mass and energy comes from Einstein’s famous equation: E=mc²
In this equation, c is the speed of light, which is a very large number
What this equation is telling us is that a small amount of mass is the equivalent of a large amount of energy—tapping into that energy is how the Sun keeps shining so long
The ultimate result of the process:
4(1H)→ 4He + energy + 2 neutrinos
The helium stays in the core
The energy is the form of gamma rays, which gradually lose their energy as they travel out from the core, emerging as visible light
The neutrinos escape without interacting
More Precisely 16-1:
Physicists recognize four fundamental forces in nature:
Gravity: very weak, but always attractive and infinite
Electromagnetic: Much stronger, but either attractive or repulsive; infinite in range
Weak nuclear force: Responsible for beta decay; short range (1-2 protons diameters); weak
Strong nuclear force: Keep nucleus together short range; very strong
More Precisely 16-2: Energy Generation in the Proton—Proton Chain
Mass of four protons: 6.6943 × 10-27kg
Mass of helium nucleus: 6.6466 ×10-27kg
Mass transformed to energy: 0.0477 × 10-27kg (about 0.71%)
Energy equivalent of that mass: 4.28 × 10-12 J
Energy produced by fusion of one kilogram of hydrogen into helium:6.40 × 1014 J
16.7 Observations of Solar Neutrinos
Typical solar neutrino detectors; resolution is very poor
Detection of solar neutrinos has been going on for more than 30 years now; there has always been a deficit in the type of neutrinos expected to be emitted by the Sun
Recent research proves that the Sun is emitting about as many neutrinos as the standard solar model predicts, but the neutrinos change into other types of neutrinos between the Sun and the Earth, causing the apparent deficit