THE SUN

PHYSICAL FEATURES

Diameter 100 x Earth--larger than orbit of moon (from known distance, and apparent size)
Volume: 1 million x Earth
Mass: 300 thousand x Earth (from orbit of planets: Newton)
Density: 300 thousand/1 million= 0.3 x Earth (comparable to Jovians)
Temperature of visible surface: 5800 K (10,000° F) (from black body spectrum)
Composition: mostly hydrogen, 9% helium; traces of other elements (from dark lines in solar spectrum, and density) -- note similarity to Jupiter
Energy output: 4x10²⁶ Watts (power=energy per unit time) produced by nuclear fusion

SOLAR ENERGY

All the energy used on Earth (with the exception of nuclear power, available since 1940's) comes directly or indirectly from the Sun. Examples:

• fossil fuels come from plants that store energy by photosynthesis
• wind to power windmills comes from convection driven by solar heat
• waterfalls powered by water evaporated by solar heat deposited as rain in the mountains

Power per unit area=1400 W/m² (solar constant)

every m² at distance of Earth receives same amount of power -- total power, or luminosity, = power per area x total area

INTERNAL STRUCTURE

Deduced from models of the solar interior. Models must:

• agree with all observable properties of the Sun
• obey known laws of physics, as applied to the conditions in the interior of the Sun.

Additional information can be deduced from solar oscillations

• vibrations of the Sun's surface, detected by their Doppler shift
• due to waves propagating through the Sun, like seismic waves through the Earth: helioseismology
• different waves have different frequencies. It's a little like trying to determine the shape of a bell by listening to the sound of its ringing!

Generally, the temperature, pressure and density of the Sun drop off as we move out from the center. Computer models must calculate the values of these parameters, as well as others, such as chemical composition, etc.

PARTS OF THE SUN

• Core: region of nuclear fusion (source of power); temperature, 15 million K (27 million ^oF); high pressure (due to weight of gas above)
• radiation zone: ("interior") energy travels through this region in form of EM radiation, occasionally scattering from nuclei.
• convection zone: in this region energy (heat) is carried by convection. It can take 100 thousand years for heat from the core to escape this zone.
• photosphere: where density of sun drops to the point that the gas is transparent to radiation. From this point, heat is carried away from the Sun as black body radiation (8 minutes to Earth). The photosphere is essentially the top layer of the convection zone.
  • granules, 1000 km across, visible in photosphere, are the tops of convection cells, cf. bands on Jupiter. (velocity determined by Doppler shift)

Note. There are no sharp surfaces in the Sun, it's all gas

SOLAR ATMOSPHERE (layers above photosphere)

• chromosphere: gas too thin to glow brightly, but visible during solar eclipse. Characteristic red color due to emission line of hydrogen.
  • spicules, thin jets of matter thrown out from the photosphere.
  • Cooler upper parts of chromosphere contribute dark lines to solar spectrum
• corona: thin hot gas above chromosphere.
  • Temperature deduced from spectral lines of elements that have lost some of their electrons; emission in X-ray region. Cause of high temperature not known.
  • Upper regions of corona are source of solar wind: far enough from Sun that the high temperature allows atmosphere to escape from Sun's gravity.

SOLAR MAGNETISM AND ACTIVE REGIONS OF THE SUN

Sunspots

• dark, cooler regions on the photosphere, first observed by Galileo.
• Usually occur in pairs.
• Frequency of occurrence varies with time: maximum approximately every 11 years (solar sunspot cycle)
• associated with magnetic field (affects spectral lines); possibly caused by interaction of magnetic field with differential rotation
• curiously absent from 1645-1715 (Maunder minimum)

Prominences

• loops or sheets of gas; tend to follow magnetic field lines near sunspot pairs.
• last for hours, days, weeks; can be larger than Earth
• cause unknown.

Flares

• like prominences, but so energetic that material is ejected from the Sun
• temperatures up to 100 million K
• flares and prominences more common near sunspot maxima

Sun's brightness varies by a few parts in a thousand near sunspot maxima -- climate connection

HISTORY OF THEORIES ABOUT THE ORIGIN OF THE SUN'S ENERGY OUTPUT

• Anaxagoras (500-428 BC) thought the Sun was a large, hot rock.
• If the Sun produced power by combustion, it could last a few thousand years.
• Kelvin and Helmholtz (19th century) propose Sun's energy comes from gravitational contraction. This would give the Sun a lifetime of 100 million years. (There probably exist bodies that give off heat from this process, called brown dwarfs)
• By the end of the 19th century, geological evidence showed that the Earth was much older; and with the discovery of radioactivity, the age of the Earth was determined to be 4.5 billion years. A new explanation was required.

NUCLEAR FUSION

What is a nucleus?

• Atoms are composed of electrons orbiting nuclei, held together by electrical forces. (19th C)
• Nuclei (10^-15 m) are very small compared to the size of atoms (10^-10 m). Discovered by Ernest Rutherford around 1900.
• Nuclei are composed of protons and neutrons. (1930's)

Nuclear fusion is a reaction between nuclei in which smaller nuclei join to form a larger nucleus. The mass of the total products of the reaction is less than the original mass. Where does the mass go? Mass is not conserved: it can be converted into energy: E=mc² (Einstein, 1905).

• c, the speed of light, is very large, so a small amount of mass yields a lot of energy
• to produce the amount of power emitted by the Sun, 600 million tons of hydrogen must be combined to form helium per second: a lot of mass, but small compared to the Sun
• at this rate, the Sun has enough fuel to last another 5 billion years

NUCLEAR POWER

Requirements for a fusion reaction are high temperature and high density: this has been achieved on Earth in the hydrogen bomb, but never in a controlled reactor (beyond very preliminary tests)

Fission is a nuclear reaction in which large, heavy nuclei such as those of uranium or plutonium are split into smaller nuclei plus byproducts. Again, the total mass of the products is less than that of the original nuclei, and the mass difference is converted into energy. Nuclear fission occurs in atom bombs, and in controlled nuclear reactors.

Problems with nuclear fission as a source of power:

• the supply of fuel is limited.
• byproducts are extremely dangerous
• the technology is expensive
• the power plants have limited lifetimes because of the radiation emitted in the reactor

Controlled fusion, if achieved, would solve the first two problems but probably not the others

THE PROTON-PROTON REACTION AND THE SOLAR NEUTRINO PROBLEM

A positron is an antimatter electron; a neutrino is a particle with no charge and almost no mass that hardly interacts with anything

The main reaction in the Sun is the proton-proton reaction; some stars also use another reaction called the carbon cycle, but the net result is the same

Solar neutrino problem. Neutrinos can pass right through the Sun, and so it should be possible to detect the neutrinos coming from the fusion reaction at the core of the Sun. (Neutrinos turn chlorine nuclei to argon, or gallium to germanium.) The results are 1/3 to 1/2 the predicted value.

Possible explanations:

1. models of the solar interior are incorrect (e.g. perhaps we haven't calculated the temperature of the core correctly)

2. our understanding of the physics of neutrinos is incorrect

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