The Sun


The Sun’s Anatomy


  • Radius: 696,000 km (109 times Earth’s radius)
  • Mass: 2×10³º kg (332,946 times Earth’s mass)
  • Temperature: 15,000,000 K – center; 5,780 K – photosphere; 2-5 million K – corona
  • Luminosity: 3.8×1026 watts
  • Rotation Period at Equator: 25.4 days
  • Photospheric Composition (by number of atoms): 92.1% hydrogen, 7.8% helium, 0.1% other elements
  • Photospheric Composition (by mass %): 73.5% hydrogen, 24.9% helium, 1.6% other elements

The Sun: Very Close, Very Average, Very Spherical, Very Active, Very Energetic, Prime of Life

VERY CLOSE: At about 93 million miles (1 astronomical unit/AU) or 8 light minutes from Earth, the Sun is up close and personal. The closest stars from the Sun are Proxima Centauri (4.25 ly) and Sirius (8.55 ly).

VERY AVERAGE: Of the spectral type G2V, the Sun’s surface temperature is about 5800 Kelvin. It is average in size (1 million miles in diameter), average in life cycle (5 billion years with 5 billion years more to go), and average in composition (76% Hydrogen and 22% Helium). Unlike most stars that are in a binary star system, the Sun is lonely.

VERY SPHERICAL: Due to an inward pushing force due to gravitational contraction and outward pushing force due to high pressure from high temperature, the sun achieves hydrostatic equilibrium.

VERY ACTIVE: Constantly changing, the Sun’s surface contains sunspots, solar storms or solar flares (AKA Corona Mass Ejection). The Sun constantly produces energy with nuclear fusion and oscillates like a bell.

VERY ENERGETIC: The Sun emits 1026 watts into space with solar winds. Nuclear fusion occurs with 657 million tons of H2 (hydrogen) fused into 653 million tons of He (helium) every second.


  • Conduction: most important in solids (e.g. pot over open flame or stove)
  • Radiation: transport of energy by motion of photons; efficiency depends on how opaqueness of the matter (e.g. heater in a cold room)
  • Convection: bulk transport of packets of matter in a liquid or gas (e.g. boiling water — hot water rises, cold water sinks)

LAYERS OF THE SUN: The four parts of the Sun are: core, photosphere, chromosphere, and corona. Inside the core at 15 million Kelvin, nuclear fusion produces energy by fusing 2 hydrogen atoms into 1 helium atom. The energy is then carried from the core by radiation and convection. The photosphere (5,800 Kelvin) is the outermost/lowest level of solar atmosphere. In 1814, German physicist Joseph von Fraunhofer discovered the set of spectral lines (AKA Fraunhofer Lines), or dark features, in the optical spectrum of the Sun. Active with plages, prominences, filaments, sunspots, and solar flares, the photosphere has granulation, or pockets (granules) of hot gases rising and pockets of cooler gases sinking. Hotter than the photosphere at 15,000 Kelvin, the chromosphere is a thin diffuse layer composed of spicules, or dynamic jets. The corona at 2 million Kelvin is only observed during a total solar eclipse. A thin layer, the corona emits mostly X-rays.

  • Radiative Zone: the sun’s center is opaque, so energy takes hundreds of thousands of years to escape
  • Convective Zone: hot gases rise, cold gases sink

Solar Atmosphere (photosphere, corona, chromosphere)

  • Photosphere: 5,800 K; 500 km thick; granulation in solar atmosphere
  • Chromosphere: 10,000 K; 1,000 km thick; red color, Balmer series emission line of hydrogen
  • Corona: 2 million K; large region of high-density plasma
Solar Magnetic Activity

SUNSPOTS: Sunspots are areas of unusually strong magnetic fields, of relatively dark and cool areas with umbra and penumbra, usually in pairs, and of sizes greater than several Earth’s in diameter. The number of sunspots varies with an 11-year cycle (Sunspot Cycle). During the Maunder Minimum from 1645 to 1715, the number of sunspots was unusually low. Moreover, during the Solar Cycle (22-year cycle), the polarity of the sunspots reverses and returns to the original state.

SOLAR FLARES: powerful, energetic eruptions that releases magnetic energy, and up to 20 million K

CORONA MASS EJECTIONS: huge flows of hot gas at 1,500 km/sec


  • Neutrinos: matter that interact very weakly with normal matter; the interior of the Sun is transparent to neutrinos (discovered in 1956 by Clyde Cowan and Frederick Reines)
  • First neutrino “telescope” at Homestead gold mine, South Dakota: used 400,000 liters of dry-cleaning fluid (perchloroethylene -C2Cl4) because a neutrino can interact with a chlorine nucleus to form an argon nucleus
  • Only 1 out of 10²² passing neutrinos reacted, once every two days
  • 1/3 of expected neutrinos detected based on understanding of the proton-proton chain
  • Other experiments: Sudbury Neutrino Observatory (1,000 tons of heavy water) and Super-Kamiokande experiment in Japan (50,000 tons of water)

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