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Our Star, the Sun. Chapter Eighteen. Guiding Questions. What is the source of the Sun’s energy? What is the internal structure of the Sun? How can astronomers measure the properties of the Sun’s interior? How can we be sure that thermonuclear reactions are happening in the Sun’s core?

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Our Star, the Sun

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Our Star, the Sun

Chapter Eighteen

Guiding Questions

  • What is the source of the Sun’s energy?

  • What is the internal structure of the Sun?

  • How can astronomers measure the properties of the Sun’s interior?

  • How can we be sure that thermonuclear reactions are happening in the Sun’s core?

  • Does the Sun have a solid surface?

  • Since the Sun is so bright, how is it possible to see its dim outer atmosphere?

  • Where does the solar wind come from?

  • What are sunspots? Why do they appear dark?

  • What is the connection between sunspots and the Sun’s magnetic field?

  • What causes eruptions in the Sun’s atmosphere?

Source of solar energy

  • The sun produces 3.9 * 1026 joules/sec (watts).

  • The earth receives 7 * 1017 watts or 2 billionths of sun’s energy.

  • The sun has been shining for about 4.5 billion years.

  • Theory of fusion energy source for sun’s radiation was developed; 1928 – 1938.

  • George Gamow, Hans Bethe, F. Houtermans, R. Atkinson, W. Pauli, W. Heisenberg developed the theory.

  • 1968; solar neutinos observed by John N. Bahcall & Raymond Davis.

Neutrino experiments

  • Brookhaven National Laboratory; 100, 000 gallons of perchloroethylene (C2Cl4); 1968.

    • Neutrino strikes Chlorine nucleus, turning it into Argon, can be separated from fluid.

    • Only 1/3 as many neutrinos as predicted.

  • Late 1980’s; M. Koshiba

    • Kamiokande experiment in Japan

    • Showed neutrinos indeed came from the sun.

  • 1998 Super Kamiokande experiment

    • Showed there were 3 types of neutrinos, explained missing 2/3 of the neutrinos.

  • 1987 Supernova occurs in Magellenic Cloud

    • Neutrinos observed in 2 different detectors.

    • Product of supernova explosion.

The Sun’s energy is generated by thermonuclearreactions in its core

  • The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E = mc2

  • Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 107 K

  • In the Sun, fusion occurs only in the dense, hot core

The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus

Start of hydrogen fusion process in the sun’s interior; 2 protons collide.

Step 2 in the fusion process involves a 3rd proton

In the final step, the end products are helium with 2 of the

original 6 hydrogen atoms recycled.

A theoretical model of the Sun shows how energygets from its center to its surface

  • Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius

  • The core is surrounded by a radiative zone extending to about 0.71 solar radius

    • In this zone, energy travels outward through radiative diffusion

  • The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure

    • In this zone, energy travels outward primarily through convection

Energy is transported by radiative diffusion from center to 0.71 solar radii

It takes 170,000 years to traverse through the body of the sun to surface.

Astronomers probe the solar interior usingthe Sun’s own vibrations

  • Helioseismology is the study of how the Sun vibrates

  • These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun

Note that only 0.8 % of the sun’s volume is < .2 solar radii

Internal solar densities and temperature, note water is 1000 kg/m3

Neutrinos reveal information about the Sun’score—and have surprises of their own

  • Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected

  • Recent neutrino experiments explain why this is so

The photosphere is the lowest of three main layersin the Sun’s atmosphere

  • The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona

  • Everything below the solar atmosphere is called the solar interior

  • The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere

Limb darkening because base of photosphere is hotter than higher layers seen near solar limb.

Convection in the photosphere produces granules

The chromosphere is characterized by spikesof rising gas

  • Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere

  • Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules

Chromospheric Spectrum

  • Chromospheric emission spectrum

    • Emission lines with some matching wavelengths of photospheric absorption lines

      • Bright yellow line produced by helium (He)

    • Chromospheric temperature up to 30,000 K at highest level

    • Gas density is lower than photosphere

  • From this, one concludes that temperature must rise rapidly up through chromosphere

  • The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density

  • The solar corona blends into the solar wind at great distances from the Sun

The corona ejects mass into space to form the solar wind

Activity in the corona includes coronal mass ejections and coronal holesUltraviolet image taken from SOHO spacecraft.

Sunspots are low-temperature regions inthe photosphere

Temperature in Umbra abt 4400 K, Penumbra abt 5000 K, 30% of light

Tracking the sun’s rotation with sunspots; 25 ¼ days at equator, 28.2 days at latitude 45, 34 days nearer the poles.

Chromospheric Flares

  • Flares - brief burst of X-rays and particle

    • Observed in monochromatic light

    • Lifetimes of about 20 minutes

    • Size about 30,000 km

    • Enhances particle density in solar wind and solar cosmic rays

Solar Flare Movie

Bright areas or Plages form just before appearance of sunspots

Filaments appear as prominences above solar limb

Bright areas are called Plages,

Sunspots are produced by a 22-year cyclein the Sun’s magnetic field

  • The Sun’s surface features vary in an 11-year cycle

  • This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity

  • The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next

  • Two such cycles make up the 22-year solar cycle

Variations in solar activity

  • 1610 Galileo observed sunspots

  • From 1645 to 1715 very few sunspots were observed

    • Historical records, Flamsteed in 1674 said that 1st since 1664.

  • “Little Ice Age”; 1300 to ~ 1850

    • Glaciers advanced in Alps; severe winters

    • Greenland colony fails

    • 1816, year without a summer follows Mt Tambora eruption

    • Weather unstable & unpredictable.

  • Astronomical evidence that sunspot minima occurs about 20% of the time in a star like the sun

  • 1958, Sunspot activity was the largest ever observed.

  • Since 1900, sun is getting hotter, about 1/3 of global warming.

The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field

These changes are caused by convection and the Sun’s differential rotation

Rotation of the Solar Interior; center of sun rotates uniformly

The Sun’s magnetic field also produces otherforms of solar activity

  • A solar flare is a brief eruption of hot, ionized gases from a sunspot group

  • A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona

Coronal Prominences

  • Prominences- Chromospheric material extending upward into corona

    • Seen against photospheric or chromospheric disk known as filaments

  • Properties

    • Much cooler than surrounding corona

    • Sizes, if quiescent, height 30,000 km, length 200,000 km, thickness 5000 km

    • Exhibit motions associated with magnetic fields up to several hundred gauss

    • Lifetimes up to 90 days


Soft X-RayJuly 7, 1998


  • Holes - lower temperature and much lower density regions

    • Sizes up to hundreds of thousands of km

    • Magnetic field lines open out to interplanetary space

    • Source of solar wind particles

    • Changeable in periods of days to weeks

  • Active regions - relatively hot and dense regions consisting of magnetic loop structures

    • Sizes up to hundreds of thousands of km

    • Magnetic field lines form large loop structures

    • Occur over chromospheric plages

  • Quiet regions - between coronal holes and coronal active regions

    • Magnetic fields weak and roughly in loop structures

Coronal Quiet Region


Coronal Hole

Coronal Active Region


Coronal Mass Ejection

Coronal mass ejections send bursts of energetic charged particles out through the solar system.

Coronal Mass Ejections

  • Shows relatively quiet corona

    • Black disk blocks photospheric and chromospheric radiation

  • 16 minutes later, huge balloon-shaped volume of high-energy gas is ejected from corona

  • Ejected material expands at typical velocities of 400 km/s

    • Ejection lasts several hours and contains trillions of tons of matter

    • Often associated with solar flares, but not always




Coronal mass ejection of 1012 kg mass ( a billion tons)

Solar events also peak at 11 yr cycle

  • Numbers of plages, filaments, solar flares and coronal mass ejects also follow the same 11-yr cycle as sunspots.

  • Coronal mass ejections (CMJ) can occur at any point in the cycle. Flares are also unpredictable.

  • Solar flares and CMJ’s produce showers of charged particles (electrons and protons).

  • Can disrupt electrical grids, radio and TV transmissions.

  • Dangerous for astronauts, satellites

  • Can produce brilliant auroras, even at low latitudes.

Space Weather

  • Space weather – study of variable emission of high-energy photons, particles, and magnetic fields and their interaction with the geosphere

  • Earth influences

    • Van Allen radiation belts

    • Spacecraft and crews

    • High-altitude aircraft

    • Electric power grid

    • Communications, land and satellite

    • Major source of natural variability in terrestrial climate

The Big Picture

  • The Sun continues to shine, while it radiates away as its luminosity, by generating energy by thermonuclear fusion of hydrogen into helium.

  • Gravitational and thermal equilibrium determine the Sun’s internal structure and its rate of energy generation.

  • The Sun’s atmosphere displays its own version of weather and climate, governed by solar magnetic fields. Solar weather has important influences on the Earth.

  • The Sun is important not only as our source of light and heat, but also because it is the only star near enough for us to study in great detail. In the coming chapters, we will use what we’ve learned about the Sun to help us understand other stars.

22-year solar cycle


CNO cycle



convective zone


coronal hole

coronal mass ejection

differential rotation





hydrogen fusion

hydrostatic equilibrium

limb darkening

luminosity (of the Sun)

magnetic-dynamo model


magnetic reconnection

negative hydrogen ion


neutrino oscillation






Key Words

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