Roger Freedman • Robert Geller • William Kaufmann III
Universe
Tenth Edition
Chapter 16
Our Star, the Sun
By reading this chapter, you will learn
16-1 The source of the Sun’s heat and light
16-2 How scientists model the Sun’s internal structure
16-3 How the Sun’s vibrations reveal what lies beneath
its glowing surface
16-4 How scientists are able to probe the Sun’s energy-
generating core
16-5 Why the gaseous Sun appears to have a sharp outer
edge
By reading this chapter, you will learn
16-6 Why the upper regions of the solar atmosphere
have an emission spectrum
16-7 The relationship between the Sun’s corona and the
solar wind
16-8 The nature of sunspots
16-9 The connection between sunspots and the Sun’s
magnetic field
16-10 How magnetic reconnection can power immense
solar eruptions
The Sun
The ProtonProton Chain
16-1: The Sun’s energy is generated by
thermonuclear reactions in its core
The Proton-Proton Chain
16-2: A theoretical model of the Sun shows how
energy gets from its center to its surface
Hydrostatic Equilibrium
Hydrostatic Equilibrium
A Theoretical Model of the Sun’s Interior
A Theoretical Model of the Sun’s Interior
16-3: Astronomers probe the solar interior
using the Sun’s own vibration
The Sun’s Internal Structure
A Sound Wave Resonating in the Sun
16-4: Neutrinos reveal information about the
Sun’s core – and have surprises of their own
A Solar Neutrino Experiment
16-5: The photosphere is the lowest of three
main layers in the Sun’s atmosphere
The Photosphere
The Origin of Limb Darkening
The Origin of Limb Darkening
Solar Granulation
Supergranules and Large Scale Convection
16-6: Spikes of rising gas extend
through the Sun’s chromosphere
The Chromosphere
The Solar Atmosphere
Spicules from Above
16-7: The corona ejects mass into
space to form the solar wind
The Solar Corona
Temperatures in the Sun’s Upper Atmosphere
Temperatures in the Sun’s Upper Atmosphere
Temperatures in the Sun’s Upper Atmosphere
The Ultraviolet Corona
16-8: Sunspots are low-temperature
regions in the photosphere
Sunspots
Tracking the Sun’s Rotation with Sunspots
The Sunspot
Cycle
The Sunspot
Cycle
Variations in the Average Latitude of Sunspots
16-9: Sunspots are produced by a 22year cycle in the Sun’s magnetic field
Sunspots have Strong Magnetic Fields
Sunspots have Strong Magnetic Fields
Magnetic Fields Deflect Moving, Electrically Charged Objects
Mapping the Sun’s Magnetic Field
Babcock’s Magnetic Dynamo Model
Rotation of the Solar Interior
16-10: The Sun’s magnetic field heats the corona,
produces flares, and causes massive eruptions
Magnetic Arches and the Magnetic Reconnection
Magnetic Arches and the Magnetic Reconnection
Magnetic Arches and the Magnetic Reconnection
The Active Sun Seen through an Hα Filter
The Sun in Ultraviolet Light
A Solar Prominence
Coronal Mass Ejection
The Sun-Earth Connection
The November 15, 1999 Transit of Mercury
Key Ideas
• Hydrogen Fusion in the Sun’s 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.
• 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.
Key Ideas
• Models of the Sun’s Interior: A theoretical description of a star’s
interior can be calculated using the laws of physics.
• The standard model of the Sun suggests that 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.
Key Ideas
• Solar Neutrinos and Helioseismology: Conditions in the solar
interior can be inferred from measurements of solar neutrinos
and of solar vibrations.
• 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.
• 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.
Key Ideas
• 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. Its spectrum is similar
to that of a blackbody at a temperature of 5800 K.
Convection in the photosphere produces granules.
Key Ideas
• Above the photosphere is a layer of less dense but higher
temperature gases called the chromosphere. Spicules extend
upward from the photosphere into the corona.
• The outermost layer of the solar atmosphere, the corona, is
made of very high-temperature gases at extremely low density.
• Activity in the corona includes coronal mass ejections and
coronal holes. The solar corona blends into the solar wind at
great distances from the Sun.
Key Ideas
• The Active Sun: 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.
• Sunspots are relatively cool regions produced by local
concentrations of the Sun’s magnetic field. 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 22year solar cycle.
Key Ideas
• 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.
• 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.