The document discusses the physical structure and properties of the Sun. It describes how the Sun generates energy through nuclear fusion reactions in its core, where hydrogen is fused into helium. This releases energy according to Einstein's equation. It also summarizes the Sun's interior structure, atmosphere, activity cycles, and how observations of neutrinos and vibrations have informed our understanding.

1. Astronomy & Space Science Association
Of
University of Kelaniya
The Sun, Our Star
Physical Structure & Properties of Sun
Eranga Jayashantha
Hon.Mentioned Diploma in Astronomy and Astrophysics ( IOAA – China )
Sri Lanka National Astronomy and Astrophysics Olympiad Team Trainer

5. Stars in different stages of their evolution may
generate energy using different nuclear reactions.
These reactions can occur in the core or in a layer
around the core. At the present, the energy of the
Sun is generated
A. in its central core by fission of heavy nuclei.
B. from gravitational energy as the Sun slowly
shrinks.
C. in its core by radioactive decay of uranium.
D. in the central core by fusion of helium nuclei and
in an outer shell by fusion of hydrogen nuclei.
E. in its central core by fusion of hydrogen nuclei.
Q16.1
Question

6. Stars in different stages of their evolution may
generate energy using different nuclear reactions.
These reactions can occur in the core or in a layer
around the core. At the present, the energy of the
Sun is generated
A. in its central core by fission of heavy nuclei.
B. from gravitational energy as the Sun slowly
shrinks.
C. in its core by radioactive decay of uranium.
D. in the central core by fusion of helium nuclei and
in an outer shell by fusion of hydrogen nuclei.
E. in its central core by fusion of hydrogen nuclei.
A16.1

15. The surface layers of the Sun are very massive.
What stops the Sun from collapsing under its own
weight?
A. The strong nuclear repulsion between the atoms
of these layers.
B. Neutrinos exert a strong outward pressure,
holding the layers up.
C. The magnetic field exerts a strong force.
D. The pressure of the very high-temperature gas
within the Sun supports the outer layers.
E. The interior of the Sun is under such high
pressure that it is solid.
Q16.7

16. The surface layers of the Sun are very massive.
What stops the Sun from collapsing under its own
weight?
A. The strong nuclear repulsion between the atoms
of these layers.
B. Neutrinos exert a strong outward pressure,
holding the layers up.
C. The magnetic field exerts a strong force.
D. The pressure of the very high-temperature gas
within the Sun supports the outer layers.
E. The interior of the Sun is under such high
pressure that it is solid.
A16.7

22. Solar Nutrino
Detector
Composition mu
Nutrino
tau
Nutrino
Electro
Nutrino
Kamiokandae (JAP) D2O
Sudbury (CAN) D2O
SAGE (FRA) 37Cl
GALEX (ITA) Ar
Homostake (USA) Cl

30. This photo shows solar granulation. The darker
areas are regions where the gas is
A. hotter.
B. cooler.
C. Doppler shifted.
D. moving laterally.
E. less dense.
Q16.9

31. This photo shows solar granulation. The darker
areas are regions where the gas is
A. hotter.
B. cooler.
C. Doppler shifted.
D. moving laterally.
E. less dense.
A16.9

53. The dark regions on this photo of the Sun are
A. the corona.
B. solar granules.
C. Zeeman effects.
D. sunspots.
E. prominences.
Q16.11

54. The dark regions on this photo of the Sun are
A. the corona.
B. solar granules.
C. Zeeman effects.
D. sunspots.
E. prominences.
A16.11

55. Stars in different stages of their evolution may
generate energy using different nuclear reactions.
These reactions can occur in the core or in a layer
around the core. At the present, the energy of the
Sun is generated
A. in its central core by fission of heavy nuclei.
B. from gravitational energy as the Sun slowly
shrinks.
C. in its core by radioactive decay of uranium.
D. in the central core by fusion of helium nuclei and
in an outer shell by fusion of hydrogen nuclei.
E. in its central core by fusion of hydrogen nuclei.
A16.1

56. The “fuel” of the Sun is ______, and the main
products of the nuclear reactions include ______.
A. hydrogen / helium, neutrinos, and gamma rays
B. helium / only neutrinos and gamma rays
C. hydrogen / neutrinos and microwaves
D. helium / neutrinos and microwaves
E. hydrogen / only neutrinos.
Q16.2

63. The End

64. 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.

65. 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.

66. 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.

67. 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.

68. 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
chromosphere along the boundaries of supergranules.
 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.

69. 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 22-year solar
cycle.

70. 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.