Astronomy & Space Science Association

Of

University of Kelaniya

The Sun, Our Star

Physical Structure & Properties of Sun

Eranga JayashanthaHon.Mentioned Diploma in Astronomy and Astrophysics ( IOAA – China )

Sri Lanka National Astronomy and Astrophysics Olympiad Team Trainer

at Institute of Physics Sri Lanka

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

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

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

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

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

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

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

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

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

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

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

The End

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

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.

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.

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.