The Sun
The Sun
-Our sun is a medium
sized star, of middle age,
at 4.6 byo and 150 million
km from earth.
-It’s diameter is 1.35
million km-we could fit
1,000,000 Earths inside.
The Sun
The Sun
• The Sun is the largest object in the solar system,
in both size and mass.
– The Sun contains more than 99 percent of all the mass
in the solar system, which allows it to control the
motions of the planets and other objects.
The Sun
Properties of the Sun
• The solar interior is gaseous throughout because
of its high temperature.
• Many of the gases are in a plasma state,
meaning that they are completely ionized and
composed only of atomic nuclei and electrons.
• The sun has 4 layers. Corona, chromosphere,
photosphere and a core.
Layers of
the Sun
Layers
again
The Sun
The Sun’s Atmosphere
The corona, which
is the top layer of
the Sun’s
atmosphere,
extends several
million kilometers
southward from the
top of the
chromosphere and
has a temperature
range of 1 million to
2 million degrees K.
• The chromosphere, which is above the
photosphere and approximately 2500 km in
thickness, has a temperature of nearly
30 000 K at the top.
The Sun
The Sun’s Atmosphere
• The photosphere, approximately 400 km in
thickness, is the lowest layer of the Sun’s
atmosphere.
– This is the visible surface of
the Sun because most of the
light emitted by the Sun
comes from this layer.
– The average temperature of
the photosphere is about
5800 K.
The Core
• The core is the center of the sun where nuclear
fusion takes place. This converts Hydrogen into
Helium. This releases enormous amounts of
heat and light.
Solar prominence
A prominence, sometimes associated with flares, is
an arc of gas that is ejected from the chromosphere,
or gas that condenses in the inner corona and rains
back to the surface.
The Sun
Solar Features
– Coronal holes, often located over sunspot groups, are
areas of low density in the gas of the corona.
– Solar flares are violent eruptions of particles and
radiation from the surface of the Sun that are associated
with sunspots.
– When these particles
reach Earth, they can
interfere with
communications and
damage satellites.
The Sun
The Sun’s Atmosphere
Solar Wind
– Gas flows outward from the corona at high speeds and
forms the solar wind.
– Solar wind consists of charged particles, or ions, that
flow outward through the entire solar system, bathing
each planet in a flood of particles.
– The charged particles are trapped in two huge rings in
Earth’s magnetic field, called the Van Allen belts, where
they collide with gases in Earth’s atmosphere, causing
an aurora.
The Sun
Solar Activity
• The Sun’s magnetic field disturbs the solar
atmosphere periodically and causes new features
to appear in a process called solar activity.
• Sunspots are cooler areas
that form on the surface of
the photosphere due to
magnetic disturbances,
which appear as dark spots.
• The sun has an axis that it rotates on every
days. (like Earth rotates on its axis)
• The cycle of sunspots is on a 11.2 year interval
The Sun
The Solar Interior
• Fusion occurs within the core of the Sun where
the pressure and temperature are extremely high.
– Fusion is the combining of lightweight nuclei,
such as hydrogen, into heavier nuclei.
• In the core of the Sun, helium is a product of the
process in which hydrogen nuclei fuse.
• At the Sun’s rate of hydrogen fusing, it is about
halfway through its lifetime, with about another
5 billion years left.
The Sun
Solar Composition
• The Sun consists of 70% hydrogen and 28%
helium, as well as a small amount of other
elements.
• This composition is
very similar to that of
the gas giant planets.
• The Sun’s
composition
represents that of the
galaxy as a whole.
The Sun
Section Assessment
1. Match the following terms with their definitions.
___
D photosphere
___
B corona
A. the middle layer of the Sun’s
atmosphere
___
A chromosphere
B. the outermost layer of the
Sun’s atmosphere
___
C sunspot
C. cooler region on the Sun’s
surface that forms due to
magnetic irregularities
D. the lowest layer of the Sun’s
atmosphere
Measuring the Stars
Objectives
• Describe star distribution and distance.
• Classify the types of stars.
• Summarize the interrelated properties of stars.
Vocabulary
– constellation
– absolute magnitude
– binary star
– luminosity
– parallax
– Hertzsprung-Russell
diagram
– apparent magnitude
– main sequence
Measuring the Stars
Groups of Stars
• Constellation-A group of stars that form a
pattern.
• There are 88 constellations.
• They are named after animals, mythological
characters, or everyday objects.
– Circumpolar constellations can be seen all
year long as they appear to move around the
north or south pole. Ex-Big Dipper
– Summer, fall, winter, and spring constellations can be
seen only at certain times of the year because of
Earth’s changing position in its orbit around the Sun.
Measuring the Stars
Groups of Stars
Binaries
– A binary star is two stars that are gravitationally bound
together and that orbit a common center of mass.
– More than half of the stars in the sky are either binary
stars or members of multiple-star systems.
– Astronomers are able to identify binary stars through
several methods.
• Accurate measurements can show that its position
shifts back and forth as it orbits the center of mass.
• In an eclipsing binary, the orbital plane of a binary
system can sometimes be seen edge-on from Earth.
Measuring the Stars
Stellar Position and Distances
• Astronomers use units of measure for long
distances.
– A light-year (ly) is the distance that light
travels in one year, equal to 9.461 × 1012 km or
9,461,000,000,000 km
Measuring the Stars
Stellar Position and Distances
• To estimate the distance of stars from Earth,
astronomers make use of the fact that nearby
stars shift in position as observed from Earth.
• Parallax is the apparent shift in position of an
object caused by the motion of the observer.
• As Earth moves from one side of its orbit to the
opposite side, a nearby star appears to be shifting
back and forth.
Measuring the Stars
Basic Properties of Stars
• The basic properties of stars include diameter,
mass, brightness, energy output (power), surface
temperature, and composition.
• The diameters of stars range from as little as
0.1 times the Sun’s diameter to hundreds of
times larger.
• The masses of stars vary from a little less than
0.01 to 20 or more times the Sun’s mass.
Measuring the Stars
Basic Properties of Stars
Magnitude
– One of the most basic observable properties of a star is
how bright it appears.
– The ancient Greeks established a classification system
based on the brightnesses of stars.
– The brightest stars were given a ranking of +1, the next
brightest +2, and so on.
Measuring the Stars
Basic Properties of Stars
Apparent Magnitude
– Apparent magnitude is based on the ancient Greek
system of classification which rates how bright a star
appears to be.
– In this system, a difference of 5 magnitudes
corresponds to a factor of 100 in brightness.
– Negative numbers are assigned for objects brighter
than magnitude +1.
Measuring the Stars
Basic Properties of Stars
Absolute Magnitude
– Apparent magnitude does not actually indicate how
bright a star is, because it does not take distance
into account.
– Absolute magnitude is the brightness an object would
have if it were placed at a distance of 10 pc.
Measuring the Stars
Basic Properties of Stars
Luminosity
– Luminosity is the energy output from the surface of a
star per second.
– The brightness we observe for a star depends on both
its luminosity and its distance.
– Luminosity is measured in units of energy emitted per
second, or watts.
– The Sun’s luminosity is about 3.85 × 1026 W.
Measuring the Stars
Spectra of Stars
• Stars also have dark absorption lines in their
spectra and are classified according to their
patterns of absorption lines.
Measuring the Stars
Spectra of Stars
H-R Diagrams
– A Hertzsprung-Russell
diagram, or H-R diagram,
demonstrates the
relationship between mass,
luminosity, temperature,
and the diameter of stars.
– An H-R diagram plots the
absolute magnitude on the
vertical axis and
temperature or spectral
type on the horizontal axis.
Measuring the Stars
Spectra of Stars
H-R Diagrams
– The main sequence, which
runs diagonally from the
upper-left corner to the
lower-right corner of an
H-R diagram, represents
about 90 percent of stars.
– Red giants are large, cool,
luminous stars plotted at the
upper-right corner.
– White dwarfs are small, dim,
hot stars plotted in the lowerleft corner.
Measuring the Stars
Spectra of Stars
H-R Diagrams
Measuring the Stars
Groups of Stars
Star Clusters
– Although stars may appear to be close to each other,
very few are gravitationally bound to one other.
– By measuring distances to stars and observing how
they interact with each other, scientists can determine
which stars are gravitationally bound to each other.
– A group of stars that are gravitationally bound to each
other is called a cluster.
• In an open cluster, the stars are not densely packed.
• In a globular cluster, stars are densely packed into
a spherical shape.
Measuring the Stars
Section Assessment
1. Match the following terms with their definitions.
___
B binary star
___
C absolute magnitude
___
A luminosity
___
D parallax
A. the energy output from the
surface of a star per second
B. when two stars are
gravitationally bound and orbit
a common center of mass
C. the brightness an object would
have if placed at a set distance
D. an apparent shift in the position
of an object caused by the
motion of the observer
Measuring the Stars
Section Assessment
2. How can astronomers measure the speed at
which a star is moving?
Spectral lines are shifted in wavelength by
motion between the source of light and the
observer. If a star is moving toward the observer,
spectral lines are blueshifted. If a star is moving
away, spectral lines are redshifted. The higher
the speed, the larger the shift.
Measuring the Stars
Section Assessment
3. Identify whether the following statements are
true or false.
______
true The full Moon has less brightness than Venus on
the absolute magnitude scale.
______
false Luminosity of stars is a relatively consistent
stellar property.
______
false Around two-thirds of the stars in the sky are
either binary stars or members of multi-star
systems.
______
true The Sun is part of the main sequence.
Stellar Evolution
Objectives
• Explain how astronomers learn about the internal
structure of stars.
• Describe how the Sun will change during its lifetime
and how it will end up.
• Compare the evolutions of stars of different masses.
Vocabulary
– nebula
– supernova
– protostar
– black hole
– neutron star
Stellar Evolution
Basic Structure of Stars
• The mass and the composition of a star
determine nearly all its other properties.
– Hydrostatic equilibrium is the
balance between gravity
squeezing inward and
pressure from nuclear
fusion and radiation
pushing outward.
– This balance, which is
governed by the mass of a
star, must hold for any stable
star; otherwise, the star would
expand or contract.
Stellar Evolution
Basic Structure of Stars
Fusion
– Inside a star, the density and temperature increase
toward the center, where energy is generated by
nuclear fusion.
– Stars on the main sequence all produce energy by
fusing hydrogen into helium, as the Sun does.
– Stars that are not on the main sequence either fuse
different elements in their cores or do not undergo
fusion at all.
Stellar Evolution
Basic Structure of Stars
Fusion
– Fusion reactions involving elements other than
hydrogen can produce heavier elements, but few
heavier than iron.
– The energy produced according to the equation
E = mc2 stabilizes a star by producing the pressure
needed to counteract gravity.
Stellar Evolution
Stellar Evolution and Life Cycles
• A star changes as it ages because its internal
composition changes as nuclear fusion reactions in
the star’s core convert one element into another.
• As a star’s core composition changes, its density
increases, its temperature rises, and its
luminosity increases.
• When the nuclear fuel runs out, the star’s internal
structure and mechanism for producing pressure
must change to counteract gravity.
Stellar Evolution
Stellar Evolution and Life Cycles
Star Formation
– A nebula (pl. nebulae) is a cloud of interstellar
gas and dust.
– Star formation begins when the nebula
collapses on itself as a result of its own gravity.
– As the cloud contracts, its rotation forces it into
a disk shape.
– A protostar is a hot condensed object that
forms at the center of the disk that will become
a new star.
Stellar Evolution
Stellar Evolution and Life Cycles
Star Formation
Stellar Evolution
Stellar Evolution and Life Cycles
Fusion Begins
– Eventually, the temperature inside a protostar becomes
hot enough for nuclear fusion reactions to begin
converting hydrogen to helium.
– Once this reaction begins, the
star becomes stable because it
then has sufficient internal heat
to produce the pressure needed
to balance gravity.
– The object is then truly a star and
takes its place on the main
sequence according to its mass.
Stellar Evolution
The Sun’s Life Cycle
• What happens during a star’s life cycle depends
on its mass.
– It takes about 10 billion years for a
star with the mass of the Sun to
convert all of the hydrogen in its
core into helium.
– When the hydrogen in its core
is gone, a star has a helium
center and outer layers made
of hydrogen-dominated gas.
– Some hydrogen continues to react
in a thin layer at the outer edge of
the helium core.
Stellar Evolution
The Sun’s Life Cycle
– The energy produced in the thin hydrogen layer forces
the outer layers of the star to expand and cool and the
star becomes a red giant.
– While the star is a red giant, it loses gas from its outer
layers while its core becomes hot enough, at 100
million K, for helium to react and form carbon.
– When the helium in the core is all used up, the star is
left with a core made of carbon.
Stellar Evolution
The Sun’s Life Cycle
A Nebula Once Again
– A star of the Sun’s mass never becomes hot enough
for carbon to react, so the star’s energy production
ends at this point.
– The outer layers expand once
again and are driven off entirely
by pulsations that develop,
becoming a shell of gas called a
planetary nebula.
– In the center of a planetary
nebula, the core of the star
remains as a white dwarf made
of carbon.
Stellar Evolution
The Sun’s Life Cycle
Pressure in White Dwarfs
– A white dwarf is stable because it is supported by the
resistance of electrons being squeezed close together
and does not require a source of heat to be maintained.
– A star that has less mass than that of the Sun has a
similar life cycle, except that helium may never form
carbon in the core, and the star ends as a white dwarf
made of helium.
Stellar Evolution
Life Cycles of Massive Stars
• A massive star begins its life high on the
main sequence with hydrogen being converted
to helium.
• A massive star undergoes
many reaction phases and
produces many elements
in its interior.
• The star becomes a red
giant several times as it
expands following the end
of each reaction stage.
Stellar Evolution
Life Cycles of Massive Stars
• As more shells are formed by the fusion of
different elements, the star expands to a larger
size and becomes a supergiant.
• A massive star loses
much of its mass during
its lifetime.
• White dwarf composition
is determined by how
many reaction phases the
star went through before
reactions stopped.
Stellar Evolution
Life Cycles of Massive Stars
Supernovae
– A star that begins with a mass between about 8 and 20
times the Sun’s mass will end up with a core that is too
massive to be supported by electron pressure.
– Once no further energy-producing reactions can
occur, the core of the star violently collapses in on
itself and protons and electrons in the core merge to
form neutrons.
– A neutron star results from the resistance of neutrons
to being squeezed, which creates a pressure that halts
the collapse of the core.
Stellar Evolution
Life Cycles of Massive Stars
Supernovae
– A neutron star has a mass of 1.5 to 3 times the Sun’s
mass but a radius of only about 10 km.
– Infalling gas rebounds when it strikes the hard surface
of the neutron star and explodes outward.
– A supernova (pl. supernovae) is a massive explosion in
which the entire outer portion of the star is blown off and
elements that are heavier than iron are created.
Stellar Evolution
Life Cycles of Massive Stars
Supernovae
Stellar Evolution
Life Cycles of Massive Stars
Black Holes
– A star that begins with more than about 20 times the
Sun’s mass will not be able to form a neutron star.
– The resistance of neutrons to being squeezed is not
great enough to stop the collapse, so the core of the
star simply continues to collapse forever, compacting
matter into a smaller and smaller volume.
– A black hole is a small, extremely dense remnant of a
star whose gravity is so immense that not even light can
escape its gravity field.
Stellar Evolution
Section Assessment
1. Match the following terms with their definitions.
___
A nebula
___
C protostar
___
D supernova
___
B black hole
A. a cloud of interstellar gas
and dust
B. small, extremely dense
remnant of a star with
immense gravity
C. a hot, condensed object that
eventually will begin nuclear
fusion.
D. a massive explosion that
blows off the outer portion of
a massive star
Stellar Evolution
Section Assessment
2. How is a neutron star different from a
white dwarf?
A white dwarf is created when the resistance of
electrons to being squeezed stops the inward
collapse of a star’s core. A neutron star is created
when the original star does not lose enough
mass to become a white dwarf. The pressure of
the collapsing core causes protons and electrons
to merge to form neutrons. The resistance of
neutrons to be being squeezed halts the collapse
of the core forming the neutron star.
Stellar Evolution
Section Assessment
3. Identify whether the following statements are
true or false.
______
false The Sun will likely produce a supernova.
______
true Black holes are likely smaller than 10 km in
diameter.
______
false Planets form from planetary nebula.
______
true All stable stars have hydrostatic equilibrium.
______
true The Sun will become a red giant in about
5 million years.
Chapter Resources Menu
Study Guide
Section 30.1
Section 30.2
Section 30.3
Chapter Assessment
Image Bank
Section 30.1 Study Guide
Section 30.1 Main Ideas
• The Sun contains most of the mass in the solar system
and is made up primarily of hydrogen and helium.
• Astronomers learn about conditions inside the Sun by a
combination of observation and theoretical models.
• The Sun’s atmosphere consists of the photosphere, the
chromosphere, and the corona.
• The Sun has a 22-year activity cycle caused by reversals
in its magnetic field polarities.
• Sunspots, solar flares, and prominences are active
features of the Sun.
• The solar interior consists of the core, where fusion of
hydrogen into helium occurs, and the radiative and
convective zones.
Section 30.2 Study Guide
Section 30.2 Main Ideas
• Positional measurements of the stars are important for
measuring distances through stellar parallax shifts.
• Stellar brightnesses are expressed in the systems of
apparent and absolute magnitude.
• Stars are classified according to the appearance of
their spectra, which indicate the surface temperatures
of stars.
• The H-R diagram relates the basic properties of stars:
class, mass, temperature, and luminosity.
Section 30.3 Study Guide
Section 30.3 Main Ideas
• The mass of a star determines its internal structure and
its other properties.
• Gravity and pressure balance each other in a star.
• If the temperature in the core of a star becomes high
enough, elements heavier than hydrogen but lighter than
iron can fuse together.
• Stars such as the Sun end up as white dwarfs. Stars up to
about 8 times the Sun’s mass also form white dwarfs after
losing mass. Stars with masses between 8 and 20 times
the Sun’s mass end as neutron stars, and more massive
stars end as black holes.
• A supernova occurs when the outer layers of the star
bounce off the neutron star core, and explode outward.
Chapter Assessment
Multiple Choice
1. Which atmospheric layer of the Sun is visible
under normal conditions?
a. photosphere
c. convective zone
b. corona
d. chromosphere
The photosphere is the lowest layer of the Sun’s
atmosphere. The chromosphere and corona are the
upper two layers of the sun’s atmosphere which are
usually only visible during a solar eclipse.
Chapter Assessment
Multiple Choice
2. The apparent shift in the position of an object
caused by the motion of the observer is
called ____.
a. luminosity
c. parallax
b. magnitude
d. a parsec
The distance to a star can be estimated from its parallax
shift. In this case, the motion of the observer is the
change in position as the Earth orbits the Sun. Using the
parallax technique, astronomers can find accurate
distances up to 500 pc.
Chapter Assessment
Multiple Choice
3. What is the first fusion reaction in all stars?
a. helium to oxygen
c. hydrogen to helium
b. silicon to iron
d. oxygen to carbon
Hydrogen to helium is the first fusion reaction in all stars.
In the next reaction, when the core consists primarily of
helium, it is fused to form oxygen. Stars will continue to
generate sequentially heavier elements, though usually
no heavier than iron, as its mass allows.
Chapter Assessment
Multiple Choice
4. What causes sun spots?
a. prominence
b. magnetic irregularities
c. solar flares
d. solar wind
Sun spots are caused by magnetic irregularities in the
photosphere. Prominence, solar flares, and solar wind
are all associated with sun spots.
Chapter Assessment
Multiple Choice
5. Which of the following star classifications
represents the highest temperature?
a. G
c. F
b. K
d. B
Stars are assigned spectral types in the following order
from hottest to coolest: O, B, A, F, G, K, and M. Each
class is subdivided into more specific divisions with
number from 0 to 9. The Sun is a type G2 star.
Chapter Assessment
Short Answer
6. What happens halfway through the solar
activity cycle?
Halfway through the 22-year solar activity
cycle, the Sun’s magnetic field reverses so
that the north magnetic pole becomes the
south magnetic pole.
Chapter Assessment
Short Answer
7. What is a binary star? How common are they?
A binary star is when two stars are
gravitationally bound together and orbit a
common center of mass. More than half of
the stars in the sky are either binary stars or
members of multi-star systems.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
true The Sun has roughly the same density
as Jupiter.
______
false A parsec is roughly one-third of a light year.
______
true About 90 percent of all stars are main
sequence stars.
______
false The Sun will someday become a white dwarf
made of helium.
______
true A neutron star has a radius of only about 10 km.
Image Bank
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