Download Stars

EARTH SCIENCE
Geology, the Environment and the Universe
Chapter 29: Stars
CHAPTER
29
Table Of Contents
Section 29.1 The Sun
Section 29.2 Measuring the Stars
Section 29.3 Stellar Evolution
Click a hyperlink to view the corresponding slides.
Exit
SECTION
29.1
The Sun
Essential Questions
• What are the layers and features of the Sun?
• How is the process of energy production in the
Sun explained?
• How are the three types of spectra defined?
SECTION
29.1
The Sun
• The Sun contains most of the mass of the solar
system and has many features typical of other
stars.
Review Vocabulary
• magnetic field: the portion of space near a
magnetic or current-carrying body where magnetic
forces can be detected
SECTION
The Sun
29.1
New Vocabulary
photosphere
solar flare
chromosphere
prominence
corona
fusion
solar wind
fission
sunspot
SECTION
29.1
The Sun
Properties of the Sun
• The Sun is the largest object in the solar system,
in both diameter and mass. The Sun contains
more than 99 percent of all the mass in the solar
system. It should not be surprising, then, that the
Sun’s mass affects the motions of the planets
and other objects.
SECTION
29.1
The Sun
Properties of the Sun
• The Sun’s average density is similar to the
densities of the gas giant planets. Computer
models show that the density in the center of the
Sun is about 1.50 × 105 kg/m3.
SECTION
29.1
The Sun
Properties of the Sun
• The Sun’s interior is gaseous throughout
because of its high temperature—about
1 × 107 K in the center. At this temperature, all of
the gases are completely ionized. This means
that the interior is composed only of atomic
nuclei and electrons, in the state of matter
known as plasma.
SECTION
29.1
The Sun
Please click the image above to view the interactive table.
SECTION
29.1
The Sun
The Sun’s Atmosphere
• The outer regions of the Sun’s atmosphere are
organized into layers, like a planetary
atmosphere separated into different levels, and
each layer emits energy at wavelengths
resulting from its temperature.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Photosphere
• The photosphere is the innermost layer of the
Sun’s atmosphere and is also its visible
surface. It has an effective temperature of
5800 K and is about 400 km thick.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Chromosphere
• Outside the photosphere is the chromosphere,
which is approximately 2500 km thick and has
an average temperature of 15,000 K.
• Usually, the chromosphere is visible only during
a solar eclipse, but astronomers can use
special filters to observe it when the Sun is not
eclipsed.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Corona
• The outermost layer of the Sun’s atmosphere,
called the corona, extends several million
kilometers from the outside edge of the
chromosphere and usually has a temperature
of about 3 to 5 million K.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Corona
• The density of the gas in the corona is very low,
which explains why the corona is so dim. It can
be seen only when the photosphere is blocked
by either special instruments, as in a
coronagraph, or by the Moon during an eclipse.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Solar wind
• Gas flows outward from the corona at high
speeds and forms the solar wind. As this wind
of charged particles, called ions, flows outward
through the entire solar system, it bathes each
planet in a flood of particles.
SECTION
The Sun
29.1
The Sun’s Atmosphere
Solar wind
• Charged particles in the solar wind are
deflected by Earth’s magnetic field and are
trapped in two huge rings, called the Van Allen
belts.
• The high-energy particles in these belts collide
with gases in Earth’s atmosphere and cause
the gases to give off light, called the aurora.
SECTION
29.1
The Sun
Solar Activity
• Some features of the Sun change over time in a
process called solar activity, which includes
fountains and loops of glowing gas. Some of this
gas has structure—a certain order in both time
and place—driven by magnetic fields.
SECTION
The Sun
29.1
Solar Activity
The Sun’s magnetic field and sunspots
• The Sun’s magnetic field disturbs the solar
atmosphere periodically and causes new
features to appear. A sunspot is a dark
spot on the surface of the photosphere that
typically lasts two months, occurs in pairs,
and has a penumbra and an umbra.
SECTION
The Sun
29.1
Solar Activity
Solar activity cycle
• Astronomers have observed that the
number of sunspots changes from minimum
to maximum and then back to minimum
again in about 11 years. At this point, the
Sun’s magnetic field reverses, so that the
north magnetic pole becomes the south
magnetic pole and vice versa.
SECTION
The Sun
29.1
Solar Activity
Solar activity cycle
• Because sunspots are caused by magnetic
fields, the polarities of sunspot pairs reverse
when the Sun’s magnetic poles reverse.
Therefore, when the polarity of the Sun’s
magnetic field is taken into account, the
length of the cycle doubles to 22 years.
SECTION
The Sun
29.1
Solar Activity
Other solar features
• Coronal holes are often located over
sunspot groups. Coronal holes are areas of
low density in the gas of the corona and are
the main regions from which the particles
that comprise the solar wind escape.
SECTION
The Sun
29.1
Solar Activity
Other solar features
• Solar flares are violent eruptions of
particles and radiation from the surface
of the Sun. Highly active solar flares are
associated with sunspots.
SECTION
The Sun
29.1
Solar Activity
Other solar features
• A prominence is an arc of gas ejected from
the chromosphere, or gas that condenses in
the Sun’s inner corona and rains back to the
surface. Prominences can reach temperatures
over 50,000 K and are associated with
sunspots.
SECTION
29.1
The Sun
The Solar Interior
• Fusion is the combination of lightweight atomic
nuclei into heavier nuclei, such as hydrogen
fusing into helium.
• This is the opposite of the process of fission,
which is the splitting of heavy atomic nuclei into
smaller, lighter nuclei, like uranium into lead.
SECTION
The Sun
29.1
The Solar Interior
Energy production in the Sun
• In the core of the Sun, helium is a product
of the process in which hydrogen nuclei
fuse.
• The mass of the helium nucleus is less
than the combined mass of the four
hydrogen nuclei, which means that mass is
lost during the process.
SECTION
The Sun
29.1
The Solar Interior
Energy production in the Sun
• Albert Einstein’s special theory of relativity
shows that mass and energy are
equivalent, and that matter can be
converted into energy and vice versa.
SECTION
The Sun
29.1
The Solar Interior
Energy production in the Sun
• The relationship between mass and energy
can be expressed as E = mc2, where E is
energy measured in joules, m is the quantity of
mass that is converted to energy measured in
kilograms, and c is the speed of light
measured in m/s.
SECTION
The Sun
29.1
The Solar Interior
Energy production in the Sun
• Einstein’s special theory of relativity explains
that the mass lost in the fusion of hydrogen to
helium is converted to energy, which powers
the Sun.
SECTION
The Sun
29.1
The Solar Interior
Energy transport
• Energy in the Sun is
transferred mostly by
radiation from the core
outward to about 75
percent of its radius. The
outer layers transfer
energy in convection
currents.
SECTION
The Sun
29.1
The Solar Interior
Energy transport
• As energy in the Sun moves outward, the
temperature is reduced from a central value of
about 1 × 107 K to its photospheric value of
about 5800 K.
• A tiny fraction of the immense amount of solar
energy eventually reaches Earth.
SECTION
The Sun
29.1
The Solar Interior
Solar energy on Earth
• Above Earth’s atmosphere, 1354 J of solar
energy is received in 1 m2/s (1354 W/m2).
However, not all of this energy reaches the
ground because some is absorbed and
scattered by the atmosphere.
SECTION
The Sun
29.1
Spectra
• A spectrum (plural, spectra) is visible light
arranged according to wavelengths. There
are three types of spectra: continuous,
emission, and absorption.
SECTION
29.1
The Sun
Spectra
• A spectrum that has no breaks in it, such as the
one produced when light from an ordinary bulb is
shined through a prism, is called a continuous
spectrum. A continuous spectrum can also be
produced by a glowing solid or liquid, or by a
highly compressed, glowing gas.
SECTION
29.1
The Sun
Spectra
• The spectrum from a noncompressed gas
contains bright lines at certain wavelengths. This
is called an emission spectrum, and the lines are
called emission lines. The wavelengths of the
visible lines depend on the element being
observed because each element has its own
characteristic emission spectrum.
SECTION
29.1
The Sun
Spectra
• A spectrum produced from the Sun’s light
shows a series of dark bands. These dark
spectral lines are caused by different chemical
elements that absorb light at specific
wavelengths. This is called an absorption
spectrum, and the lines are called absorption
lines.
SECTION
29.1
The Sun
Solar Composition
• Using the lines of the
absorption spectra like
fingerprints, astronomers
have identified the
elements that compose
the Sun. The Sun is
composed primarily of
hydrogen and helium
with small amounts of
other gases.
SECTION
29.1
The Sun
Solar Composition
• The Sun’s composition represents that of the
galaxy as a whole. Most stars have
proportions of the elements similar to the Sun.
SECTION
Section Check
29.1
The Sun has used only about 3 percent of
its hydrogen.
a. true
b. false
SECTION
Section Check
29.1
Which is the visible surface of the Sun?
a. photosphere
b. corona
c. chromosphere
d. prominence
SECTION
Section Check
29.1
What is formed by gas flowing outward
at a high speed from the Sun’s corona?
a. prominence
b. solar flare
c. coronal hole
d. solar wind
SECTION
29.2
Measuring the Stars
Essential Questions
• How are distances between stars measured?
• What is the difference between brightness and
luminosity?
• What are the properties used to classify stars?
SECTION
29.2
Measuring the Stars
• Stellar classification is based on measurement
of light spectra, temperature, and composition.
Review Vocabulary
• wavelength: the distance from one point
on a wave to the next corresponding point
SECTION
29.2
Measuring the Stars
New Vocabulary
constellation
absolute magnitude
binary star
luminosity
parsec
Hertzsprung-Russell diagram
parallax
main sequence
apparent magnitude
SECTION
29.2
Measuring the Stars
Patterns of Stars
• Long ago, many civilizations looked at the
brightest stars and named groups of them after
animals, mythological characters, or everyday
objects. These groups of stars are called
constellations.
• Today, astronomers group stars by the
88 constellations named by ancient peoples.
SECTION
29.2
Measuring the Stars
Patterns of Stars
• Some constellations are visible throughout the
year, depending on the observer’s location.
Constellations that appear to rotate around one
of the poles are called circumpolar
constellations. Ursa Major, which contains the
Big Dipper, is a circumpolar constellation for
most of the northern hemisphere.
SECTION
29.2
Measuring the Stars
Patterns of Stars
• Unlike circumpolar constellations, the other
constellations can be seen only at certain times
of the year because of Earth’s changing position
in its orbit around the Sun.
SECTION
29.2
Measuring the Stars
Patterns of Stars
• The most familiar constellations are the ones
that are part of the zodiac. These twelve
constellations can be seen in both the northern
and southern hemispheres.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Star clusters
• By measuring distances to stars and observing
how their gravities 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.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Star clusters
• An open cluster is a group of stars that are not
densely packed.
• A globular cluster is a group of stars that are
densely packed into a spherical shape.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Binaries
• When only two stars are gravitationally bound
together and orbit a common center of mass,
they are called binary stars.
• More than half of the stars in the sky are either
binary stars or members of multiple-star
systems.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Binaries
• Most binary stars appear to be single stars to
the human eye, even with a telescope. The
two stars are usually too close together to
appear separately, and one of the two is often
much brighter than the other.
SECTION
29.2
Measuring the Stars
Visualizing Star Groupings
• When you look into the night sky, the stars seem
to be randomly spaced from horizon to horizon.
Upon closer inspection, you begin to see groups
of stars that seem to cluster in one area.
SECTION
29.2
Measuring the Stars
Please click the image above to view the video.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Doppler shifts
• The most common way to tell that a star is
one of a binary pair is to find subtle
wavelength shifts, called Doppler shifts.
• Scientists use Doppler shifts to determine the
speed and direction of a star’s motion.
SECTION
Measuring the Stars
29.2
Patterns of Stars
Doppler shifts
• When a star moves toward the observer, the
light emitted by the star shifts toward the blue
end of the electromagnetic spectrum. When a
star moves away
from the observer,
its light shifts
toward the red.
SECTION
29.2
Measuring the Stars
Please click the image above to view the video.
SECTION
29.2
Measuring the Stars
Stellar Positions and Distances
• Astronomers use two units of measure for long
distances. One is the light-year (ly). A lightyear is the distance that light travels in one
year, equal to 9.461 × 1012 km. Astronomers
often use a unit larger than a light-year—a
parsec (pc), which is equal to 3.26 ly, or 3.086
× 1013 km.
SECTION
Measuring the Stars
29.2
Stellar Positions and Distances
Parallax
• When estimating the distance of stars from
Earth, astronomers must account for the fact
that nearby stars shift in position as observed
from Earth. This apparent shift in position
caused by the motion of the observer is called
parallax.
SECTION
29.2
Measuring the Stars
Please click the image above to view the video.
SECTION
Measuring the Stars
29.2
Stellar Positions and Distances
Parallax
• The distance to a star can be estimated
from its parallax shift by measuring the
angle of the change. With advancements in
technology, such as the Hipparcos satellite,
astronomers can find accurate distances up
to 100 pc by using the parallax technique.
SECTION
29.2
Measuring the Stars
Basic Properties of Stars
• The basic properties of a star are mass,
diameter, and luminosity, which are all related to
each other. Temperature is another property and
is estimated by finding the spectral type of a
star.
SECTION
29.2
Measuring the Stars
Basic Properties of Stars
• Temperature controls the nuclear reaction rate
and governs the luminosity, or absolute
magnitude. Apparent magnitude is how bright
the stars and planets appear in the sky from
Earth.
SECTION
29.2
Measuring the Stars
Basic Properties of Stars
Magnitude
• Absolute magnitude is how bright a star would
appear if it were placed at a distance of 10 pc.
The absolute magnitude compared to the
apparent magnitude is used to find the distance
to a star.
SECTION
Measuring the Stars
29.2
Basic Properties of Stars
Magnitude
• The classification of stars by absolute
magnitude allows comparisons that are based
on how bright the stars would appear at equal
distances from an observer. The disadvantage
of absolute magnitude is that it can be difficult
to determine unless the actual distance to a
star is known.
SECTION
Measuring the Stars
29.2
Basic Properties of Stars
Magnitude
• Apparent magnitudes do not give an actual
measure of energy output. To measure the
energy output from the surface of a star per
second, called its power or luminosity, an
astronomer must know both the star’s
apparent magnitude and how far away it is.
SECTION
Measuring the Stars
29.2
Basic Properties of Stars
Magnitude
• Luminosity is measured in units of energy
emitted per second, or watts. The Sun’s
luminosity is about 3.85 × 1026 W. The values
for other stars vary widely, from about 0.0001
to more than 1 million times the Sun’s
luminosity. No other stellar property varies as
much.
SECTION
Measuring the Stars
29.2
Classification of Stars
Temperature
• Stars are assigned spectral types in the
following order: O, B, A, F, G, K, and M. Each
class is subdivided into more specific divisions
with numbers from 0 to 9.
SECTION
Measuring the Stars
29.2
Classification of Stars
Temperature
• The classes were originally based only on the
pattern of spectral lines, but astronomers later
discovered that the classes also correspond to
stellar temperatures, with the O stars being
the hottest and the M stars being the coolest.
Thus, by examination of a star’s spectrum, it is
possible to estimate its temperature.
SECTION
Measuring the Stars
29.2
Classification of Stars
Temperature
• Temperature is also related to luminosity and
absolute magnitude. Hotter stars put out more
light than stars with lower temperatures.
• Distance can be determined by calculating a
star’s luminosity based on its temperature.
SECTION
Measuring the Stars
29.2
Classification of Stars
Composition
• All stars, including the Sun, have nearly
identical compositions, despite differences in
their spectra. The differences in the
appearance of their spectra are almost entirely
a result of temperature differences.
SECTION
Measuring the Stars
29.2
Classification of Stars
Composition
• Typically, about 73 percent of a star’s mass is
hydrogen, about 25 percent is helium, and the
remaining 2 percent is composed of all the
other elements.
SECTION
Measuring the Stars
29.2
Classification of Stars
H-R diagrams
• A Hertzsprung-Russell diagram (H-R
diagram) is a graph that relates stellar
characteristics— class, mass, temperature,
diameter, and luminosity.
• Absolute magnitude is plotted on the vertical
axis and temperature or spectral type is
plotted on the horizontal axis.
SECTION
29.2
Measuring the Stars
Please click the image above to view the interactive table.
SECTION
Measuring the Stars
29.2
Classification of Stars
H-R diagrams
• Most stars occupy the region in the diagram
called the main sequence, which runs
diagonally from the upper-left corner, where
hot, luminous stars are represented, to the
lower-right corner, where cool, dim stars are
represented.
SECTION
29.2
Measuring the Stars
Please click the image above to view the interactive table.
SECTION
29.2
Measuring the Stars
Classification of Stars
Main sequence
• While stars are in the main sequence, they are
fusing hydrogen in their cores. As stars evolve
off the main sequence, they begin to fuse
helium in their cores and burn hydrogen
around the core edges.
SECTION
29.2
Measuring the Stars
Classification of Stars
Main sequence
• A star’s mass determines almost all its other
properties, including its main-sequence
lifetime. The more massive a star is, the
higher its central temperature and the more
rapidly it burns its hydrogen fuel.
SECTION
29.2
Measuring the Stars
Classification of Stars
Main sequence
• Red giants are large, cool, luminous stars.
They are so large—more than 100 times the
size of the Sun in some cases—that Earth
would be swallowed up if the Sun were to
become a red giant.
SECTION
29.2
Measuring the Stars
Classification of Stars
Main sequence
• Small, dim, hot stars are called white dwarfs. A
white dwarf is about the size of Earth but has a
mass about as large as the Sun’s.
SECTION
Section Check
29.2
Most stars are part of multiple-star
systems.
a. true
b. false
SECTION
Section Check
29.2
What type of binary star system is
discovered by observing Doppler shifts?
a. eclipsing
b. spectroscopic
c. open cluster
d. globular cluster
SECTION
Section Check
29.2
What does the number of lines on a star’s
spectra indicate?
a. distance
b. age
c. temperature
d. magnitude
SECTION
29.3
Stellar Evolution
Essential Questions
• What is the relationship between mass and a
star’s evolution?
• What are the features of massive and regular
star life cycles?
• How is the universe affected by the life cycles of
stars?
SECTION
29.3
Stellar Evolution
• The Sun and other stars follow similar life
cycles, leaving the galaxy enriched with
heavy elements.
Review Vocabulary
• evolution: a radical change in composition
over a star’s lifetime
SECTION
Stellar Evolution
29.3
New Vocabulary
nebula
pulsar
protostar
supernova
neutron star
black hole
SECTION
Stellar Evolution
29.3
Basic Structure of Stars
Mass effects
• The more massive a star is, the greater the
gravity pressing inward, and the hotter and
more dense the star must be inside to
balance its own gravity. The temperature
inside a star governs the rate of nuclear
reactions, which in turn determines the star’s
energy output—its luminosity.
SECTION
29.3
Stellar Evolution
Basic Structure of Stars
Mass effects
• The balance between gravity squeezing inward
and outward pressure is maintained by heat
due to nuclear reactions and compression.
• This balance, governed by the
mass of the star, is called
hydrostatic equilibrium, and it
must hold for any stable star.
SECTION
Stellar Evolution
29.3
Basic Structure of Stars
Fusion
• The density and temperature increase toward
the center of a star, where energy is generated
by nuclear fusion.
SECTION
29.3
Stellar Evolution
Stellar Evolution
• Eventually, when its nuclear fuel runs out, a
star’s internal structure and mechanism for
producing pressure must change to counteract
gravity. The changes a star undergoes during its
evolution begin with its formation.
SECTION
Stellar Evolution
29.3
Stellar Evolution
Star formation
• The formation of a star begins with a cloud of
interstellar gas and dust, called a nebula
(plural, nebulae), which collapses on itself as a
result of its own gravity.
• As the cloud contracts, its rotation forces it into
a disk shape with a hot, condensed object at
the center, called a protostar.
SECTION
Stellar Evolution
29.3
Stellar Evolution
Star formation
• Friction from gravity continues to increase the
temperature of the protostar, until the
condensed object reaches the ignition
temperature for nuclear reactions and
becomes a new star.
SECTION
29.3
Stellar Evolution
Please click the image above to view the video.
SECTION
Stellar Evolution
29.3
Stellar Evolution
Fusion begins
• The first nuclear fusion reaction to ignite in a
protostar is always the conversion of
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.
SECTION
29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
• 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. Thus, such a star has a
main-sequence lifetime of 10 billion years. From
here, the next step in the life cycle of a small
mass star is to become a red giant.
SECTION
Stellar Evolution
29.3
Life Cycles of Stars Like the Sun
Red giant
• When the hydrogen in a star’s core is gone, it
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. The energy produced
in this layer forces the outer layers of the star
to expand and cool.
SECTION
29.3
Stellar Evolution
Please click the image above to view the video.
SECTION
Stellar Evolution
29.3
Life Cycles of Stars Like the Sun
Red giant
• While a star is a red giant, it loses gas from
its outer layers. Meanwhile, the core of the
star becomes hot enough, at 100 million K,
for helium to react and form carbon. When
the helium in the core is depleted, the star is
left with a core made of carbon.
SECTION
Stellar Evolution
29.3
Life Cycles of Stars Like the Sun
The final stages
• A star with the same mass as the Sun never
becomes hot enough for carbon to fuse, so its
energy production ends. The outer layers
expand again and are expelled by pulsations
that develop in the outer layers. The shell of
gas is called a planetary nebula.
SECTION
Stellar Evolution
29.3
Life Cycles of Stars Like the Sun
The final stages
• In the center of a planetary nebula, the core of
the star becomes exposed as a small, hot
object about the size of Earth. The star is then
a white dwarf made of carbon.
SECTION
29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
Internal pressure in white dwarfs
• A white dwarf is stable despite its lack of
nuclear reactions because it is supported by
the resistance of electrons being squeezed
together. This pressure counteracts gravity
and can support the core as long as the mass
of the remaining core is less than about 1.4
times the mass of the Sun.
SECTION
29.3
Stellar Evolution
Life Cycles of Massive Stars
• A more massive star begins its life with hydrogen
being converted to helium, but it is much higher
on the main sequence. The star’s lifetime in this
phase is short because the star is very luminous
and uses up its fuel quickly. When the white
dwarf cools and loses its luminosity, it becomes
an undetectable black dwarf.
SECTION
Stellar Evolution
29.3
Life Cycles of Massive Stars
Supergiant
• A massive star undergoes many more reaction
phases and thus produces a rich stew of many
elements in its interior. The star becomes a red
giant several times as it expands following the
end of each reaction stage.
SECTION
Stellar Evolution
29.3
Life Cycles of Massive Stars
Supergiant
• As more shells are formed
by the fusion of different
elements in a massive
star, the star expands to a
larger size and becomes a
supergiant. These stars
are the source of heavier
elements in the universe.
SECTION
29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
• 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
reactions in the core of the star have created
iron, no further energy-producing reactions
can occur, and the core of the star violently
collapses in on itself.
SECTION
29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
• A neutron star is a collapsed, dense core of
a star that forms quickly while its outer layers
are falling inward. It has a diameter of about
20 km and a mass 1.4 to 3 times that of the
Sun, and it contains mostly neutrons.
SECTION
29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
• A pulsar is a spinning neutron star that
exhibits a pulsing pattern.
SECTION
29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
• When the outer layers of a star collapse into
the neutron core, the central mass of neutrons
creates a pressure that causes this mass to
explode outward as a supernova, leaving a
neutron star.
SECTION
Stellar Evolution
29.3
Life Cycles of Massive Stars
Black holes
• A star that begins with more than 20 times the
Sun’s mass will be too massive to form a
neutron star. The resistance of neutrons to
being squeezed is not great enough to stop
the collapse. The core of the star continues to
collapse, compacting matter into a smaller
volume.
SECTION
Stellar Evolution
29.3
Life Cycles of Massive Stars
Black holes
• 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.
SECTION
29.3
Section Check
The diagram depicts a star that is stable
and will not expand or contract. What is
this balance called?
a. electrostatic equilibrium
b. hydrostatic equilibrium
c. gravitational equilibrium
d. luminosity equilibrium
SECTION
Section Check
29.3
The density of a neutron star is
comparable to that of an atomic nucleus.
a. true
b. false
SECTION
29.3
Section Check
If light cannot escape a black hole, how
do astronomers locate black holes?
Answer: Because light cannot escape, a black
hole is invisible. However, gases spiraling into a
black hole emit X rays. Astronomers can locate
the black hole by looking for those X-ray
emissions.
CHAPTER
Stars
29
Resources
Earth Science Online
Study Guide
Chapter Assessment Questions
Standardized Test Practice
Click on a hyperlink to view the corresponding feature.
SECTION
The Sun
29.1
Study Guide
• The Sun contains most of the mass of the solar
system and has many features typical of other
stars.
• Most of the mass in the solar system is found in
the Sun.
• The Sun’s average density is approximately
equal to that of the gas giant planets.
SECTION
The Sun
29.1
Study Guide
• The Sun has a layered atmosphere.
• The Sun’s magnetic field causes sunspots and
other solar activity.
• The fusion of hydrogen into helium provides the
Sun’s energy and composition.
SECTION
Measuring the Stars
29.2
Study Guide
• Stellar classification is based on measurement of
light spectra, temperature, and composition.
• Most stars exist in clusters held together by their
gravity.
• The simplest cluster is a binary.
SECTION
Measuring the Stars
29.2
Study Guide
• Parallax is used to measure distances to stars.
• The brightness of stars is related to their
temperature.
• Stars are classified by their spectra.
• The H-R diagram relates the basic properties of
stars: class, temperature, and luminosity.
SECTION
Stellar Evolution
29.3
Study Guide
• The Sun and other stars follow similar life cycles,
leaving the galaxy enriched with heavy
elements.
• The mass of a star determines its internal
structure and its other properties.
• Gravity and pressure balance each other in a
stable star.
SECTION
Stellar Evolution
29.3
Study Guide
• If the temperature in the core of a star becomes
high enough, elements heavier than hydrogen
can fuse together.
• A supernova occurs when the outer layers of the
star bounce off the neutron star core, and
explode outward.
CHAPTER
Stars
29
Chapter Assessment
The diagram shows a star with a helium
core. At which stage of its life cycle is this
star?
a. main sequence
b. red giant
c. white dwarf
d. helium-carbon
CHAPTER
Stars
29
Chapter Assessment
Which is the outermost layer of the Sun?
a. corona
b. prominence
c. chromosphere
d. photosphere
CHAPTER
29
Stars
Chapter Assessment
What is the difference between absolute
magnitude and apparent magnitude?
Possible answer: Apparent magnitude is how
bright a star appears to be from Earth. Absolute
magnitude takes the star’s distance into account.
CHAPTER
29
Stars
Chapter Assessment
How is parallax used to determine the distance
from Earth to a star?
CHAPTER
29
Stars
Chapter Assessment
Answer: As Earth orbits the Sun, nearby stars
appear to shift position in the sky when compared
with more distant stars. The closer the star, the
greater the shift. By measuring the angle of the
change, astronomers can estimate the distance to
the star.
CHAPTER
29
Stars
Chapter Assessment
What causes the dark bands in a star’s
spectrum?
Answer: The various chemical elements that make
up the star absorb light at specific wavelengths.
This causes dark bands to appear in the star’s
spectrum.
CHAPTER
Stars
29
Standardized Test Practice
Where does the Sun’s energy come from?
a. radioactive decay
b. X-ray emissions
c. fusion reactions
d. nuclear fission
CHAPTER
Stars
29
Standardized Test Practice
What do astronomers measure to determine a
star’s motion relative to Earth’s?
a. wavelength shift
b. absolute magnitude
c. angle of parallax
d. apparent magnitude
CHAPTER
Stars
29
Standardized Test Practice
Which do astronomers use to classify a star?
a. age and size
b. position
c. color and size
d. spectral type
CHAPTER
Stars
29
Standardized Test Practice
At which part of its life cycle is a Sun-sized star
with a carbon core?
a. protostar
b. main sequence
c. beginning stages
d. final stages
CHAPTER
Stars
29
Standardized Test Practice
Which property takes a star’s distance into
account?
a. apparent magnitude
b. absorption spectra
c. absolute magnitude
d. emission spectra