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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 12
The Lives of the Stars from Birth
Through Middle Age
WHAT DO YOU THINK?
1.
2.
3.
4.
How do stars form?
Are stars still forming today? If so,
where?
Do more massive stars shine longer than
less massive ones? What is your
reasoning?
When stars like the Sun stop fusing
hydrogen and helium in their cores, do
the stars get smaller or larger?
In this chapter you will discover…
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how stars form
what a stellar “nursery” looks like
how astronomers use the physical properties of stars to
learn about stellar evolution
the remarkable transformations of older stars into giants
how the Hertzsprung-Russell (H-R) diagram is your
guide to the stellar life cycle
how pairs of orbiting stars change each other
(a) 1935
Everything Ages
(b) 1994
Stars and the Interstellar Medium
This open cluster,
called the Pleiades,
can easily be seen
with the naked eye in
the constellation
Taurus (the Bull). The
blue glow surrounding
the stars of the
Pleiades is a reflection
nebula created as
some of the stars’
radiation scatters off
preexisting dust grains
in their vicinity (a
reflection nebula).
Stars and the Interstellar Medium
The same region of
the sky in a falsecolor infrared .
Image taken by the
Spitzer Space
Telescope. Gases are
seen here to exist in
more areas than can be
detected in visible light.
We see an emission nebula via:
A. reflected blue light from a nearby star or
stars.
B. blue light emitted by hot (excited)
hydrogen atoms.
C. red light emitted by hot (excited)
hydrogen atoms.
D. reflected red light from a nearby star.
Stars and the Interstellar Medium
H-R diagram for 500 stars in
the Pleiades . Most of the
cool, low-mass stars have
arrived at the main
sequence, indicating that
hydrogen fusion has begun
in their core. The cluster has
a diameter of about 5 ly, is
about 100 million years old.
A Connection to Interstellar Space
The charred layer created by overcooking this beef
contains compounds of carbon and hydrogen, called
polycyclic aromatic hydrocarbons. These molecules
are also found in interstellar clouds.
Interstellar Reddening
Dust in interstellar space scatters more short-wavelength (blue)
light passing through it than longer-wavelength colors.
Therefore, stars and other objects seen through interstellar
clouds appear redder than they would otherwise.
Interstellar Reddening
Light from these two nebulae pass through different amounts of
interstellar dust and therefore they a have different colors.
Because NGC 3603 is farther away, its color is completely
dominated by the H line, while NGC 3576 has some H
A Dark Nebula
The dark nebula Barnard 86 is located in Sagittarius. It is
visible in this photograph simply because it blocks out light
from the stars beyond it. The bluish stars to the left of the dark
nebula are members of a star cluster called NGC 6520.
A Gas- and Dust-Rich Region of Orion
Giant molecular clouds
in Orion and Monoceros
as seen in the radio part
of the spectrum. The
intensity of carbon
monoxide (CO)
emission is displayed by
colors in the order of the
rainbow, from violet for
the weakest to red for
the strongest. Black
indicates no detectable
emission.
A Gas- and Dust-Rich Region of Orion
A variety of nebulae appear in the sky around
Alnitak, the easternmost star in the belt of Orion.
To the left of Alnitak is a bright, red emission
nebula called NGC 2024. The glowing gases in
emission nebulae are excited by UV radiation
from young, massive stars. Dust grains obscure
part of NGC 2024, giving the appearance of black
streaks, while the distinctively shaped dust cloud,
called the Horsehead Nebula, blocks the light
from the background nebula IC 434. The
Horsehead Nebula is part of a larger complex of
dark interstellar matter, seen in the lower left of
this image. Above and to the left of the Horsehead
Nebula is the reflection nebula NGC 2023, whose
dust grains scatter blue light from stars between
us and it more effectively than any other color. All
of this nebulosity lies about 1600 ly from Earth,
while the star Alnitak is only 815 ly away from us.
A Supernova Remnant
(a) X-ray image of the Cygnus Loop, the remnant of a supernova that occurred nearly
20,000 years ago. The expanding spherical shell of gas now has a diameter of about
120 ly.
(b) This visible-light Hubble Space Telescope image of part of the Cygnus Loop shows
emission from different atoms false-color-coded with blue from oxygen, red from
sulfur, and green from hydrogen.
Core of the Rosette Nebula – Sweeping Dust
Protostar in a Bok Globule
(a) This visible-light image shows a small dark nebula
(equivalently, Bok globule) . (b) When viewed in the
infrared, a protostar is visible within the nebula.
Protostars are not seen in visible light
telescopes because:
A. they don’t emit any radiation
B. they are surrounded by clouds of gas
and dust
C. they only emit infrared radiation
D. they are all moving away from Earth so
fast that their visible light is Doppler shifted
into the infrared
A Cluster of Protostars – Same Technique
Pre–Main-Sequence Stars
Seen in infrared, the two
large bright objects in the
center of this image are
pre–main-sequence stars.
They have recently shed
their cocoons of gas and
dust but still have strong
stellar winds that create
their irregular shapes. The
two stars are an optical
double; that is, they are
not orbiting each other.
Summary so far
Nebulae containing gas and dust are
plentiful
 Protostars and stars are seen in such
nebulae
 Star formation is ongoing
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A Brown Dwarf - a “Failed Star”
Gliese 229B was the first confirmed brown dwarf ever observed. With a
surface temperature of about 1000 K, its spectrum is similar to that of Jupiter.
Gliese 229B is in orbit around its companion Gliese 229A on the left. The two
bodies are separated by about 43 AU. Gliese 229B has from 20 to 50 times
the mass of Jupiter, but the brown dwarf is compressed to the same size as
Jupiter. The spike is not real – it stems from an electronics overload .
A brown dwarf is best described as:
A. a low mass object that doesn’t fuse in its
core
B. a low mass main sequence star
C. a high mass main sequence star
D. an object of dust too small to classify as
a planet
Pre–Main-Sequence Evolutionary Tracks
This H-R diagram shows evolutionary tracks based on models of seven
stars having different masses. The dashed lines indicate the stage
reached after the indicated number of years of evolution. The birth line,
shown in blue, is the location where each protostar stops accreting matter
and becomes a pre–main-sequence star..
A Stellar Nursery Full of Brown Dwarfs
Besides containing more than 100 young stars, the rho Ophiuchi cloud,
located 540 ly away in the constellation Ophiuchus, contains at least 30
brown dwarfs. By studying these objects, astronomers expect to learn more
about early stellar evolution. This infrared image is color coded, with red
indicating 7.7-µm radiation and blue indicating 14.5-µm radiation.
Mass Loss from a Supermassive Star
Within the Quintuplet Cluster is one of the brightest known stars, called the Pistol.
Astronomers calculate that the Pistol formed nearly 3 million years ago and
originally had 100–200 solar masses. The structure of the gas cloud suggests the
star ejected the gas we see in two episodes, 6000 and 4000 years ago. The gas
from any previous ejections is so thinly spread now that we cannot see it. The
nebula shown in the inset is more than 4 ly (1.25 pc) across—it would stretch
from the Sun nearly to the closest star, Proxima Centauri. The image of the
Quintuplet Cluster was taken in the infrared. The name Pistol was given to the
star based on early, low-resolution radio images of its gas, which initially looked
like an old-fashioned pistol aimed to the left near the top of the inset.
Mass Loss from a Supermassive Star
The largest, most massive known star, LBV 1806-20, is 5 million times
brighter and apparently some 150 times more massive than the Sun. This
drawing shows the star’s color and its size compared to the Sun.
An H II Region
The Eagle Nebula, M16, surrounds a star cluster. Star formation is presently
occurring in M16. Several bright, hot O and B stars are responsible for the
ionizing radiation that causes the gases to glow. Inset: Star formation is
occurring inside these dark pillars of gas and dust. Intense ultraviolet radiation
from existing massive stars off to the right of this image is evaporating the
dense cores in the pillars, thereby prematurely terminating star formation
there. Newly revealed stars are visible at the tips of the columns.
The Orion Nebula
The middle “star” in Orion’s sword is
actually the Orion Nebula, part of a
huge system of interstellar gas and
dust in which new stars are now
forming. This nebula’s mass is about
300 solar masses.
Left inset: This view at visible
wavelengths shows the inner
regions of the Orion Nebula. At
the lower left are four massive
stars, the brightest members of
the Trapezium star cluster, which
cause the nebula to glow.
Right inset: This view shows numerous
infrared objects—many of which are stars in
the early stages of formation—along with
shock waves caused by matter flowing out of
protostars faster than the speed of sound
waves in the nebula. Shock waves from the
Trapezium stars may have helped trigger the
formation of the protostars in this view.
The Evolution of an OB Association
High-speed particles and ultraviolet radiation from young O and B stars produce a
shock wave that compresses gas farther into the molecular cloud, stimulating new
star formation deeper in the cloud. Meanwhile, older stars are left behind. Inset:
Stars forming around a massive star 2500 ly away in the constellation Monoceros’s
Cone Nebula. The stars (small dots on the right side of the inset) arrayed around
the bright, massive central star are believed to have formed as a result of the
central star compressing surrounding gas with high-speed particles and radiation.
The younger stars are just 0.04–0.08 ly from the central star.
Plotting the Ages of Stars
This photograph shows a
region of ionized hydrogen
and the young star cluster
NGC 2264 in the
constellation Monoceros.
The red nebulosity is located
about 2600 ly from Earth
and contains numerous stars
that are about to begin
hydrogen fusion in their
cores.
Plotting the Ages of Stars-Cluster Only 2 Million yr
Each dot plotted on this HR diagram represents a
star in NGC 2264 whose
luminosity and surface
temperature have been
measured. Note that most
of the cool, low-mass stars
have not yet arrived at the
main sequence.
Calculations of stellar
evolution indicate that this
star cluster started forming
about 2 million years ago.
Why are A-type main sequence stars hotter
than G-type main sequence stars?
A. A-type stars have cores of metal, whereas
G-type stars do not
B. A-type stars have more fusion on their
surface than G-type stars
C. A-type stars have more fusion in their
cores than G-type stars
D. A-type stars fuse in their cores and near
their surfaces, while G-type stars only fuse
in their cores.
A Summary of the Star Formation Process
Some Differences From Sun: Fully Convective Star
This drawing shows how the helium created in the cores of
red dwarfs rises into the outer layers of the star by convection,
while the hydrogen from the outer layers descends into the
core. This process continues until the entire star is helium.
The star is on the main sequence
It fuses hydrogen to helium, just as our
Sun does. (Some stars have a variation.)
 It spends 80-90% of its lifetime on main
sequence.
 It very slowly brightens.
 … then life gets exciting
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Evolution of Stars Off the Main Sequence
(a) Hydrogen fusion occurs in the core of main-sequence stars. (b) When the
core is converted into helium, fusion there ceases and then begins in a shell
that surrounds the core. The star expands into the giant phase. This newly
formed helium sinks into the core, which heats up. (c) Eventually, the core
reaches 108 K, whereupon core helium fusion begins. This activity causes the
core to expand, slowing the hydrogen shell fusion and thereby forcing the
outer layers of the star to contract.
What is Helium Fusion
+ 4He + 4He  12C Three Heliums fuse to Carbon
4He + 12C  16O Some of the C picks up one more He
It takes 3 He’s: two of them won’t create any energy, no
matter how you fuse them
It takes a temperature of 100 million K
It begins with a spike for low-mass stars
 4He
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A Mass-Loss Star
A red giant star is shedding its outer layers, thereby creating this
reflection nebula, labeled IC 2220 and called Toby Jug, located
in the constellation Carina. The star is embedded inside the
nebula and is not visible in this image.
Red giants burn helium via nuclear
fusion in their core. The ash (end
product) of this nuclear fusion is:
A.
B.
C.
D.
iron.
hydrogen.
lithium and carbon.
carbon and oxygen.
The Sun Today and as a Giant
In about 5 billion years, when the Sun expands to become a giant, its diameter will
increase a hundredfold from what it is now, while its core becomes more compact.
Today, the Sun’s energy is produced in a hydrogen-fusing core whose diameter is
about 200,000 km. When the Sun becomes a giant, it will draw its energy from a
hydrogen-fusing shell that surrounds a compact helium-rich core. The helium core
will have a diameter of only 30,000 km. The Sun’s diameter will be about 100 times
larger, and it will be about 2000 times more luminous as a giant than it is today.
Red Giant Stars
This composite of visible and infrared images shows red
giant stars in the open cluster M50 in the constellation
of Monoceros (the Unicorn).
Post–Main-Sequence Evolution
The luminosity of the Sun changes as our star evolves. It began as a
protostar with decreasing luminosity. On the main sequence today, it
gradually brightens. Giant-phase evolution occurs more rapidly, with
faster and larger changes of luminosity. Note the change in scale of
the horizontal axis scale at 12 billion years.
Post–Main-Sequence Evolution
Model-based
evolutionary tracks of
five stars are shown on
this H-R diagram. In
the high-mass stars,
core helium fusion
ignites smoothly where
the evolutionary tracks
make a sharp turn
upward into the giant
region of the diagram.
The Instability Strip
The instability strip
occupies a region between
the main sequence and the
giant branch on the H-R
diagram. A star passing
through this region along its
evolutionary track becomes
unstable and pulsates.
Analogy for Cepheid Variability
(a) As pressure builds up in this pot, the force on the lid (analogous to a
Cepheid’s outer layers) increases. (b) When the pressure inside the pot is
sufficient, it lifts the lid off (expands the star’s outer layers) and thereby
allows some of the energy inside to escape. This process cycles (two
cycles are shown here), as do the luminosity and temperature of Cepheid
stars.
The Period-Luminosity Relation for Cepheids
The period of a Cepheid variable is directly related to its average luminosity:
The more luminous the Cepheid, the longer its period and the slower its
pulsations. Type I Cepheids (δ Cephei stars) are brighter, more massive, and
more metal-rich stars than Type II Cepheids. The greater brightness of the
Type I Cepheids is a result of their higher mass.
A Globular Cluster
This cluster, M10, is about 85 ly across and is located in the constellation
Ophiuchus (the Serpent Holder), roughly 16,000 ly from Earth. Most of the
stars here are either red giants or blue horizontal-branch stars with both
core helium fusion and hydrogen shell fusion.
An H-R Diagram of a Globular Cluster
Each dot on this graph represents the absolute magnitude and surface
temperature of a star in the globular cluster M55. Note that the upper
half of the main sequence is missing. The horizontal-branch stars are
stars that recently experienced the helium flash in their cores and now
exhibit core helium fusion and hydrogen shell fusion.
Mass, Temperature, Luminosity, and Lifetime
1. High-mass stars consume their fuel MUCH faster.
2. The age of a cluster can be determined by examining an
H-R diagram of its stars.
Structure of the H-R Diagram
and Main Sequence Turnoff
The black bands indicate where data from various star clusters fall on the H-R
diagram. The ages of turnoff points (in years) are listed in red alongside the main
sequence. The age of a cluster can be estimated from the location of the turnoff point,
where the cluster’s most massive stars are just now leaving the main sequence.
The Evolution of a Theoretical Cluster of 100 Stars
Spectra of a Metal-Poor and a Metal-Rich Star
These spectra compare (a) a metal-poor (Population II) and (b)
a metal-rich (Population I) star (the Sun) of the same surface
temperature. Numerous spectral lines prominent in the solar
spectrum are caused by elements heavier than hydrogen and
helium. Note that corresponding lines in the metal-poor star’s
spectrum are weak or absent. Both spectra cover a wavelength
range that includes two strong hydrogen absorption lines,
labeled Hγ (434 nm) and Hδ (410 nm).
Detached, Semidetached, Contact, and Over-Contact Binaries
(a) In a detached binary, neither star fills its Roche lobe. (b) If one star fills
its Roche lobe, the binary is semidetached. Mass transfer is often observed
in semidetached binaries. (c) In a contact binary, both stars fill their Roche
lobes. (d) The two stars in an over-contact binary both overfill their Roche
lobes. The two stars actually share the same outer atmosphere.
Three Close Binaries
Sketches of and light curves for three
eclipsing binaries are shown. The phase
denotes the fraction of the orbital period
from one primary minimum to the next.
(a) Algol, also known as β Persei, is a
semidetached binary. The deep eclipse
occurs when the giant star (right) blocks
the light from the smaller, but more
luminous, main-sequence star. (b) β
Lyrae is a semidetached binary in which
mass transfer has produced an accretion
disk that surrounds the detached star.
This disk is so thick and opaque that it
renders the secondary star almost
invisible. (c) W Ursae Majoris is an overcontact binary. Both stars therefore share
their outer atmospheres. The short, 8-h
period of this binary indicates that the
stars are very close to each other.
Mass Exchange Between Close Binary Stars
This sequence of drawings shows how close binary stars
can initially be isolated but, as they age, grow and
exchange mass. Such mass exchange leads to different
fates than if the same stars had evolved in isolation.
Summary of Key Ideas
Protostars and Pre–Main-Sequence Stars
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Enormous, cold clouds of gas and dust, called giant
molecular clouds, are scattered about the disk of the
Galaxy.
Star formation begins when gravitational attraction
causes clumps of gas and dust, called protostars, to
coalesce in Bok globules within a giant molecular cloud.
As a protostar contracts, its matter begins to heat and
glow. When the contraction slows down, the protostar
becomes a pre–main-sequence star. When the pre–
main-sequence star’s core temperature becomes high
enough to begin hydrogen fusion and stop contracting, it
becomes a main-sequence star.
Protostars and Pre–Main-Sequence Stars
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The most massive pre–main-sequence stars take the
shortest time to become main-sequence stars (O and B
stars).
In the final stages of pre–main-sequence contraction,
when hydrogen fusion is about to begin in the core, the
pre–main-sequence star may undergo vigorous
chromospheric activity that ejects large amounts of
matter into space. G, K, and M stars at this stage are
called T Tauri stars.
A collection of a few hundred or a few thousand newborn
stars formed in the plane of the Galaxy is called an open
cluster. Stars escape from open clusters, most of which
eventually dissipate.
Main-Sequence and Giant Stars
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The Sun has been a main-sequence star for 4.6 billion years
and should remain so for about another 5 billion years. Less
massive stars than the Sun evolve more slowly and have
longer main-sequence lifetimes. More massive stars than the
Sun evolve more rapidly and have shorter main-sequence
lifetimes.
Main-sequence stars with a solar mass between 0.08 and 0.4
convert all of their mass into helium and then stop fusing.
Their lifetimes last hundreds of billions of years, so none of
these stars has yet left the main sequence.
Core hydrogen fusion ceases when hydrogen in the core of a
main-sequence star with a solar mass greater than 0.4 is
gone, leaving a core of nearly pure helium surrounded by a
shell where hydrogen fusion continues. Hydrogen shell fusion
adds more helium to the star’s core, which contracts and
becomes hotter. The outer atmosphere expands considerably,
and the star becomes a giant.
Main-Sequence and Giant Stars
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When the central temperature of a giant reaches about
100 million K, the thermonuclear process of helium
fusion begins. This process converts helium to carbon,
then to oxygen. In a massive giant, helium fusion begins
gradually. In a less massive giant, it begins suddenly in a
process called helium flash.
The age of a stellar cluster can be estimated by plotting
its stars on an H-R diagram. The upper portion of the
main sequence disappears first, because more massive
main-sequence stars become giants before low-mass
stars do.
Giants undergo extensive mass loss, sometimes
producing shells of ejected material that surround the
entire star.
Relatively young stars are metal rich (Population I);
ancient stars are metal poor (Population II).
Clusters of Stars
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Groups of between a few hundred and a few thousand
stars, formed together from a single interstellar cloud in
the disk of our Galaxy, are called open clusters.
Star in open clusters go their separate ways.
Groups of hundreds of thousands to millions of stars
formed together from a common interstellar cloud are
called globular clusters.
Stars in globular clusters remain bound together.
Variable Stars
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When a star’s evolutionary track carries it through a
region called the instability strip in the H-R diagram, the
star becomes unstable and begins to pulsate.
RR Lyrae variables are low-mass, pulsating variables
with short periods. Cepheid variables are high-mass,
pulsating variables exhibiting a regular relationship
between the period of pulsation and luminosity.
Mass can be transferred from one star to another in
close binary systems. When this occurs, the evolution of
the two stars changes.
Key Terms
accretion disk
birth line
Bok globule
brown dwarf
Cepheid variable
contact binary
core helium fusion
dark nebulae
dense core
detached binary
electron degeneracy
pressure
emission nebula
evolutionary track
giant molecular
cloud
globular cluster
H II regions
helium flash
horizontal-branch star
hydrogen shell fusion
instability strip
interstellar extinction
interstellar medium
interstellar reddening
Jeans instability
molecular cloud
nebula (plural nebulae)
OB association
open cluster
over-contact binary
Pauli exclusion principle
period-luminosity relation
Population I star
Population II star
pre–main-sequence
star
protostar
red dwarf
reflection nebula
Roche lobe
RR Lyrae variable
semidetached binary
supernova remnant
T Tauri stars
turnoff point
Type I Cepheid
Type II Cepheid
variable stars
zero-age main sequence
(ZAMS)
WHAT DID YOU THINK?
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How do stars form?
Stars form from the collective gravitational
attraction of a clump of gas and dust inside a
giant molecular cloud.
WHAT DID YOU THINK?
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Are stars still forming today? If so, where?
Yes. Astronomers have seen stars that have just
arrived on the main sequence, as well as
infrared images of gas and dust clouds in the
process of forming stars. Most stars in the Milky
Way form in giant molecular clouds in the disk of
the Galaxy.
WHAT DID YOU THINK?
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Do more massive stars shine longer than less
massive ones? What is your reasoning?
No. Lower-mass stars last longer because the
lower gravitational force inside them causes
fusion to take place at slower rates compared to
the fusion inside higher-mass stars. These latter
stars therefore use up their fuel more rapidly
than do lower mass stars.
WHAT DID YOU THINK?


When stars like the Sun stop fusing hydrogen
and helium in their cores, do the stars get
smaller or larger?
They get larger. Such stars start fusing hydrogen
and helium outside their cores. This new fusion,
closer to the star’s surface, is able to push the
star’s outer layers out farther than they had been
before.