Download CHAPTER 1 The Formation and Structure of Stars

CHAPTER 1
Astronomy: Horizons
10th edition
Michael Seeds
The Formation and
Structure of Stars
CHAPTER 1
The Formation and
Structure of Stars
• In this chapter, you will learn how gravity
creates stars from the thin gas of space and
how nuclear reactions inside stars generate
energy.
• You will learn how the flow of that energy
outward toward the surface of the star
balances gravity and makes the stars stable.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• It is a common misconception to
imagine that space is empty—
a vacuum.
– In fact, if you glance at Orion’s sword on a
winter evening, you can see the Great Nebula in
Orion—a glowing cloud of gas and dust.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Such evidence shows that the space
between the stars is filled with lowdensity gas and dust called the
interstellar medium.
– About 75 percent of the mass of the gas is hydrogen.
– Around 25 percent is helium.
– There are also traces of carbon, nitrogen, oxygen,
calcium, sodium, and heavier atoms.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
– Roughly 1 percent of the mass is made up of
microscopic dust called interstellar dust.
– The dust seems to be made mostly of carbon and
silicates (rocklike minerals) mixed with or coated
with frozen water.
– The average distance between dust grains is
about 150 m.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• How do astronomers know there is an
interstellar medium, and how do they
know its properties?
– In some cases, the interstellar medium is easily visible
as clouds of gas and dust—as in the case of the Great
Nebula in Orion.
– Astronomers call such
a cloud a nebula—from
the Latin word for cloud.
– Such nebulae are visible
evidence of an
interstellar medium.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• There are three kinds of nebulae.
• There are three points to note about
nebulae.
• One, very hot stars can excite clouds of gas
and dust to emit light.
– This reveals that
the clouds contain
mostly hydrogen gas
at very low densities.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Two, where dusty clouds reflect the light of
slightly cooler stars, you see evidence that
the dust in the clouds is made up of very
small particles.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Three, some dense clouds of gas and dust
are detectable only where they are
silhouetted against background regions
filled with stars or bright nebulae.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• If a cloud is less dense, the starlight may be
able to penetrate it, and stars can be seen
through the cloud.
– However, the stars look dimmer because the dust in the
cloud scatters some of the light.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Shorter wavelengths are scattered more
easily than longer wavelengths, so redder
photons are more likely to make it through
the cloud.
• Thus, the stars look slightly redder than
they should—an effect called interstellar
reddening.
– This is the same
process that makes
the setting sun look
redder.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Distant stars are dimmed and reddened by
intervening gas and dust—clear evidence of
an interstellar medium.
– At near-infrared wavelengths, stars are more easily
seen through the dusty interstellar medium because
those longer wavelengths are scattered less often.
– The interstellar medium is very cold—10 to 50 K.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• X-ray observations can detect regions of
very hot gas apparently produced by
exploding stars.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Infrared observations can detect dust in
the interstellar medium.
– Although the dust grains are very small and very cold,
there are huge numbers of grains in a cloud, and each
grain emits infrared radiation..
– Some molecules in the
cold gas emit in the
infrared—so infrared
observations can
detect very cold clouds
of gas.
CHAPTER 1
The Formation and
The Interstellar Medium Structure of Stars
• Ultraviolet observations have been able to
map the distribution of hydrogen in the
interstellar medium.
• Furthermore, radio astronomers can study
the radio emissions of specific molecules in
the interstellar medium.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• The thermal energy in clouds of gas
causes them to resist collapse.
– Temperature is a measure of the motion of the atoms
or molecules in a material—in a hot gas, the atoms
move more rapidly than do those in a cool gas.
– Although the interstellar clouds are very cold, even at
a temperature of only 10 K, the average hydrogen
atom moves about 0.5 km/s (1,100 mph).
– This thermal motion would make the cloud drift apart
if gravity were too weak to hold it together.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Other factors can help a cloud resist its
own gravity.
• Observations show that clouds are
turbulent with currents of gas pushing
through and colliding with each other.
• Also, magnetic fields in clouds may resist
being squeezed.
– The thermal motion of the atoms, turbulence in a
cloud, and magnetic fields resist gravity, and only the
densest clouds are likely to contract.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• The densest interstellar clouds contain
from 103 to 105 atoms/cm3, include from a
few hundred thousand to a few million solar
masses, and have temperatures as low as
10 K.
– In such clouds, hydrogen can exist as molecules (H2)
rather than as atoms.
– These clouds are called molecular clouds, and the
largest are called giant molecular clouds.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Stars form in these clouds when the
densest parts of the clouds become
unstable and contract under the
influence of their own gravity.
– Most clouds, though, do not appear to be
gravitationally unstable and will not contract to
form stars on their own.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Nevertheless, a
stable cloud colliding
with a shock wave
—the astronomical
equivalent of a sonic
boom—can be
compressed and
disrupted into
fragments.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Theoretical
calculations show
that some of these
fragments can
become dense
enough to collapse
under the influence
of their own gravity
and form stars.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Thus, most interstellar clouds may not
collapse and form stars—until they
are triggered by the compression of a
shock wave.
– Happily for this hypothesis, space is filled with
shock waves.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Supernova explosions produce shock
waves that compress the interstellar
medium.
– Recent observations show young stars forming at the
edges of such shock waves.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Another source of shock waves may be
the birth of very hot stars.
– A massive star is so luminous and hot that it emits vast
amounts of ultraviolet photons.
– When such a star is born, the sudden blast of light,
especially ultraviolet radiation, can ionize and drive
away nearby gas—forming a shock wave that could
compress nearby clouds and trigger further star
formation.
– Even the collision of two interstellar clouds can
produce a shock wave and trigger star formation.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Once begun, star formation can spread like
a grass fire.
• Astronomers have found a number of giant
molecular clouds in which stars are forming
in a repeating cycle.
– Both high-mass and low-mass stars form in such a
cloud.
– However, when the massive stars form, their intense
radiation or eventual supernovae explosions push
back the surrounding gas and compress it.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• This compression can trigger the formation
of more stars, some of which will also be
massive.
– Thus, a few massive stars can drive a continuing cycle
of star formation in a giant molecular cloud.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Although low-mass stars do form in
such clouds along with massive stars,
they also form in smaller clouds of gas
and dust.
– However, as they have lower luminosities and do
not develop quickly into supernovae, low-mass
stars alone cannot drive a continuing cycle of
star formation.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• Such a collapsing cloud of gas does
not form a single object.
– Due to instabilities, it fragments—producing 10 to
1,000 stars.
• Stars held in a stable group by their
combined gravity are called a star
cluster.
CHAPTER 1
The Formation of Stars from The Formation and
the Interstellar Medium Structure of Stars
• An association is a group of stars that
are not gravitationally bound to one
another.
– Thus, an association drifts apart in a few million
years.
– The youngest associations are rich in young
stars, including O and B stars ie. the hottest stars.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• To continue the story of star formation,
you need to focus on a single fragment
of a collapsing cloud as it forms a star.
– You might be wondering how the unimaginably
cold gas of an interstellar cloud can heat up to
form a star.
– The answer is gravity.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• Once part of a cloud is triggered to
collapse, gravity draws each atom toward
the center.
• At first, the atoms fall unopposed—they
hardly ever collide with each other.
• Collisions convert the inward velocities of
the atoms into random motions.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• The initial collapse of the gas forms a
dense core of gas.
• As more gas falls in, a warm protostar
develops buried deep in the dusty
gas.
• When a star or protostar changes, it is
said to evolve.
• You can follow that evolution in the H-R
diagram.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• The dusty cloud also hides the protostar
from sight during its contraction.
– If you could see it, it would be a luminous red object a
few thousand times larger than the sun.
– You would plot it in
the red-giant region of
the H-R diagram.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• The hot gas inside the protostar
resists gravity.
• Throughout its contraction, the
protostar converts its gravitational
energy into thermal energy.
– Half of this thermal energy radiates into space.
– The remaining half, though, raises the internal
temperature.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• As the internal temperature climbs, the
collapsing gas becomes ionized—
becoming a mixture of positively charged
atomic nuclei and free electrons.
– When the center gets hot enough, nuclear reactions
begin generating energy, the protostar halts its
contraction, and—having absorbed part of its cocoon
of gas and dust and blown away the rest—it becomes
a stable, main-sequence star.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
• The time a
protostar takes to
contract from a
cool interstellar
gas cloud to a
main-sequence
star depends on
its mass.
– The more massive
the star, the stronger
its gravity and the
faster it contracts.
CHAPTER 1
The Formation and
The Formation of Protostars Structure of Stars
– The sun took about 30
million years to reach
the main sequence.
– In contrast, a 15-solarmass star can contract
in only 160,000 years.
– Conversely, a star of 0.2
solar mass takes 1
billion years to reach the
main sequence.
CHAPTER 1
The Formation and
Observations of Star Formation Structure of Stars
• Unfortunately, a protostar is not easy
to observe.
– The protostar stage is less than 0.1 percent of a
star’s total lifetime.
– Protostars form deep inside clouds of dusty gas
that absorb any light the protostar might emit.
CHAPTER 1
The Formation and
Observations of Star Formation Structure of Stars
• Only when the
protostar is hot
enough to drive
away its
enveloping cloud
of gas and dust
do you see it.
– The birth line in the
H-R diagram shows
where contracting
protostars first
become visible.
CHAPTER 1
The Formation and
Observations of Star Formation Structure of Stars
• Protostars cross
the birth line
shortly before they
reach the main
sequence.
– Thus, the early
evolution of a
protostar is hidden
from sight.
CHAPTER 1
The Formation and
Observations of Star Formation Structure of Stars
• Although astronomers cannot see
protostars at visible wavelengths before
they cross the birth line, you can detect
them in the infrared.
– The dust absorbs light from the protostar and grows
warm.
– Warm dust radiates great amounts of infrared.
– Infrared observations made by orbiting telescopes
reveal many bright sources of infrared radiation that are
protostars buried in dust clouds.
CHAPTER 1
The Formation and
Observations of Star Formation Structure of Stars
– Observations of small globules of gas and
dust within larger nebulae show how star
formation begins.
CHAPTER 1
The Formation and
Fusion in Stars Structure of Stars
• All main-sequence stars fuse hydrogen
into helium to generate energy.
– The sun fuses hydrogen using a chain of reactions—
the proton-proton chain.
– Upper main-sequence stars are more massive than
the sun and can fuse hydrogen more efficiently.
– Also, aging stars fuse fuels other than hydrogen.
CHAPTER 1
The Formation and
The CNO Cycle Structure of Stars
• Main-sequence stars more massive than
the sun fuse hydrogen into helium using the
CNO (carbon–nitrogen–oxygen) cycle.
– This is a hydrogen-fusion
process that uses carbon,
nitrogen, and oxygen as
stepping-stones.
CHAPTER 1
The Formation and
The CNO Cycle Structure of Stars
• The cycle begins with a carbon nucleus,
transforms it first into a nitrogen nucleus,
then into an oxygen nucleus, and then back
to a carbon nucleus.
– The carbon is unchanged
in the end.
– However, along the way,
four hydrogen nuclei are
fused to make a helium
nucleus plus energy, just
as in the proton–proton
chain.
CHAPTER 1
The Formation and
The CNO Cycle Structure of Stars
• The cycle requires a higher temperature
because it begins with a carbon nucleus
combining with a hydrogen nucleus.
– A carbon nucleus has
a charge six times higher
than hydrogen—so the
Coulomb barrier is high.
– Thus, temperatures higher
than 16,000,000 K are
required to make the cycle
work.
CHAPTER 1
The Formation and
The CNO Cycle Structure of Stars
• The center of the sun is not quite hot
enough.
• In stars more massive than about 1.1
solar masses, the cores are hotter.
– Those stars use the CNO cycle and not the
less efficient proton–proton chain.
CHAPTER 1
The Formation and
Heavy-Element Fusion Structure of Stars
• At later stages in the life of a star, when
it has exhausted its hydrogen fuel, it may
fuse other nuclear fuels.
– The ignition of these fuels also requires high
temperatures—because the nuclei have large positive
charges.
– The particles must travel at high velocities—to
overcome the electrostatic repulsion and force the
particles close enough together to react.
– Helium fusion requires a temperature of at least 100
million K.
CHAPTER 1
The Formation and
Heavy-Element Fusion Structure of Stars
• You can summarize the helium-fusion
process in two steps.
4He
+ 4He → 8Be + γ
8Be
+ 4He → 12C + γ
– As a helium nucleus is called an alpha particle, these
reactions are commonly known as the triple-alpha
process.
CHAPTER 1
The Formation and
Heavy-Element Fusion Structure of Stars
• At temperatures above 600,000,000 K,
carbon fuses rapidly in a complex network
of reactions, as depicted in the figure.
– Each arrow represents
a different nuclear
reaction.
CHAPTER 1
The Formation and
Heavy-Element Fusion Structure of Stars
• Reactions at still higher temperatures
can convert magnesium, aluminum,
and silicon into yet heavier atoms.
– These reactions involving heavy elements will
be important in the study of the deaths of
massive stars.
CHAPTER 1
The Pressure–Temperature Thermostat
The Formation and
Structure of Stars
• Nuclear reactions in stars manufacture
energy and heavy atoms under the
supervision of a built-in thermostat that
keeps the reactions from erupting out of
control.
– That thermostat is the relation between gas
pressure and temperature.
CHAPTER 1
The Pressure–Temperature Thermostat
The Formation and
Structure of Stars
• In a star, nuclear reactions generate just
enough energy to balance the inward pull
of gravity.
• Consider what would happen if the
reactions produced too much energy.
– As the star balances gravity by generating energy,
the extra energy would force it to expand.
– The expansion would lower the central temperature
and density and slow the nuclear reactions until the
star regained stability.
CHAPTER 1
The Pressure–Temperature Thermostat
The Formation and
Structure of Stars
• Thus, the star has a built-in regulator that
keeps the nuclear reactions from occurring
too rapidly.
• The same thermostat keeps the reactions
from dying down.
– Suppose the nuclear reactions began making too little
energy.
– Then, the star would contract slightly—increasing the
central temperature and density and increasing the
nuclear energy generation.
CHAPTER 1
The Pressure–Temperature Thermostat
The Formation and
Structure of Stars
• The stability of a star depends on
the relation between gas pressure
and temperature.
• Nuclear fusion at the centers of stars
heats their interiors, creates high gas
pressures, and thus balances the inward
forces of gravity.
– If an increase or decrease in temperature
produces a corresponding change in pressure,
then the thermostat is functioning correctly—
and the star is stable.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
• When you think about a star, it is
helpful to think of it as if it were made
up of layers.
– The weight of each layer must be supported by the
layer below.
– The deeper layers must support the weight of all the
layers above.
– As the inside of a star is made up of gas, the weight
pressing down on a layer must be balanced by the
gas pressure in the layer.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
• If the pressure is too low, the weight from
above will compress the layer.
• If the pressure is too high, the layer will
expand and lift the layers above.
– This balance between weight and pressure is called
hydrostatic equilibrium.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
• The figure shows this
hydrostatic balance in
the imaginary layers of
a star.
– The weight pressing down on
each layer is shown by
lighter red arrows—which
grow larger with increasing
depth because the weight
grows larger.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
– The pressure in each layer
is shown by darker red
arrows—which must grow
larger with increasing depth
to support the weight.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
• The pressure in a gas depends on the
temperature and density of the gas.
– Near the surface, there is little weight pressing
down.
– So, the pressure need not be high.
– Deeper in the star, the pressure must be higher.
– That means that the temperature and density of
the gas must also be higher.
CHAPTER 1
The Formation and
Hydrostatic Equilibrium Structure of Stars
• Hydrostatic equilibrium informs you that
stars have to be hot inside to maintain the
pressure needed to support their own
weight.
– The layers are hot because energy flows
outward from the core of the star, where it is
made by nuclear fusion.
• Hydrostatic equilibrium is closely related
to the pressure–temperature thermostat.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• The law of energy transport states
that energy must flow from hot
regions to cooler regions by
conduction, convection, or radiation.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• Conduction is the most familiar form of heat
flow.
• If you hold the bowl of a metal spoon in a
candle flame, the handle of the spoon
grows warmer.
– Heat, in the form of motion
among the molecules of the
spoon, is conducted from
molecule to molecule up the
handle—until the molecules
under your fingers begin to
move faster and you sense
heat.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• Thus, conduction requires close
contact between the molecules.
– As the particles (atoms) in most stars are not in
close contact, conduction is unimportant.
– Conduction is significant, though, in white dwarfs,
which have tremendous internal densities.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• Energy transport by radiation is another
familiar experience.
• If you put your hand beside a candle flame,
you can feel the heat.
– What you feel are infrared photons
radiated by the flame.
– As photons are packets of energy,
your hand grows warm as it absorbs
them.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• Radiation is the principal means
of energy transport in the sun’s
interior.
– Photons are absorbed and reemitted in
random directions over and over as they
work their way outward through the radiative
zone.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• This heat-driven circulation of a fluid is
convection, the third way energy can
move in a star.
– The rising wisp of smoke above
a candle flame is carried by convection.
– Energy is carried upward in convection
currents as rising hot gas.
– Energy flows outward as convection
in the sun’s outer layers—its
convective zone.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• Convection is important in stars both
because it carries energy and because
it mixes the gas.
– Convection currents flowing through the layers of a
star tend to homogenize the gas—giving it a uniform
composition throughout the convective zone.
– This mixing constantly refreshes the fuel supply of the
nuclear reactions—just as the stirring of a campfire
makes it burn more efficiently.
CHAPTER 1
The Formation and
Energy Transport Structure of Stars
• The four laws of stellar structure can
inform you of how stars are born, how
they live, and how they die.
CHAPTER 1
The Formation and
Main-Sequence Stars Structure of Stars
• When a contracting protostar begins to fuse
hydrogen, it stops contracting and becomes
a stable main-sequence star.
– The most massive stars are
so hot they light up
the remaining nearby gas
in a beautiful
nebula—
as if
announcing
their birth.
CHAPTER 1
The Formation and
Main-Sequence Stars Structure of Stars
• However, that gas is quickly blown
away—and the stars begin their long,
uneventful lives as main-sequence
stars.
– If you discount the peculiar white dwarfs, then
90 percent of all stars are main-sequence
stars.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• Observations of the temperature and
luminosity of stars show that main-sequence
stars obey a simple rule—the more massive
a star is, the more luminous it is.
– This rule, the mass–luminosity relation, is the key to
understanding the stability of main-sequence stars.
– In fact, the mass–luminosity relation is predicted by
theories of stellar structure—giving astronomers direct
observational confirmation of those theories.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• To understand the mass–luminosity
relation, you must consider the law of
hydrostatic equilibrium, which states that
pressure balances weight, and the
pressure–temperature thermostat, which
regulates energy production.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• A star that is more massive than the
sun has more weight pressing down
on its interior.
– So, the interior must have a high pressure to
balance that weight.
– That means the massive star’s automatic
pressure–temperature thermostat must keep the
gas in its interior hot and the pressure high.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• A star less massive than the sun has
less weight on its interior and thus
needs less internal pressure.
– So, its pressure–temperature thermostat is set
lower.
– Massive stars are more luminous because they
must support more weight by making more
energy.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• This shows you why the main sequence
must have a lower end.
• Stars with masses below about 0.08 solar
mass cannot raise their central temperature
high enough to begin hydrogen fusion.
– Called brown dwarfs, they are only about a dozen
times larger than Earth.
– Although they are still warm from contraction, they do
not generate energy by hydrogen fusion.
– They have contracted as far as they can and are
slowly cooling off.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• Brown dwarfs fall in the gap between lowmass M stars and massive planets like
Jupiter.
– They would look red to your eyes.
– However, they emit most of their energy in the
infrared.
• The warmer brown dwarfs fall in spectral
class L and the cooler in spectral class T.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• Molecules can form in the atmospheres of
red dwarfs.
• Brown dwarfs, however, are so cool that
solid particles of silicates, metals, and other
minerals can condense in cloud layers.
– Unlike stars, brown dwarfs appear to have
weather.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• As they are so small and cool, brown
dwarfs have very low luminosities and
are difficult to find.
– Nevertheless, a few hundred are known.
– They may be as common as M stars.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• The evidence shows that nature does
indeed make brown dwarfs when a
forming star does not have enough mass
to begin hydrogen fusion.
• This observational detection of objects at
the lower end of the main sequence
further confirms the theories of stellar
structure.
CHAPTER 1
The Formation and
The Mass–Luminosity Relation Structure of Stars
• Now that you know how stars form
and how they maintain their stability
through the mass–luminosity
relation, you can predict the
evolution of main-sequence stars.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• While a star is on the main sequence, it is
stable—so you might think its life would be
totally uneventful.
• However, a main-sequence star balances
its gravity by fusing hydrogen.
• As the star gradually uses up its fuel, that
balance must change.
– Thus, even stable main-sequence stars are changing
as they consume their hydrogen fuel.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• You have learned that hydrogen fusion
combines four nuclei into one.
• Thus, as a main-sequence star fuses its
hydrogen, the total number of particles in
its interior decreases.
– Each newly made helium nucleus can exert the same
pressure as a hydrogen nucleus.
– However, as the gas contains fewer nuclei, its total
pressure is less.
– This unbalances the gravity–pressure stability, and
gravity squeezes the core of the star more tightly.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• As the core contracts, its temperature
increases and the nuclear reactions run
faster—releasing more energy.
– This additional energy flowing outward through the
envelope forces the outer layers to expand.
– As the star becomes larger, it becomes more
luminous.
– Eventually, the expansion begins to cool the surface.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• Due to these
gradual changes
in mainsequence stars,
the main
sequence is not
a sharp line
across the H–R
diagram, but
rather a band.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• Stars begin their
stable lives
fusing hydrogen
on the lower
edge of the band,
which is known
as the zero-age
main sequence
(ZAMS).
– Gradual changes in
luminosity and
surface temperature
move them upward
and slightly to the
right.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
– By the time they reach
the upper edge of the
main sequence, they
have exhausted nearly
all the hydrogen in their
centers.
– Thus, you find mainsequence stars
scattered throughout
the band at various
stages of their mainsequence lives.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• The sun is a typical main-sequence star.
• As it undergoes these gradual changes,
Earth will suffer.
– When the sun began its main-sequence life about 5
billion years ago, it was only about 70 percent as
luminous as it is now.
– By the time it leaves the main sequence in another 5
billion years, it will have twice its present luminosity.
– Long before that, its rising luminosity will raise Earth’s
average temperature, melt the polar caps, modify
Earth’s climate, and ultimately drive away the oceans
and atmosphere.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• Life on Earth will probably not
survive these changes in the sun.
– Nevertheless, you have a billion years or
more to prepare.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• The average star spends 90 percent of
its life on the main sequence.
• This explains why 90 percent of all true
stars are main-sequence stars.
– You are most likely to see a star during that long,
stable period while it is on the main sequence.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• The number of years a star spends on
the main sequence depends on its mass.
– Massive stars consume fuel rapidly and live short lives.
– For example, a 25-solar-mass star will exhaust its
hydrogen and die in only about 7 million years.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
– In contrast, low-mass stars conserve their fuel and
shine for billions of years.
– The sun has enough fuel to last about 10 billion years.
– The red dwarfs, although they have little fuel, use it up
very slowly and may be able to survive for 100 billion
years or more.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• Nature makes more low-mass stars
than high-mass stars.
• This fact, however, is not sufficient to
explain the vast numbers of low-mass
stars that fill the sky.
– The additional factor is stellar lifetime.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
• As low-mass stars live long lives, there
are more of them than massive stars.
– In the figure, the
lower-main-sequence
stars are much more
common than the
massive O and
B stars.
CHAPTER 1
The Formation and
The Life of a Main-Sequence Star Structure of Stars
– The main-sequence K and M stars are so faint they are
difficult to locate.
– However, they are very
common.
– The O and B stars are
luminous and easy to
locate.
– Due to their fleeting lives,
though, there are never
more than a few visible
on the main sequence
at any one time.
CHAPTER 1
The Formation and
Building Scientific Arguments Structure of Stars
• Observational evidence shows that
the luminosity of a main-sequence
star depends on its mass.
– The more massive a star is, the more
luminous it is.
CHAPTER 1
The Formation and
Building Scientific Arguments Structure of Stars
• You can understand that if you
remember that a star supports itself by
being hot inside.
– The high temperature produces high internal
pressure—which balances the weight of the
layers pressing inward.
– That is called hydrostatic equilibrium.
CHAPTER 1
The Formation and
Building Scientific Arguments Structure of Stars
• A massive star has a stronger
gravitational field and it has more mass
to support.
– So, the pressure needed to keep it from collapsing is
very high.
– That means the internal temperature must be high,
and that means the star must generate energy
rapidly.
– All that energy—leaking outward through the surface
of the star—makes the star more luminous.