Download 12-1 MAIN-SEQUENCE STARS

CHAPTER12
STELLAR EVOLUTION
12-1 MAIN-SEQUENCE STARS
What happens as a star uses up its hydrogen?
Astronomers compute stellar models of the interiors of stars based on four simple laws of
stellar structure. Two of the laws are the conservation of mass law and the conservation of
energy law. The third, the law of hydrostatic equilibrium, says that the star must balance the
weight of its layers by its internal pressure. The fourth law says that energy can flow outward
only by conduction, convection, or radiation.
Mathematical stellar models show how rapidly a star uses its fuel in each layer, and that
allows astronomers to step forward in time and follow the evolution of the star as it ages.
There is a main sequence because stars support their weight by hydrogen fusion. As energy
flows outward, it heats the gas of the star, and pressure in the gas balances the inward pull of
gravity.
The mass–luminosity relation is explained by the requirement that a star support the weight of
its layers by its internal pressure. The more massive a star is, the more weight it must support,
and the higher its internal pressure must be. To keep its pressure high, it must be hot and
generate large amounts of energy. Thus, the mass of a star determines its luminosity.
Extremely massive stars are quite rare. Contracting gas clouds tend to fragment and produce
pairs, groups, or clusters of stars and not extremely massive stars. Also, massive stars tend to
blow matter away in strong stellar winds, and that rapidly reduces their mass.
St
a
r
sl
e
s
sma
s
s
i
v
et
h
a
n0.
08s
ol
a
rma
s
sc
a
n
’
tg
e
th
ote
n
ought
of
u
s
ehy
dr
og
e
n
.Obj
e
c
t
sl
e
s
s
massive than this become brown dwarfs and slowly cool as they radiate away their thermal
energy. Many brown dwarfs have been observed.
The zero-age main sequence is the line in the H–R diagram where contracting stars begin
fusing hydrogen and reach stability. As a star ages, it moves slightly upward and to the right,
making the main sequence a band.
Massive stars, having more weight to support, reach stability higher on the main sequence
than do lower-mass stars.
The life expectancy of a main-sequence star depends on its mass. The more massive stars use
their fuels rapidly and remain on the main sequence only a few million years. The sun will last
a total of about 10 billion years, and the least massive stars could survive for over 100
12-2 POST-MAIN-SEQUENCE EVOLUTION
What happens when a star exhausts its hydrogen?
When a main-sequence star exhausts its hydrogen, its core contracts, and it begins to fuse
hydrogen in a shell around its core. The outer parts of the star—its envelope—swell, and the
star becomes a giant. Because of this expansion, the surface of the star cools, and it moves
toward the right in the H–R diagram. The most massive stars move across the top of the
diagram as supergiants.
Because the core of a giant star contracts and the envelope expands, the nuclear fusion is
confined to a very small volume at the center of the low-density star.
When the core of the star becomes hot enough, helium fusion, known as the triple alpha
process, begins first in the core and then in a shell. This causes the star to describe a loop in
the giant region of the H–R diagram.
If the matter in the core becomes degenerate before helium ignites, the pressure of the gas
does not depend on its temperature, and when helium ignites, the core explodes in the helium
flash. Although the helium flash is violent, the star absorbs the extra energy and quickly
brings the helium-fusing reactions under control.
Helium fusion produces carbon and oxygen. Other nuclear reactions, which are not important
sources of energy, slowly cook the matter during helium fusion and produce small amounts of
heavy elements.
If a star is massive enough, it can ignite carbon and other fuels after helium fusion. Each fuel
fuses more rapidly, producing heavier and heavier elements.
12-3 EVIDENCE OF EVOLUTION: STAR CLUSTERS
What evidence do astronomers have that stars really do evolve?
Because all the stars in a cluster have about the same chemical composition and age, you can
see the effects of stellar evolution in the H–R diagram of a cluster. Massive stars evolve faster
than low-mass stars, so in a given cluster the most massive stars leave the main sequence first.
There are two types of star clusters. Open clusters contain 10 to 1000 stars and have an open,
transparent appearance. Globular clusters contain 105 to 106 stars densely packed into a
spherical shape. The open clusters tend to be young to middle-aged, but globular clusters tend
to be very old—11 billion years or more. Also, globular clusters tend to be poor in elements
heavier than helium.
You can judge the age of a cluster by looking at the turnoff point, the location on the main
sequence where the stars turn off to the right and become giants. The life expectancy of a star
at the turnoff point equals the age of the cluster.
H–R diagrams of old star clusters such as globular clusters show how giant stars ignite helium
fusion in their cores and evolve to the left in the diagram along the horizontal branch.
12-4 EVIDENCE OF EVOLUTION: VARIABLE STARS
Variable stars are those that change in brightness. Intrinsic variable stars change in brightness
because of internal processes and not because they are member of eclipsing binaries.
The Cepheid and RR Lyrae variable stars provide evidence that stars evolve. These intrinsic
variable stars lie in an instability strip in the H–R diagram; they contain a layer in their
envelopes that stores and releases energy as they expand and contract. Stars outside the
instability strip do not pulsate, because the layer is too deep or too shallow to make the stars
unstable.
The Cepheids obey a period–luminosity relation because more massive stars, which are more
luminous, are larger and pulsate more slowly.
Some Cepheids have periods that are slowly changing, showing that the evolution of the star
i
sc
h
a
n
gi
ngt
h
es
t
a
r
’
sr
a
di
u
sa
n
dt
hu
si
t
spe
r
i
odofpu
l
s
a
t
i
on
.
CHAPTER13
THE DEATH OF STARS
13-1 LOWER-MAIN-SEQUENCE STARS
How will the sun die?
Red dwarfs less massive than about 0.4 solar mass are completely mixed. They cannot ignite a
hydrogen-fusion shell, so they cannot become giant stars. Because they have little weight to
support and can fuse nearly all of their hydrogen fuel, they will remain on the main sequence
for many times the present age of the universe.
Medium-mass stars between about 0.4 and 4 solar masses, including the sun, become cool
giants and fuse helium but cannot fuse carbon.
Medium-mass stars lose mass into space while they are giant stars, a process that is helped by
thermal pulses in their helium-fusion shells. Finally such stars collapse to become white
dwarfs.
If the collapsed star becomes hot enough, it can ionize the gas it has ejected first as a slow
wind and later as a fast wind to turn on a planetary nebula.
Why are there so many white dwarfs?
White dwarfs are the cores of sunlike stars that have collapsed until their gas becomes
degenerate. They have no nuclear fuels and slowly cool off; they will eventually become black
dwarfs.
Because white dwarfs are small, dense, and generate no fusion energy, they are sometimes
called compact objects, along with neutron stars and black holes.
A white dwarf cannot support its own weight by its degenerate pressure if its mass is greater
than the Chandrasekhar limit of 1.4 solar masses.
Medium-mass stars up to about 8 solar masses can lose enough mass to die as white dwarfs.
This explains the large number of white dwarfs found in space.
13-2 THE EVOLUTION OF BINARY STARS
What happens if an evolving star is in a binary system?
Two stars orbiting each other control two regions of space called Roche lobes. The surface of
these lobes is called the Roche surface. The Lagrangian points mark places of stability in the
system, and mass can flow from one star to the other through the inner Lagrangian point.
Close binary stars evolve in complex ways because they can transfer mass from one star to the
other. This explains why some binary systems contain a main-sequence star more massive
than its giant companion—the Algol paradox.
Mass is transferred from an evolving star through an accretion disk around the receiving star.
Accretion disks can become hot enough to emit light and even X rays.
Mass transferred onto the surface of a white dwarf can build up a layer of fuel that erupts in a
nova explosion. A white dwarf can erupt repeatedly so long as mass transfer continues to form
new layers of fuel.
13-3 THE DEATHS OF MASSIVE STARS
How do massive stars die?
Stars more massive than about 8 solar masses cannot lose mass fast enough to reduce their
mass low enough to die by ejecting a planetary nebula and collapsing into a white dwarf. Such
massive stars must die more violent deaths.
The massive stars on the upper main sequence fuse nuclear fuels up to iron but cannot
generate further nuclear energy because iron is the most tightly bound of all atomic nuclei.
When a massive star forms an iron core, the core collapses and triggers a supernova explosion
known as a Type II supernova.
A type I supernova has no hydrogen lines in its spectra. A type Ia supernova can occur when
mass transferred onto a white dwarf pushes it over the Chandrasekhar limit and it collapses
suddenly, fusing all of its carbon at once in a burst of carbon deflagration. Type Ia supernova
have no hydrogen lines because white dwarfs contain very little hydrogen.
A Type Ib supernova occurs when a massive star loses its outer layers of hydrogen in a binary
system. Later its iron core can collapse, and it can explode as a type Ib supernova, which lacks
hydrogen lines.
Atoms heavier than helium but lighter than iron are produced inside massive stars and blown
out into the interstellar medium by supernova explosions. Iron and elements heavier than iron
are produced by fusion reactions during supernova explosions.
Supernova remnants are the expanding shells of gas ejected by supernova explosions. They
are filled with hot gas. The Crab Nebula is a supernova remnant formed by the supernova of
AD 1054.
The hazy glow in the Crab Nebula is produced by synchrotron radiation and is evidence that
some energy source remains in the nebula. Observations have found a neutron star there.
The supernova of 1987 is believed to have produced by a peculiar type II supernova explosion.
Neutrinos were detected and are evidence that the iron core collapsed and presumably formed
a neutron star, although it has not yet been detected.
Supernova explosions near Earth could alt
e
rt
h
eoz
on
el
a
y
e
r
,c
h
a
ng
eEa
r
t
h’
sc
l
i
ma
t
e
,a
n
d
produce extinctions as are seen in the fossil record.
CHAPTER14
NEUTRON STARS AND BLACK HOLES
14-1 NEUTRON STARS
How did scientists predict the existence of neutron stars?
When a supernova explodes, the core collapses to very small size. Theory predicts that
protons and electrons will combine to form a degenerate neutron gas.
The collapsing core cannot support itself as a white dwarf if its mass is greater than 1.4 solar
masses, the Chandrasekhar limit. If its mass lies between 1.4 solar masses and about 3 solar
masses, it can halt its contraction and form a neutron star.
A neutron star is supported by the pressure of the degenerate gas of neutrons. Theory predicts
that a neutron star should be about 10 km in radius, spin very fast because it conserves angular
momentum as it contracts, have a high temperature, and have a powerful magnetic field.
What is the evidence that neutron stars really exist?
Pulsars, rapidly pulsing radio sources, were discovered in 1967 and were eventually
understood to be spinning neutron stars. The discovery of a pulsar in the supernova remnant
called the Crab Nebula was a key link in the story.
Pulsars do not really blink. As described by the lighthouse model, pulsars are spinning neutron
stars that emit beams of radiation that sweep around the sky; if the beams sweep over Earth,
pulses can be detected.
A spinning neutron star slows as it radiates its energy into space. Most of the energy emitted
by a pulsar is carried away as a pulsar wind.
Sudden decreases in period called glitches appear to be caused by breaks in the rigid neutron
star crust or by changes in internal circulation.
Some neutron stars called magnetars are powered by intense magnetic fields. This may
explain the anomalous X-ray pulsars, which are energetic but rotate slowly. Shifts in these
magnetic fields can break the rigid crust and may explain the soft gamma-ray repeaters (SGR).
Many pulsars have been found in binary systems. In some, mass flows into a hot accretion
disk around the neutron star and causes the emission of X rays.
Observations of the first binary containing two neutron stars revealed that the system is losing
energy by radiating gravitational radiation.
The fastest pulsars, the millisecond pulsars, appear to be old pulsars that have been spun up to
high speed by mass flowing from binary companions.
Planets have been found orbiting at least one neutron star. They may be the remains of a
companion star that was mostly devoured by the neutron star.
14-2
BLACK HOLES
How did scientists predict the existence of black holes?
If the collapsing core of a supernova has a mass greater than 3 solar masses, then degenerate
neutrons cannot stop the contraction, and it must contract to a very small size—perhaps to a
singularity, an object of zero radius. Near such an object, gravity is so strong that not even
light can escape, and the region is called a black hole.
The outer boundary of a black hole is the event horizon; no event inside is detectable. The
radius of the event horizon is the Schwarzschild radius, amounting to only a few kilometers
for a black hole of a few solar masses.
Once matter falls into a black hole, it loses all of its properties except for mass, electrical
charge, and angular momentum.
Rotating black holes are called Kerr black holes after the mathematician who solved the
equations that describe them. The solution predicts a region called the ergosphere. A particle
breaking up inside the ergosphere can extract energy from the black hole if half falls in and
half escapes.
What is the evidence that black holes really exist?
If you were to leap into a black hole, your friends who stayed behind would see two
relativistic effects. They would see your clock slow relative to their own clock because of time
dilation. Also, they would see your light redshifted to longer wavelengths because of a
gravitational redshift.
You would not notice these effects, but you would feel powerful tidal forces that would
deform and heat your mass until you grew hot enough to emit X rays. Any X rays your mass
emitted before your mass reached the event horizon could escape.
To search for black holes, astronomers look for binary star systems in which mass flows into a
compact object and emits X rays. If the mass of the compact object is greater than about 3
solar masses, then the object cannot be a neutron star and is presumably a black hole. A
number of such objects have been located.
14-3
COMPACT OBJECTS WITH DISKS AND JETS
What happens when matter falls into a neutron star or black hole?
X-ray bursters appear to be binary systems in which mass transfer deposits matter on the
surface of a neutron star. When helium fusion ignites, the surface explodes and produces a
burst of X rays.
Astronomers can observe rapid flickering called quasi-periodic oscillations (QPOs) produced
by blobs of gas orbiting very rapidly and spiraling inward in accretion disks. Bursts of energy
are seen when such matter hits the surface of neutron stars, but no bursts are seen if the matter
approaches the event horizon around a black hole.
Rapidly spinning accretion disks around neutron stars or black holes can twist magnetic fields
into tubes and eject narrow, powerful jets of radiation and matter in a process that is not yet
well understood.
Gamma-ray bursters appear to be related to violent events involving neutron stars and black
holes. Many bursts appear to arise during hypernovae (also called collapsars), the collapse of
the most massive stars to form black holes and eject powerful beams of gamma rays. Some
bursts may be caused by the merger of two neutron stars, but such events have not been
confirmed by direct observations.