Transcript DTU 8e Chap 13 The Deaths of Stars

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 13
The Deaths of Stars
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
Will the Sun someday cease to shine brightly? If so,
how will this occur?
What is a nova? How does it differ from a supernova?
What are the origins of the carbon, silicon, oxygen,
iron, uranium, and other heavy elements on Earth?
What are cosmic rays? Where do they come from?
What is a pulsar?
In this chapter you will discover…
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what happens to stars when core helium fusion ceases
how heavy elements are created
the characteristics of the end of stellar evolution
why some stars go out relatively gently and others go out
with a bang
the incredible density of the matter in neutron stars and
how these objects are observed
Post–Main-Sequence Evolution of Low-Mass Stars
(a) A typical evolutionary track on the H-R diagram as a star makes the
transition from the main sequence to the giant phase. The asterisk (*) shows
the helium flash occurring in a low-mass star. (b) After the helium flash, the star
converts its helium core into carbon and oxygen. While doing so, its core
reexpands, decreasing shell fusion. As a result, the star’s outer layers
recontract. (c) After the helium core is completely transformed into carbon and
oxygen, the core recollapses, and the outer layers reexpand, powered up the
asymptotic giant branch by hydrogen shell fusion and helium shell fusion.
The Structure of an Old Low-Mass Star
Near the end of its life, a low-mass star, like the Sun, travels up
the AGB and becomes a supergiant. (The Sun will be about as
large as the diameter of Earth’s orbit.) The star’s inert core, the
hydrogen-fusing shell, and the helium-fusing shell are
contained within a volume roughly the size of Earth.
Evolution from Supergiants to White Dwarfs
Masses in Msun
Ltr
A
B
C
Mn Pl White
Seq Neb Dwarf
3.0 1.8
1.5 0.7
0.8 0.2
1.2
0.8
0.6
The evolutionary tracks of three low-mass supergiants are shown as they
eject planetary nebulae. The loops indicate periods of instability and
adjustment for the white dwarfs. The dots on this graph represent the central
stars of planetary nebulae whose surface temperatures and luminosities have
been determined. The crosses are white dwarfs for which similar data exist.
Some Shapes of Planetary Nebulae
An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster
M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth.
Some Shapes of Planetary Nebulae
The Helix Nebula, NGC 7293, located in the constellation Aquarius, about
700 ly (215 pc) from Earth, has an angular diameter equal to about half that
of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue
gas is rich in oxygen.
Some Shapes of Planetary Nebulae
NGC 6826 shows, among other features, lobes of nitrogenrich gas (red). The process by which they were ejected is as
yet unknown. This planetary nebula is located in Cygnus.
Some Shapes of Planetary Nebulae
MZ 3 (Menzel 3), in the constellation Norma (the Carpenter’s
Square), is 3000 ly (900 pc) from Earth. The dying star, creating
these bubbles of gas, may be part of a binary system.
Formation of a Bipolar Planetary Nebula
Bipolar planetary nebulae may form in two steps.
Astronomers hypothesize that (a) first, a doughnut-shaped
cloud of gas and dust is emitted from the star’s equator...
Formation of a Bipolar Planetary Nebula
…(b) followed by outflow that is channeled by the original gas
to squirt out perpendicularly to the plane of the doughnut.
Formation of a Bipolar Planetary Nebula
The Hourglass Nebula appears to be a “textbook” example of such a system.
The bright ring is believed to be the doughnut-shaped region of gas lit by
energy from the planetary nebula. The Hourglass is located about 8000 ly
(2500 pc) from Earth.
Sirius and Its White Dwarf Companion
Sirius, the brightest-appearing star in the night sky, is actually
a double star. The smaller star, Sirius B, is a white dwarf,
seen here at the five o’clock position in the glare of Sirius.
The spikes and rays around the bright star, Sirius A, are
created by optical effects within the telescope.
Sirius and Its White Dwarf Companion
The White Dwarf is the BRIGHT source, at 25 or 30 thousand K
Nova Herculis 1934
These two pictures show a nova (a) shortly after peak
brightness as a magnitude –3 star and (b) 2 months later,
when it had faded to magnitude +12. Novae are named after
the constellation and year in which they appear.
The Light Curve of a Nova
This graph shows the history (light curve) of Nova Cygni 1975, a
nova that was observed to blaze forth in the constellation of
Cygnus in September 1975. The rapid rise in magnitude followed
by a gradual decline is characteristic of many novae, although
some oscillate in intensity as they become dimmer.
A nova is believed to occur when which
of the following pairs of stars are in a
binary system?
A.
B.
C.
D.
white dwarf, main sequence star
white dwarf, neutron star
neutron star, red giant
a pair of supergiants
White dwarves are composed primarily of:
A.
B.
D.
E.
helium
neutrons
carbon and oxygen
iron
Supernova Light Curves – Fade Slower
A Type Ia supernova, which gradually declines in brightness, is
caused by an exploding white dwarf in a close binary system. A
Type II supernova is caused by the explosive death of a
massive star and usually has alternating intervals of steep and
gradual declines in brightness.
A Type Ia Supernova results from:
A. A white dwarf being swallowed up by a
black hole
B. A white dwarf exceeding the
Chandrasekhar mass limit.
C. created if a star stops burning helium
and contracts.
D. left behind after a Type II supernova
explosion.
The Structure of an Old High-Mass Star
Near the end of its life, a high-mass star becomes a supergiant
with a diameter almost as wide as the orbit of Jupiter. The
star’s energy comes from six concentric fusing shells, all
contained within a volume roughly the same size as Earth.
Which type of star is forming iron in its
core?
A. supergiant
B. giant
C. main sequence
D. white dwarf
Why is Iron the end-of-the-line?
Mass Loss by a Supermassive Star
This gas and dust was ejected by the massive star HD
148937 in the constellation Norma. Located about 4200
ly (1300 pc) away, this star has 40 solar masses and is
more than halfway through its 6-million-year lifespan.
Supernovae Proceed Irregularly
Images (a) and (b) are computer simulations showing the chaotic flow of
gas deep inside the star as it begins to explode as a supernova. This
uneven flow helps account for the globs of iron and other heavy elements
emitted from deep inside, as well as the lopsided distribution of all elements
in the supernova remnant, as shown in (c), (d), and (e). These three
pictures are X-ray images of supernova remnant Cassiopeia A taken by
Chandra at different wavelengths.
The Gum Nebula
The Gum Nebula is the largest known supernova remnant. It spans 60°
across the sky and is centered roughly on the southern constellation of
Vela. The nearest portions of this expanding nebula are only 300 ly from
Earth. The supernova explosion occurred about 11,000 years ago, and
the remnant now has a diameter of about 2300 ly. Only the central
regions of the nebula are shown here.
Cassiopeia A
Supernova remnants, such as Cassiopeia A, are typically strong sources
of X rays and radio waves. (a) A radio image produced by the Very Large
Array (VLA). (b) A corresponding X-ray picture of Cassiopeia A taken by
Chandra. The opposing jets of silicon, probably guided by powerful
magnetic fields, were ejected early in the supernova, before the iron-rich
jets were released. Radiation from the supernova that produced this
nebula first reached Earth 300 years ago. The explosion occurred about
10,000 ly from here.
Cosmic Ray Shower
Cosmic rays from space slam into particles in the atmosphere,
breaking them up and sending them Earthward. These debris are
called secondary cosmic rays. This process of impact and
breaking up continues as secondary cosmic rays travel downward,
creating a cosmic ray shower, as depicted in this artist’s
conception of four such events.
Supernova 1987A
A supernova was discovered in a nearby galaxy called the Large Magellanic
Cloud (LMC) in 1987. This photograph shows a portion of the LMC that
includes the supernova and a huge H II region called the Tarantula Nebula. At
its maximum brightness, observers at southern latitudes saw the supernova
without a telescope. (Insets) The star before and after it exploded.
Shells of Gas Around SN 1987A
(a) Intense radiation from the supernova explosion caused three
rings of gas surrounding SN 1987A to glow in this HST image.
This gas was ejected from the star 20,000 years before the star
detonated. All three rings lie in parallel planes. The inner ring is
about 1.3 ly across.
A Recording of a Pulsar – Light Curve in Radio
This chart recording shows the intensity of radio emissions
from one of the first pulsars to be discovered, PSR 0329 + 54.
Note that some of the pulses are weak and others are strong.
Nevertheless, the spacing between pulses is so regular (0.714
s) that it is more precise than most clocks on Earth.
The Crab Nebula and Pulsar
(a) This nebula, named for the crablike appearance of its filamentary
structure in early visible-light telescope images, is the remnant of a
supernova seen in A.D. 1054. The distance to the nebula is about 6000
ly, and its present angular size (4 by 6 arcmin) corresponds to linear
dimensions of about 7 by 10 ly. Observations at different wavelengths
give astronomers information about the nebula’s chemistry, motion,
history, and interactions with preexisting gas and dust.
The Crab Nebula and Pulsar
(b) The insets show the Crab pulsar in its “on” and “off” states. Both its
visible flashes and X-ray pulses have identical periods of 0.033 s.
Analogy for How Magnetic Field Strengths Increase
When growing, these wheat stalks cover a much larger area
than when they are harvested and bound together. A star’s
magnetic field behaves similarly, as the collapsing star carries
the field inward, thereby increasing its strength.
A Rotating, Magnetized Neutron Star
Calculations reveal that many neutron stars rotate rapidly and possess
powerful magnetic fields. Charged particles are accelerated near a neutron
star’s magnetic poles and produce two oppositely directed beams of
radiation. As the star rotates (going from a to b is half a rotation period), the
beams sweep around the sky. If Earth happens to lie in the path of a beam
(a, but not b), we see the neutron star as a pulsar.
The size of a white dwarf is closest to
which of the following?
A.
B.
C.
D.
about 1 A.U. in diameter
about the size of the Sun
about the size of the Earth
about 10 kilometers in diameter
The size of a neutron star is closest to
which of the following?
A.
B.
C.
D.
about 1 A.U. in diameter
about the size of the Sun
about the size of the Earth
about 10 kilometers in diameter
A Glitch Interrupts the Vela Pulsar’s Spindown Rate
An isolated pulsar radiates energy, which causes it to slow down. This
“spin down” is not always smooth. As it slows down, it becomes more
spherical, and so its spinning, solid surface must readjust its shape.
Because the surface is brittle, this readjustment is often sudden, like the
cracking of glass, which causes the spin rate of the pulsar to suddenly
jump. Such an event, shown here for the Vela pulsar in 1975, changes the
pulsar’s rotation period.
Double Pulsar
This artist’s conception shows two pulsars that orbit their center of mass.
The double pulsar they represent is called PSR J0737-3039, which is about
1500 ly from Earth in the constellation Puppis. One pulsar has a 23-ms
period and the other has a 2.8-s period. The two orbit once every 2.4 h.
A neutron star is:
A. Left behind after a Type Ia supernova
explosion.
B. created if a star stops burning hydrogen
and contracts.
C. created if a star stops burning helium
and contracts.
D. left behind after a Type II supernova
explosion.
X-Ray Pulses from Centaurus X-3
This graph shows the intensity of X rays detected by Uhuru as Centaurus X3 moved across the satellite’s field of view. The variation in the height of the
pulses was a result of the changing orientation of Uhuru’s X-ray detectors
toward the source as the satellite rotated. The short pulse period suggests
that the source is a rotating neutron star.
A Model of a Pulsating X-Ray Source
Gas transfers from an ordinary star to the neutron star. The infalling
gas is funneled down onto the neutron star’s magnetic poles, where it
strikes the star with enough energy to create two X ray–emitting hot
spots. As the neutron star spins, beams of X rays from the hot spots
sweep around the sky.
A pulsar is best described as:
A. a rapidly rotating white dwarf
B. a rapidly rotating neutron star
C. A white dwarf which expands and
contracts, similarly to a Cepheid Variable
D. A neutron star which expands and
contracts, similarly to a Cepheid Variable
X Rays from an X-Ray Burster
A burster emits X rays with a constant low intensity
interspersed with occasional powerful bursts. This burst
was recorded in September 1975 by an X-ray telescope
that was pointed toward the globular cluster NGC 6624.
A Summary of Stellar Evolution
The evolution of isolated stars depends primarily on their masses. The higher
the mass, the shorter the lifetime. Stars less massive than about 8 solar
masses can eject enough mass to become white dwarfs. High-mass stars can
produce Type II supernovae and become neutron stars or black holes. The
horizontal (time) axis is not to scale, but the relative lifetimes are accurate.
A Summary of Stellar Evolution
The cycle of stellar evolution is summarized in this figure.
Summary of Key Ideas
Low-Mass Stars and Planetary
Nebulae
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A low-mass (below 8 solar masses) main-sequence star
becomes a giant when hydrogen shell fusion begins. It
becomes a horizontal-branch star when core helium
fusion begins. It enters the asymptotic giant branch and
becomes a supergiant when helium shell fusion starts.
Stellar winds during the thermal pulse phase eject mass
from the star’s outer layers.
The burned-out core of a low-mass star becomes a
dense carbon-oxygen body, called a white dwarf, with
about the same diameter as that of Earth. The maximum
mass of a white dwarf (the Chandrasekhar limit) is 1.4
solar masses.
Low-Mass Stars and Planetary
Nebulae
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Explosive hydrogen fusion may occur in the surface
layer of a white dwarf in some close binary systems,
producing sudden increases in luminosity that we call
novae.
An accreting white dwarf in a close binary system can
also become a Type Ia supernova when carbon fusion
ignites explosively throughout such a degenerate star.
High-Mass Stars and Supernovae
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After exhausting its central supply of hydrogen and
helium, the core of a high-mass (above 8 solar masses)
star undergoes a sequence of other thermonuclear
reactions. These reactions include carbon fusion, neon
fusion, oxygen fusion, and silicon fusion. This last fusion
eventually produces an iron core.
A high-mass star dies in a supernova explosion that
ejects most of the star’s matter into space at very high
speeds. This Type II supernova is triggered by the
gravitational collapse and subsequent bounce of the
doomed star’s core.
Neutrinos were detected from Supernova 1987A, which
was visible to the naked eye. Its development supported
theories of Type II supernovae.
Neutron Stars, Pulsars, and (perhaps)
Quark Stars
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The core of a high-mass main-sequence star containing
between 8 and 25 solar masses becomes a neutron star. The
cores of slightly more massive stars may become quark stars. A
neutron star is a very dense stellar corpse consisting of closely
packed neutrons in a sphere roughly 20 km in diameter. The
maximum mass of a neutron star, called the OppenheimerVolkov limit, is about 3 solar masses.
A pulsar is a rapidly rotating neutron star with a powerful
magnetic field that makes it a source of periodic radio and other
electromagnetic pulses. Energy pours out of the polar regions of
the neutron star in intense beams that sweep across the sky.
Some X-ray sources exhibit regular pulses. These objects are
believed to be neutron stars in close binary systems with
ordinary stars.
Explosive helium fusion may occur in the surface layer of a
companion neutron star, producing a sudden increase in X-ray
radiation, called an X-ray burster.
Key Terms
asymptotic giant branch
(AGB) star
Chandrasekhar limit
cosmic ray
cosmic ray shower
glitch
helium shell flash
helium shell fusion
lighthouse model
magnetar
neutron degeneracy
pressure
neutron star
nova (plural novae)
nucleosynthesis
photodisintegration
planetary nebula
primary cosmic ray
pulsar
quark
secondary cosmic ray
Type Ia supernova
Type II supernova
X-ray burster
WHAT DID YOU THINK?
Will the Sun someday cease to shine
brightly? If so, how will this occur?
 Yes. The Sun will shed matter as a
planetary nebula in about 6 billion years
and then cease nuclear fusion. Its remnant
white dwarf will dim over the succeeding
billions of years.
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WHAT DID YOU THINK?
What is a nova? How does it differ from a
supernova?
 A nova is a relatively gentle explosion of
hydrogen gas on the surface of a white
dwarf in a binary star system. Supernovae,
on the other hand, are explosions that
cause the nearly complete destruction of
massive stars.
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WHAT DID YOU THINK?
What are the origins of the carbon, silicon,
oxygen, iron, uranium, and other heavy
elements on Earth?
 These elements are created during stellar
evolution by supernovae, and by colliding
neutron stars.
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WHAT DID YOU THINK?
What are cosmic rays? Where do they
come from?
 Cosmic rays are high-speed particles
(mostly hydrogen and other atomic nuclei)
in space. Many of them are thought to
have been created as a result of
supernovae.
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WHAT DID YOU THINK?
What is a pulsar?
 A pulsar is a rotating neutron star in which
the magnetic field’s axis does not coincide
with the rotation axis. The beam of
radiation it emits periodically sweeps
across our region of space.
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