DTU9ePPTChap13 - Faculty Lounge : Astronomy

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Transcript DTU9ePPTChap13 - Faculty Lounge : Astronomy

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
The Deaths of Stars
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…
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. The inner layers are not shown to scale here.
Evolution from Supergiants to White Dwarfs
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
Since Sirius A (11,000 K) and Sirius B (30,000 K) are
hot blackbodies, they are strong emitters of X rays.
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 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.
Supernova Light Curves
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.
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.
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. The white and colored spots are unrelated
stars. (b) When the progenitor star of SN 1987A was still a red supergiant, a
slowly moving wind from the star filled the surrounding space with a thin gas.
When the star contracted into a blue supergiant, it produced a faster-moving
stellar wind. The interaction between the fast and slow winds somehow caused
gases to pile up along an hourglass-shaped shell surrounding the star. The burst
of ultraviolet radiation from the supernova ionized the gas in the rings, causing
the rings to glow. The supernova itself, at the center of the hourglass, glows
because of energy released from radioactive decay.
A Recording of a Pulsar
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.
Model of a Neutron Star’s Interior
This drawing shows the
theoretical model of a 1.4-solarmass neutron star. The neutron
star has a superconducting,
superfluid core 9.7 km in radius,
surrounded by a 0.6-km-thick
mantle of superfluid neutrons. The
neutron star’s crust is only 0.3-km
thick (the length of four football
fields) and is composed of heavy
nuclei and free electrons. The
thicknesses of the layers are not
shown to scale.
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 angular momentum 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.
X-Ray Pulses from Centaurus X-3
This graph shows the intensity of X rays detected by Uhuru
as Centaurus X-3 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.
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. Highmass 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
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
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
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
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
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
helium shell flash
helium shell fusion
lighthouse model
neutron degeneracy
neutron star
nova (plural novae)
planetary nebula
primary cosmic ray
secondary cosmic ray
Type Ia supernova
Type II supernova
X-ray burster
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.
What is a nova? How does it differ from a
 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.
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.
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
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.