The document discusses the evolution and deaths of stars. It describes how low-mass stars like the Sun will evolve into red giants and planetary nebulae over billions of years. More massive stars may explode as supernovae, producing neutron stars or black holes and spreading heavy elements throughout space. Neutron stars can be observed as pulsars that emit beams of radiation. The origins of elements on Earth and phenomena like cosmic rays and pulsars are also examined.
4. Structure of an Old Low-Mass StarStructure 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.
5. Structure of an Old High-Mass StarStructure of an Old High-Mass Star
6. Supernovae Proceed IrregularlySupernovae Proceed Irregularly
Computer simulations showing how chaotic the supernova
is deep inside the star as it begins to explode.
8. Rotating, Magnetized Neutron StarRotating, Magnetized Neutron Star
Charged particles are accelerated near a neutron star’s
magnetic poles and produce two oppositely directed
beams of radiation.
9. WHAT DO YOU THINK?WHAT DO YOU THINK?
Will the Sun someday stop shining? If
so, how?
What is a nova?
Where do heavy elements on the Earth
like carbon, silicon, oxygen, iron, and
uranium come from?
What are cosmic rays?
What is a pulsar?
10. You will discover…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,
while others go with a bang
the incredible densities of neutron stars
and how they are observed
13. Sirius and White DwarfSirius and White Dwarf
Sirius B, a white dwarf,
at the five o’clock
position
Both are hot
blackbodies and strong
emitters of X rays
14. Nova Herculis 1934Nova Herculis 1934
Shortly after peak brightness
as a magnitude –3 star.
Two months later, magnitude +12.
17. Supernovae Proceed IrregularlySupernovae Proceed Irregularly
Chaos helps account for the lopsided distribution of all elements in
this supernova remnant. X-ray images of a supernova remnant
taken by Chandra.
18. Gum NebulaGum Nebula
The Gum Nebula, created by a supernova 11,000 years ago, is
the largest known supernova remnant. It now has a diameter of
about 2300 ly.
19. Cassiopeia ACassiopeia A
An X-ray picture of Cassiopeia
A taken by Chandra
Radio image produced by
the Very Large Array (VLA)
24. Crab Nebula and PulsarCrab Nebula and Pulsar
Observations at different wavelengths give astronomers
information about the nebula’s chemistry, motion, history, and
interactions with preexisting gas and dust.
25. Crab Nebula and PulsarCrab Nebula and Pulsar
The Crab’s visible flashes and X-ray pulses have identical periods
of 0.033 seconds.
26. How Magnetic Field Strengths IncreaseHow Magnetic Field Strengths Increase
As a star collapses, it carries the magnetic field
inward, thereby increasing its strength.
27. Neutron Star’s InteriorNeutron Star’s Interior
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.
28. Glitch & the Vela Pulsar’sGlitch & the Vela Pulsar’s
Spindown RateSpindown Rate
30. X-Ray Pulses from Centaurus X-3X-Ray Pulses from Centaurus X-3
X ray intensity as detected by Uhuru
31. Model of a Pulsating X-RayModel of a Pulsating X-Ray
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.
32. X Rays from an X-Ray BursterX Rays from an X-Ray Burster
33. Summary of Stellar EvolutionSummary of Stellar Evolution
The evolution of isolated stars depends on their masses.
35. WHAT DID YOU THINK?WHAT DID YOU THINK?
Will the Sun someday cease to exist? If so, how?
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?
A nova is a relatively gentle explosion of hydrogen gas on the surface of a white
dwarf in a binary star system.
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?
Cosmic rays are high-speed particles (mostly hydrogen and other atomic nuclei)
in space. Many of them are believed to have been created as a result of
supernovae.
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 sweeps across our
region of space.
36. Key TermsKey Terms
asymptotic giant branch (AGB) star
Chandrasekhar limit
cosmic ray
cosmic ray shower
glitch
helium shell flash
helium shell fusion
lighthouse model
neutron degeneracy pressure
neutron star
nova (plural novae)
photodisintegration
planetary nebula
pulsar
quark
secondary cosmic ray
supernova
Type Ia supernova
Type II supernova
white dwarf
X-ray burster
Editor's Notes
NASA’s High Energy Astrophysics Science Archive Research Center is a good place to find information on white dwarfs, supernovae and other astrophysical phenomena: heasarc.gsfc.nasa.gov/. Their Imagine the Universe web site has resources useful to teachers and others: imagine.gsfc.nasa.gov/index.html . The Imagine the Universe educator’s web site discusses the unique magnetar discovered in July 2003: imagine.gsfc.nasa.gov/docs/features/news/28jan04.html
The SEDS web site, www.seds.org/messier/planetar.html, has information on planetary nebulae and how to observe them.
The Chandra X-ray Observatory site, operated by the Smithsonian Astrophysical Observatory, provides useful information on high energy astrophysics: chandra.harvard.edu/index.html.
FIGURE 13-4 Some Shapes of Planetary Nebulae
The outer shells of dying low-mass stars are ejected in
a wonderful variety of patterns. (a) NGC 7293, the
Helix Nebula, is located in the constellation Aquarius. The star
that ejected these gases is seen at the center of the glowing
shell. This nebula, located about 700 ly (215 pc) from Earth, has
an angular diameter equal to about half that of the full Moon.
(b) NGC 6826 shows jets of gas (in red) whose origin is as yet
unknown. (c) Mz 3 (Menzel 3), in the constellation Norma (the
Carpenter’s Level), is 3000 ly (900 pc) from Earth. The dying
star, creating these bubbles of gas, may be part of a binary
system. (a: NASA, NOAO, ESA, The Hubble Helix Nebula Team, M.
Meixner/STScI, and T. A. Rector/NRAO; b: Howard Bons, STScI/Robin
Ciardullo, Pennsylvania State University/NASA; c: AURA/STScI/NASA)
FIGURE 13-3 Evolution from Supergiants to White
Dwarfs The evolutionary tracks of three low-mass,
supergiants are shown as they eject planetary nebulae.
The table gives their masses as supergiants, the amount of mass they
lose as planetary nebulae, and their remaining (white dwarf) masses.
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.
FIGURE 13-2 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
the Earth’s orbit.) The star’s core, the
hydrogen-fusing shell, and the heliumfusing
shell are contained within a volume
roughly the size of the Earth.
FIGURE 13-9 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 the Earth.
FIGURE 13-10 Supernovae Proceed Irregularly
Images (a) and (b) are computer simulations showing
how chaotic the supernova is deep inside the star as it
begins to explode. This 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). The latter three are X-ray images
of a supernova remnant taken by Chandra at different
wavelengths. (a and b: Courtesy of Adam Burrows, University of
Arizona and Bruce Fryxell, NASA/GSFC; c, d, and e: U. Hwang et al.,
NASA/GSFC)
FIGURE 13-14 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. (European Southern
Observatory; insets: Anglo-Australian
Observatory/David Malin Images)
FIGURE 13-20 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,
the beams sweep around the sky. If the Earth happens to lie in
the path of a beam, we see the neutron star as a pulsar.
NASA’s High Energy Astrophysics Science Archive Research Center has a great deal of resources on neutron stars and pulsars: heasarc.gsfc.nasa.gov/. Their Imagine the Universe educator’s web site discusses the unique magnetar discovered in July 2003:
imagine.gsfc.nasa.gov/docs/features/news/28jan04.html
FIGURE 13-1 Post–Main-Sequence Evolution of
Low-Mass Stars (a) The 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.
FIGURE 13-5 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, (b) followed by outflow
that is channeled by the original gas to squirt out perpendicular
to the plane of the doughnut. (c) 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. (c: R. Sahai and J. Trauer, JPL; WFPC-2
Science Team; and NASA)
FIGURE 13-6 Sirius and Its White
Dwarf Companion (a) 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. (b) Since Sirius A
(11,000 K) and Sirius B (30,000 K) are hot
blackbodies, they are strong emitters of X
rays. (a: R. B. Minton; b: NASA/SAO/CXC)
FIGURE 13-7 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. (UCO/Lick Observatory)
FIGURE 13-8 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.
FIGURE 13-10 Supernovae Proceed Irregularly
Images (a) and (b) are computer simulations showing
how chaotic the supernova is deep inside the star as it
begins to explode. This 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). The latter three are X-ray images
of a supernova remnant taken by Chandra at different
wavelengths. (a and b: Courtesy of Adam Burrows, University of
Arizona and Bruce Fryxell, NASA/GSFC; c, d, and e: U. Hwang et al.,
NASA/GSFC)
FIGURE 13-11 The Gum Nebula The Gum Nebula is the
largest known supernova remnant. It spans 60º 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 the Earth. The supernova explosion occurred about
11,000 years ago, and its remnant now has a diameter of about
2300 ly. Only the central regions of the nebula are shown here.
(Royal Observatory, Edinburgh)
FIGURE 13-12 Cassiopeia A Supernova
remnants such as Cassiopeia A are typically
strong sources of X rays and radio waves.
(a) An X-ray picture of Cassiopeia A taken by Chandra. (b) A
corresponding radio image produced by the Very Large Array
(VLA). Radiation from the supernova that produced this nebula
first reached Earth 300 years ago. The explosion occurred about
10,000 ly from here. (a: NASA/CSC/SAO; b: Very Large Array)
FIGURE 13-13 Cosmic Ray Shower Cosmic rays from space
slam into particles in the atmosphere, breaking up the latter 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 computer simulation. (Clem Pryke/
University of Chicago)
FIGURE 13-15 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 Hubble Space Telescope
image. This gas was ejected from the star 20,000 years before it
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.
(Robert P. Kirshner and Peter Challis, Harvard-Smithsonian Center for
Astrophysics; STScI)
SN 1987A continues to provide surprises nearly two decades after it appeared. NASA and the Chandra site (chandra.harvard.edu/index.html) are good sources of information.
Supernovae, neutron stars and pulsars continue to capture the public’s interest. PBS frequently has programs on these objects. Their web site usually has supplemental resources and transcripts of programs. The NOVA series, “Runaway Universe,” had a segment on supernovae: www.pbs.org/wgbh/nova/universe/supernova.html. The earlier NOVA program, “Death of a Star,” while dated, is an excellent glimpse into astronomers’ excitement over the event. Your local video store or college library may be able to locate a copy.
FIGURE 13-16 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.
FIGURE 13-17 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
seconds) that it is more precise than most clocks on Earth.
FIGURE 13-18 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 gamma-ray image
is only of the central region of the nebula. (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 second.
(a: NASA; b: The FORS Team, VLT, European Southern Observatory; left
insets: Lick Observatory; right insets: Einstein Observatory, Harvard-
Smithsonian Center for Astrophysics)
FIGURE 13-18 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 gamma-ray image
is only of the central region of the nebula. (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 second.
(a: NASA; b: The FORS Team, VLT, European Southern Observatory; left
insets: Lick Observatory; right insets: Einstein Observatory, Harvard-
Smithsonian Center for Astrophysics)
FIGURE 13-19 Analogy for How Magnetic Field Strengths
Increase When planted, 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: Corbis; b: Oscar
Burriel/Photo Researchers, Inc.)
FIGURE 13-21 Model of a Neutron Star’s Interior This
drawing shows the theoretical model of a 1.4-M 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
(such as neutrons) and free electrons. The thicknesses of the
layers are not shown to scale.
FIGURE 13-22 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
circular and so its spinning, solid surface must readjust its shape.
Since 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 and is
called a glitch.
FIGURE 13-23 Double Pulsar This artist’s conception shows
two pulsars orbiting 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-millisecond period and the other has a 2.8-second period.
The two orbit once every 2.4 hours. (Michael Kramer/Jodrell Bank
Observatory, University of Manchester)
FIGURE 13-24 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 successive
pulses are separated by 4.84 seconds. 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.
FIGURE 13-25 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.
FIGURE 13-26 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.
FIGURE 13-27 A Summary of Stellar Evolution
(a) The evolution of isolated stars depends on their
masses. The higher the mass, the shorter the lifetimes.
The scale on the left indicates the mass of a star when it
is on the main sequence. The scale on the right gives the
mass of the resulting stellar corpse. Stars less massive than
about 8 M can eject enough mass to become white dwarfs.
High-mass stars can produce Type II supernovae and become
neutron stars, quark stars, or black holes. The horizontal (time)
axis is not to scale, but the relative lifetimes are accurate.
(b) The cycle of stellar evolution is summarized in this figure.
(b: top inset, Infrared Space Observatory, NASA; right inset,
Anglo-Australian Observatory/J. Hester and P. Scowen, Arizona
State University/NASA; bottom inset, NASA; left inset, NASA;
middle inset, Anglo-Australian Observatory)
FIGURE 13-27 A Summary of Stellar Evolution
(a) The evolution of isolated stars depends on their
masses. The higher the mass, the shorter the lifetimes.
The scale on the left indicates the mass of a star when it
is on the main sequence. The scale on the right gives the
mass of the resulting stellar corpse. Stars less massive than
about 8 M can eject enough mass to become white dwarfs.
High-mass stars can produce Type II supernovae and become
neutron stars, quark stars, or black holes. The horizontal (time)
axis is not to scale, but the relative lifetimes are accurate.
(b) The cycle of stellar evolution is summarized in this figure.
(b: top inset, Infrared Space Observatory, NASA; right inset,
Anglo-Australian Observatory/J. Hester and P. Scowen, Arizona
State University/NASA; bottom inset, NASA; left inset, NASA;
middle inset, Anglo-Australian Observatory)
NASA’s High Energy Astrophysics Science Archive Research Center is a good place to find information on white dwarfs, supernovae and other astrophysical phenomena: heasarc.gsfc.nasa.gov/. Their Imagine the Universe web site has resources useful to teachers and others: imagine.gsfc.nasa.gov/index.html .
The SEDS web site, www.seds.org/messier/planetar.html, has information on planetary nebulae and how to observe them.
The Chandra X-ray Observatory site, operated by the Smithsonian Astrophysical Observatory, provides useful information on high energy astrophysics: chandra.harvard.edu/index.html.
Summary of Key Ideas
• Stars with different masses fuse different elements.
• Stars lose mass via stellar winds throughout their lives.
Low-Mass Stars and Planetary Nebulae
• A low-mass (below 8 M) main-sequence star becomes a
giant when hydrogen shell fusion begins. It becomes a horizontalbranch
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 the Earth. The maximum mass of a white
dwarf (the Chandrasekhar limit) is 1.4 solar masses.
• 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 supernova when carbon fusion ignites explosively
throughout such a degenerate star. Such a detonation is called
a Type Ia supernova.
High-Mass Stars and Supernovae
• After exhausting its central supply of hydrogen and helium,
the core of an intermediate- or high-mass (above 8 solar masses) star
undergoes a sequence of other thermonuclear reactions. These
include carbon fusion, neon fusion, oxygen fusion, and silicon
fusion. This last fusion eventually produces an iron core.
• An intermediate- or 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 of the doomed star’s core.
• Neutrinos were detected from Supernova 1987A, which was
visible to the naked eye.
Neutron Stars, Pulsars, and Quark Stars
• A high-mass star’s dead core becomes a neutron star, a quark
star, or even a black hole (Chapter 14). 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 Oppenheimer–Volkov limit, is about 3 solar masses.
• A quark star may occur when neutrons dissolve into quarks,
which can oppose the force of gravity slightly more strongly than
can neutrons. The upper limit to quark star masses is believed to
be only slightly above the Oppenheimer–Volkov limit.
• 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
thought 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 the sudden increase in Xray
radiation called an X-ray burster.