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- 1. Chapter 13:Chapter 13: TheDeaths of Stars The Helix Nebula
- 2. Planetary NebulaePlanetary Nebulae NGC7293, Helix Nebula NGC 6826 Menzel 3
- 3. From Supergiants toWhite DwarfsFrom Supergiants to White Dwarfs
- 4. Structure of anOld 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 anOld High-Mass StarStructure of an Old High-Mass Star
- 6. Supernovae Proceed IrregularlySupernovaeProceed Irregularly Computer simulations showing how chaotic the supernova is deep inside the star as it begins to explode.
- 7.
- 8. Rotating, Magnetized NeutronStarRotating, 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 YOUTHINK?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…Youwill 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
- 11. Post–Main-Sequence Evolution ofPost–Main-SequenceEvolution of Low-Mass StarsLow-Mass Stars
- 12. Bipolar Planetary NebulaBipolarPlanetary Nebula
- 13. Sirius and WhiteDwarfSirius 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 1934NovaHerculis 1934 Shortly after peak brightness as a magnitude –3 star. Two months later, magnitude +12.
- 15. Nova Cygni 1975NovaCygni 1975
- 17. Supernovae Proceed IrregularlySupernovaeProceed 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 TheGum 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 AnX-ray picture of Cassiopeia A taken by Chandra Radio image produced by the Very Large Array (VLA)
- 20. Cosmic Ray ShowerCosmicRay Shower
- 21. Shells of GasAround SN 1987AShells of Gas Around SN 1987A
- 22. Supernova Light CurvesSupernovaLight Curves
- 23. Recording a PulsarRecordinga Pulsar PSR 0329+54
- 24. Crab Nebula andPulsarCrab 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 andPulsarCrab Nebula and Pulsar The Crab’s visible flashes and X-ray pulses have identical periods of 0.033 seconds.
- 26. How Magnetic FieldStrengths IncreaseHow Magnetic Field Strengths Increase As a star collapses, it carries the magnetic field inward, thereby increasing its strength.
- 27. Neutron Star’s InteriorNeutronStar’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 & theVela Pulsar’sGlitch & the Vela Pulsar’s Spindown RateSpindown Rate
- 29. Double PulsarDouble Pulsar Artist’sconception of PSR J0737-3039
- 30. X-Ray Pulses fromCentaurus X-3X-Ray Pulses from Centaurus X-3 X ray intensity as detected by Uhuru
- 31. Model of aPulsating 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 froman X-Ray BursterX Rays from an X-Ray Burster
- 33. Summary of StellarEvolutionSummary of Stellar Evolution The evolution of isolated stars depends on their masses.
- 34. Summary of StellarEvolutionSummary of Stellar Evolution
- 35. WHAT DID YOUTHINK?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 asymptoticgiant 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
- #2 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.
- #3 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)
- #4 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.
- #5 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.
- #6 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.
- #7 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)
- #8 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)
- #9 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
- #12 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.
- #13 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)
- #14 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)
- #15 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)
- #16 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.
- #18 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)
- #19 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)
- #20 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)
- #21 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)
- #22 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.
- #23 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.
- #24 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.
- #25 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)
- #26 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)
- #27 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.)
- #28 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.
- #29 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.
- #30 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)
- #31 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.
- #32 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.
- #33 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.
- #34 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)
- #35 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.
- #37 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.