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Neil F. Comins • William J. Kaufmann III Discovering the Universe

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1 Neil F. Comins • William J. Kaufmann III Discovering the Universe
Ninth Edition CHAPTER 13 The Deaths of Stars This X-ray image of Tycho’s supernova, first seen as a visible-light object by Tycho Brahe in 1572, was taken in 2003 by the Chandra X-ray Observatory. Gas and dust with temperatures in the millions of kelvins (shown in red and green) are expanding outward at about 10 million km/h, following a shell of high-energy electrons (blue). (NASA/CXC/Rutgers/J. Warren, J. Hughes, et al.)

2 WHAT DO YOU THINK? 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?

3 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

4 Post–Main-Sequence Evolution of Low-Mass Stars
FIGURE 13-1 Post–Main-Sequence Evolution of Low-Mass Stars 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 re-expands, 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. In (c), the star has an inert core of Carbon (atomic number = 6, mass = 12) and oxygen (atomic number = 8, mass =16). A higher-mass star could fuse these, but a low-mass star never reaches the necessary temperature of 600 million K. So, with an inert core, the star expands again, reminiscent of the expansion as a red giant. Now its called an Asymptotic Giant Branch star (AGB). (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.

5 The Structure of an Old Low-Mass Star
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 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. At this point, some instabilities in the fusion layers cause some pulses that blow off outer layers. We’ll see what this produces, and what gets left behind. 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.

6 Evolution from Supergiants to White Dwarfs
Masses in Msun Ltr Mn Pl White Seq Neb Dwarf A B C FIGURE 13-3 Evolution from Supergiants to White Dwarfs The evolutionary tracks of three low-mass supergiants are shown as they eject lanetary nebulae. The table gives their masses as supergiants, the amount of mass they lose as planetary nebulae, and their remaining (white dwarf) masses. 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. The numbers in the table illustrate that the lower the mass, the lower the percentage of material blown off as a planetary nebula. Look out for that term: a “planetary nebula” has NOTHING to do with planets. When the first ones were seen, they were reminiscent of planets and received that misleading name. It’s too hard to change. 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.

7 Some Shapes of Planetary Nebulae
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) 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. (WIYN/NOAL/NSF) 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.

8 Some Shapes of Planetary Nebulae
FIGURE 13-4 Some Shapes of Planetary Nebulae (b) 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. (NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; Howard Bons, STScI/Robin Ciardullo, Pennsylvania State University/NASA) 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.

9 Some Shapes of Planetary Nebulae
FIGURE 13-4 Some Shapes of Planetary Nebulae (c) NGC 6826 shows, among other features, lobes of nitrogen-rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus. (NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; AURA/STScI/NASA) NGC 6826 shows, among other features, lobes of nitrogen-rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus.

10 Some Shapes of Planetary Nebulae
FIGURE 13-4 Some Shapes of Planetary Nebulae (d) 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. (NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO) In all these cases, the central star is visible. It is a white dwarf. A white dwarf is roughly the mass of the Sun packed into something roughly the size of the Earth. Typical density 1 million times the density of water. A teaspoon full (in its environment) has the mass of an SUV. Surface gravity 300 thousand times Earth’s. Surface temperature usually hot (for a star). 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.

11 Formation of a Bipolar Planetary Nebula
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... 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...

12 Formation of a Bipolar Planetary Nebula
FIGURE 13-5 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. …(b) followed by outflow that is channeled by the original gas to squirt out perpendicularly to the plane of the doughnut.

13 Formation of a Bipolar Planetary Nebula
FIGURE 13-5 Formation of a Bipolar Planetary Nebula (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. (R. Sahai and J. Trauer, JPL; WFPC-2 Science Team; and NASA) 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.

14 Sirius and Its White Dwarf Companion
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. (R. B. Minton) Two interesting properties of white dwarfs are: (1) The more mass is added, the smaller they become – the extra gravity causes more compression, which actually decreases the radius. (2) The electron degeneracy pressure which supports them has a limit, associated with the fact that electrons cannot exceed the speed of light. This very important limit is called the Chandrasekhar Mass (Limit) and is about 1.4 Msun 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.

15 Sirius and Its White Dwarf Companion
FIGURE 13-6 Sirius and Its White Dwarf Companion (b) Since Sirius A (11,000 K) and Sirius B (30,000 K) are hot blackbodies, they are strong emitters of X rays. (NASA/SAO/CXC) Skelton: I checked the Chandra website. Sirius B is the brighter X-ray source. Sirius A, The main-seq star is visible only because the ultraviolet blocker isn’t perfect. The White Dwarf is the BRIGHT source, at 25 or 30 thousand K

16 Nova Herculis 1934 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) Novae (plural of Nova) fade over a few weeks, as the light curve shows. Observationally, they are similar to supernovae (which fade more slowly). Physically, they are entirely different. Novae involve a white dwarf in a close orbit. The white dwarf is carbon and oxygen. Its very strong gravitational field compresses the hydrogen accreted from the companion. When the shell gets thick enough, the hydrogen-to-helium reactions start. The heat generated causes all the rest of the accreted hydrogen to fuse abruptly. The explosion is mild by astrophysical standards, and the white dwarf remains afterward. 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.

17 The Light Curve of a Nova
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 The rapid rise in magnitude followed by a gradual decline is characteristic of many novae, although some oscillate in intensity as they become dimmer. 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 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 A nova is believed to occur when which of the following pairs of stars are in a binary system?
A. white dwarf, main sequence star B. white dwarf, neutron star C. neutron star, red giant D. a pair of supergiants

19 White dwarves are composed primarily of:
A. helium B. neutrons D. carbon and oxygen E. iron

20 Supernova Light Curves – Fade Slower
FIGURE 13-9 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. In a supernova, the star explodes. We will discuss only Type Ia and Type II. Type IA supernovae do not involve a high-mass star. The situation is that a white dwarf has a mass near the Chandrasekhar Limit, and also has a companion to accrete from. If its accretion puts it over the limit, it will collapse (remember, adding mass to a white dwarf makes it smaller). The collapse heats it up to the point that some of the nuclear reactions that fuse carbon start. These release energy: more heat, more reactions, more energy – and an explosion similar in magnitude to the “Type II” (discussed later). The Type Ia supernovae are extremely important in astronomy. Because every one is “identical” to every other one, all have the same peak luminosity. This allows the distance to be made, in a manner similar to how it as done for Cepheids. 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.

21 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.

22 The Structure of an Old High-Mass Star
FIGURE 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. High-mass (we will say over 8 Msun) stars do have enough gravity, mass, pressure, and temperature (into billions K) to fuse heavier elements. The key is that nuclear reactions must liberate energy (“exothermic”) to prevent gravity from crushing them. The last exothermic reactions are those which create iron (atomic number = 26, mass = 56). The iron core continues to build up. Iron is the “end of the line.” Sounds dramatic? It is. When the iron core reaches the Chandrasekhar mass limit, electron degeneracy pressure no longer supports it. It collapses. Electrons combine with protons to form neutrons. At some point of collapse, the neutrons start to “touch”. The repulsion causes a bounce. This shock wave from the bounce slams into the higher layers (which are also collapsing). The result is a supernova explosion, which includes thousands of nuclear reactions, including ones which build up elements heavier than iron due to the plentiful supply of neutrons. The central part becomes a neutron star or a black hole. 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.

23 Which type of star is forming iron in its core?
A. supergiant B. giant C. main sequence D. white dwarf

24 Why is Iron the end-of-the-line?
Both insets are simply expansions of areas of the overall plot. The vertical axis, “Rest Mass per nucleon” represents the stability of a nucleus. Reactions which liberate energy are those which go to lower values, like a rock rolling downhill. (a) The full diagram shows a maximum stability (minimum of curve) around mass numbers 50 to 60. This explains why fusing light nuclei (H to He to C) liberates energy, but splitting heavy nuclei (Uranium-235) also releases energy. (b) The middle inset zooms in on the low mass numbers in the first several stages of fusion. Fusing hydrogen to helium takes up most of the available decrease, which is why the main sequence stage is most of a star’s lifetime. (c) The small inset zooms in to show that the exact minimum is at mass 56, which corresponds to iron. Slide by R. T. Skelton.

25 Mass Loss by a Supermassive Star
FIGURE Mass Loss by a Supermassive Star Gas and dust ejected by the massive star HD 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. (Gemini Observatory/AURA) This gas and dust was ejected by the massive star HD 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.

26 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. FIGURE 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. (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)

27 The Gum Nebula FIGURE 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. (Royal Observatory, Edinburgh) 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.

28 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. FIGURE 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. (a: NASA/ CXC/GSFC/U. Hwang et al.; b: Very Large Array)

29 Most of its life is in the Hydrogen Fusion stage, what is happening internally while the star is externally on the Main Sequence. Once it completes Helium Fusion, time scales shorten considerably.

30 Cosmic Ray Shower FIGURE 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. (Simon Swordy [U. Chicago], NASA) 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.

31 Supernova 1987A FIGURE Supernova 1987A A supernova was discovered in a nearby galaxy called the Large Magellanic Cloud (LMC) in 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) This was exciting because supernovae in our galaxy are seen about once per 200 years. The rate in our galaxy is believed to be one per 30 or 40 years, with most of them obscured by dust. A supernova was discovered in a nearby galaxy called the Large Magellanic Cloud (LMC) in 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.

32 Shells of Gas Around SN 1987A
FIGURE 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 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. (Robert P. Kirshner and Peter Challis, Harvard-Smithsonian Center for Astrophysics; STScI) (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.

33 A Recording of a Pulsar – Light Curve in Radio
FIGURE 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. In a Type II supernova, an outer shell is blown off and a central object remains behind. That central object might be a neutron star or a black hole (or possibly a quark star). We will focus first on neutron stars. A neutron star is an object approximately 10 km in radius having a mass around 1.8 Msun, made up mostly of neutrons. As extreme as surface gravity and density were on a white dwarf, they are a million or more times greater for a neutron star. These pulses represent the spin of the neutron star. The idea of a neutron star was proposed by Walter Bade and Fritz Zwicky in However, such an object seemed so improbable and undetectable that it astronomers forgot about it. On the initial discovery of radio pulses by Jocelyn Bell in 1967, it was seriously considered that this might be an alien civilization broadcasting to the universe. 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.

34 The Crab Nebula and Pulsar
FIGURE 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 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. (Main photo: NASA/CXC/SAO; insets, clockwise from top: NRAO; 2MASS/UMass/IPAC-Caltech/NASA/NSF; Palomar Observatory; NASA/CXC/SAO.) (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 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.

35 The Crab Nebula and Pulsar
FIGURE 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 s. (NASA/CXC/SAO) The pulsations depend on the neuron star spinning and having an extremely strong magnetic field which is NOT aligned with its spin axis, and also on Earth being in its beam as it rotates. (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 s.

36 Analogy for How Magnetic Field Strengths Increase
FIGURE 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. The collapsing star carries the field inward, thereby increasing its strength. (a: Corbis; b: Oscar Burriel/Photo Researchers, Inc.) The cause for a pulsar (neutron star) extreme magnetic field is illustrated. The total amount of magnetic flux is constant, but it gets compressed, resulting in a great increase in intensity. The magnetic field on a neutron star can be a million times the strongest field that can be created in a laboratory dedicated to producing high fields. 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.

37 A Rotating, Magnetized Neutron Star
FIGURE 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. So a PULSAR is a NEUTRON STAR with: (a) a spin; (b) a strong magnetic field, misaligned with the spin; and (c) oriented so that our solar system is in its beam. 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.

38 The size of a white dwarf is closest to which of the following. A
The size of a white dwarf is closest to which of the following? A. about 1 A.U. in diameter B. about the size of the Sun C. about the size of the Earth D. about 10 kilometers in diameter

39 The size of a neutron star is closest to which of the following. A
The size of a neutron star is closest to which of the following? A. about 1 A.U. in diameter B. about the size of the Sun C. about the size of the Earth D. about 10 kilometers in diameter

40 Model of a Neutron Star’s Interior
This drawing shows the theoretical model of a 1.4-solar-mass 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. FIGURE Model of a Neutron Star’s Interior This drawing shows the theoretical model of a 1.4 solar mass 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.

41 A Glitch Interrupts the Vela Pulsar’s Spindown Rate
FIGURE 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. Recall that the planets bulge around their equators because of centrifugal force from the spin. So do neutron stars. As energy (and angular momentum) are radiated away, the neutron star spins down, and the centrifugal force decreases and the neutron star becomes more spherical. The glitch indicates that the readjustment of shape is abrupt, as though the neutron star is solid. More than we need for Astro 101 for those interested. The surface density is a million times the density of water. 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.

42 Double Pulsar FIGURE Double Pulsar This artist’s conception shows two pulsars that orbit their center of mass. The double pulsar they represent is called PSR J , 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 hours. (Michael Kramer/Jodrell Bank Observatory, University of Manchester) Deeper than we need for the course, but it was unexpected that two neutron stars would still be bound in orbit around each other after two supernova explosions This artist’s conception shows two pulsars that orbit their center of mass. The double pulsar they represent is called PSR J , 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.

43 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. 

44 X-Ray Pulses from Centaurus X-3
FIGURE 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. 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.

45 A Model of a Pulsating X-Ray Source
FIGURE 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. 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.

46 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

47 X Rays from an X-Ray Burster
FIGURE 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 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.

48 A Summary of Stellar Evolution
FIGURE A Summary of Stellar Evolution The evolution of isolated stars depends 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. Note the spelling should be “neutron star”. 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.

49 A Summary of Stellar Evolution
FIGURE A Summary of Stellar Evolution (b) The cycle of stellar evolution is summarized in this figure. (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) Each cycle produces more heavy elements, so each successive generation is more enriched in heavy elements. The cycle of stellar evolution is summarized in this figure.

50 Summary of Key Ideas

51 Low-Mass Stars and Planetary Nebulae
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.

52 Low-Mass Stars and Planetary Nebulae
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.

53 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.

54 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 Oppenheimer-Volkov 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.

55 Key Terms neutron star asymptotic giant branch nova (plural novae)
(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

56 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.

57 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.

58 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.

59 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.

60 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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