Discovering the Universe Ninth Edition

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Ninth Edition CHAPTER 13 The Deaths of Stars

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 (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 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.

6. Evolution from Supergiants to White Dwarfs The evolutionary tracks of three low-mass supergiants are shown as they eject planetary nebulae. The loops indicate periods of instability and adjustment for the white dwarfs. The dots on this graph represent the central stars of planetary nebulae whose surface temperatures and luminosities have been determined. The crosses are white dwarfs for which similar data exist.

7. Some Shapes of Planetary Nebulae An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth.

8. Some Shapes of Planetary Nebulae The Helix Nebula, NGC 7293, located in the constellation Aquarius, about 700 ly (215 pc) from Earth, has an angular diameter equal to about half that of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue gas is rich in oxygen.

9. Some Shapes of Planetary Nebulae 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 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 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 …(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 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 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 Since Sirius A (11,000 K) and Sirius B (30,000 K) are hot blackbodies, they are strong emitters of X rays.

16. 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.

17. 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. 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.

19. 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.

20. Mass Loss by a Supermassive Star This gas and dust was ejected by the massive star HD 148937 in the constellation Norma. Located about 4200 ly (1300 pc) away, this star has 40 solar masses and is more than halfway through its 6-million-year lifespan.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. Shells of Gas Around SN 1987A (a) Intense radiation from the supernova explosion caused three rings of gas surrounding SN 1987A to glow in this HST image. This gas was ejected from the star 20,000 years before the star detonated. All three rings lie in parallel planes. The inner ring is about 1.3ly 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.

27. 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.

28. 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.

29. The Crab Nebula and Pulsar (b) The insets show the Crab pulsar in its “on” and “off” states. Both its visible flashes and X-ray pulses have identical periods of 0.033 s.

30. Analogy for How Magnetic Field Strengths Increase When growing, these wheat stalks cover a much larger area than when they are harvested and bound together. A star’s magnetic field behaves similarly, as the collapsing star carries the field inward, thereby increasing its strength.

31. 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.

32. 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.

33. 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.

34. Double Pulsar This artist’s conception shows two pulsars that orbit their center of mass. The double pulsar they represent is called PSR J0737-3039, which is about 1500 ly from Earth in the constellation Puppis. One pulsar has a 23-ms period and the other has a 2.8-s period. The two orbit once every 2.4 h.

35. 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’sX-ray detectors toward the source as the satellite rotated. The short pulse period suggests that the source is a rotating neutron star.

36. 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.

37. 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.

38. A Summary of Stellar Evolution The evolution of isolated stars depends primarily on their masses. The higher the mass, the shorter the lifetime. Stars less massive than about 8 solar masses can eject enough mass to become white dwarfs. High-mass stars can produce Type II supernovae and become neutron stars or black holes. The horizontal (time) axis is not to scale, but the relative lifetimes are accurate.

39. A Summary of Stellar Evolution The cycle of stellar evolution is summarized in this figure.

40. Summary of Key Ideas

41. 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.

42. 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.

43. 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.

44. 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.

45. Key Terms 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 asymptotic giant branch (AGB) star Chandrasekhar limit cosmic ray cosmic ray shower glitch helium shell flash helium shell fusion lighthouse model magnetar neutron degeneracy pressure

46. 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.

47. 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.

48. 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.

49. 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.

50. 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.