Week #9 Notes:

1. Week #9 Notes: The Death of Stars: Recycling

2. Introduction • The more massive a star is, the shorter its stay on the main sequence. • The most massive stars may be there for only a few million years. • A star like the Sun, on the other hand, is not especially massive and will live on the main sequence for about ten billion years. • Since it has taken over four billion years for humans to evolve, it is a good thing that some stars can be stable for such long times.

3. Introduction • This week, we will first discuss what will happen when the Sun dies: It will follow the same path as other single lightweight stars, stars born with up to about 10 (but possibly as low as 8) times the mass of the Sun. • They will go through planetary nebula (see figure) and white-dwarf stages. • Then we will discuss the death of more massive stars, greater than about 8 or 10 times the Sun’s mass, which we can call heavyweight stars. • They go through spectacular stages. • Some wind up in such a strange final state—a black hole—that we devote the entire next chapter to it.

4. The Death of the Sun: • Most solitary (that is, single) stars containing less than about 8 –10 times the Sun’s mass will have the same fate. (Later we will see that stars in tightly bound binary systems can end their lives in a different manner.) • Since the Sun is a typical star in this mass range, we focus our attention on stars of one solar mass when describing the various evolutionary stages of stars.

5. Red Giants • As fusion exhausts the hydrogen in the center of a star (after about 10 billion years on the main sequence, for the Sun), its core’s internal pressure diminishes because temperatures are not yet sufficiently high to fuse helium into heavier elements. • Gravity pulls the core in, heating it up again. • Hydrogen begins to “burn” more vigorously in the now hotter shell around the core. (The process is once again nuclear fusion, not the chemical burning we have on Earth.)

6. Red Giants: • The new energy causes the outer layers of the star to swell by a factor of 10 or more. • The solar surface will be relatively cool for a star, only about 3000 K, so it will appear reddish. • Such a star is called a red giant. • Red giants appear at the upper right of temperature-luminosity (Hertzsprung-Russell) diagrams. • The Sun will be in this stage, or on the way to it after the main sequence, for about a billion years, only 10 per cent of its lifetime on the main sequence. • Red giants are so luminous that we can see them at quite a distance, and a few are among the brightest stars in the sky. • Arcturus in Boötes and Aldebaran in Taurus are both red giants.

7. Red Giants: • The contracting core eventually becomes so hot that helium will start fusing into carbon and oxygen nuclei, but this stage will last only a brief time (for a star), perhaps 100 million years. • During this time, the star becomes slightly cooler and fainter. • Not being hot enough to fuse carbon nuclei, the core once again contracts and heats up, generating energy and causing the surrounding shells of helium and hydrogen to fuse even faster than before. • This input of energy makes the outer layers expand again, and the star becomes an even larger red giant.

8. Planetary Nebulae: • As the star grows still larger during the second red-giant phase, the loosely bound outer layers can continue to drift outward until they leave the star. • Perhaps the outer layers escape as a shell of gas, in a relatively gentle ejection that we can think of as a “cosmic burp.” • Or perhaps they drift off gradually (in the form of a “wind”), and a second round of gas sometimes comes off at a more rapid pace. • This second round of gas plows into the first round, creating a visible shell (see figure).

9. Planetary Nebulae • We know of about a thousand such glowing shells of gas in our Milky Way Galaxy. • Each shell contains roughly 20 per cent of the Sun’s mass. • They are exceptionally beautiful. • In the small telescopes of a hundred years ago, though, they appeared as faint greenish objects, similar to the planet Uranus. • These objects were thus named planetary nebulae. • The remaining part of the star in the center is the star’s exposed hot core, which reaches temperatures of 100,000 K and so appears bluish. • It is known as the “central star of a planetary nebula.” • It is on its way to becoming a white dwarf (see next section).

10. Planetary Nebulae • We now know that planetary nebulae generally look greenish because the gas in them emits mainly a few strong spectral emission lines that include greenish ones, specifically lines of doubly ionized oxygen (see figure).

11. Planetary Nebulae • The best-known planetary nebula is the Ring Nebula in the constellation Lyra (see figure, top). • It is visible in even a medium-sized telescope as a tiny apparent smoke ring in the sky. • Only photographs reveal the vivid colors. • The Dumbbell Nebula is another famous example. • The Helix Nebula (see figure, bottom) is so close to us that it covers about half the apparent diameter in the sky as the full moon, though it is much fainter. • Each planetary-nebula stage in the life of a Sun-like star lasts only about 50,000 years; after that time, the nebula spreads out and fades too much to be seen at a distance.

12. Planetary Nebulae • The Hubble Space Telescope has viewed planetary nebulae with a resolution about 10 times better than most images from the ground, and has revealed new glories in them. • Its infrared camera provided views of different aspects of some of the planetary nebulae (see figure). • To our surprise, planetary nebulae turn out to be less round than previously thought. • The stars aren’t losing their mass symmetrically in all directions.

13. White Dwarfs • Through a series of winds and planetary-nebula ejections, all stars that are initially up to 8 (or perhaps even 10) times the Sun’s mass manage to lose most of their mass. • The remaining stellar core is less than 1.4 times the Sun’s mass. (The Sun itself will have only 0.6 of its current mass at that time, in about 6 billion years.) • This results in a type of star called a white dwarf (see figure).

14. White Dwarfs • When its remaining mass (about 0.6 solar masses) is compressed into a volume 100 times smaller across, which is a million times smaller in volume, the density of matter goes up incredibly. • A single teaspoonful of a white dwarf would weigh about 5 tons! • A white dwarf ’s mass cannot exceed 1.4 times the Sun’s mass; it would become unstable and either collapse or explode. • This theoretical maximum was worked out by an Indian university student, S. Chandrasekhar (usually pronounced “chan dra sek´ har” in the United States), en route to England in 1930. It is called the “Chandrasekhar limit.” • A major NASA spacecraft, the Chandra X-ray Observatory, is named after him.

15. White Dwarfs • Because they are so small, white dwarfs are very faint and therefore hard to detect. • Only a few single ones are known. • We find most of them as members of binary systems. • Even the brightest star, Sirius (the Dog Star), has a white-dwarf companion, which is named Sirius B and sometimes called “The Pup” (see figure).

16. White Dwarfs • A very odd and interesting system of white dwarfs was discovered with the Chandra X-ray Observatory. From the periodicity of the x-ray observations (see figure, left), the system is thought to contain two white dwarfs orbiting each other so closely thatthe orbit has a period of only five minutes (see figure, right)!

17. Summary of the Sun’s Evolution • The entire “post-main-sequence” evolution of the Sun, a representative solitary low-mass star, can be tracked in a temperature-luminosity diagram (see figure), or Hertzsprung-Russell diagram • The Sun “moves” through the diagram, but of course we really mean that the combination of luminosity and surface temperature changes with time, and that this changing set of values is reflected by a “trajectory” in the diagram.

18. Binary Stars and Novae • Single stars evolve in a simple manner. • In particular, their main-sequence lifetime depends primarily on their mass. • Most stars, however, are in binary systems, and the stars can exchange matter. • Surrounding each star is a region known as the Roche lobe, in which its gravity dominates over that of the other star (see figure). • The Roche lobes of the two stars join at a point between them, forming a “figure-8” shape. (Édouard Roche was a 19thcentury French mathematician.)

19. Binary Stars and Novae • Consider two main-sequence stars. • As the more massive star evolves to the red giant phase, it fills its Roche lobe, and gas can flow from this “donor” star toward the lower-mass companion (see figure, top). • The recipient star can gain considerable mass, and it subsequently evolves faster than it would have as a single star. • Note that the flowing matter forms an accretion disk around the recipient star because of the rotation of the system (see figure, bottom).

20. Binary Stars and Novae • If one star is already a white dwarf and the companion (donor) fills its Roche lobe (for example, on its way to the red giant phase), a nova can result (see figure). • For millennia, apparently new stars (novae, pronounced “no´vee” or “no´vay,” the plural of nova) have occasionally become visible in the sky.

21. Binary Stars and Novae • Actually, however, a nova is an old star that brightens by a factor of a hundred to a million (corresponding to 5 to 15 magnitudes) in a few days or weeks. • It then fades over the course of weeks, months, or years. • The ejected gas may eventually become visible as an expanding shell.

22. Core-Collapse Supernovae • Stars that are more than 8 –10 times as massive as the Sun whip through their main-sequence lifetimes at a rapid pace. • These prodigal stars use up their store of hydrogen very quickly. • A star containing 15 times as much mass as the Sun may take only 10 million years from the time it reaches the main sequence until it fully uses up the hydrogen in its core. • This timescale is 1000 times faster than that of the Sun. • For these massive stars, the outer layers expand as the helium core contracts. • The star has become so large that we call it a red supergiant. • Betelgeuse, the star that marks the shoulder of Orion, is the best-known example (see figure).

23. Core-Collapse Supernovae • Eventually, the core temperature reaches 100 million degrees, and the triple-alpha process begins to transform helium into carbon. • Some of the carbon nuclei then fuse with a helium nucleus (alpha particle) to form oxygen. • The carbon-oxygen core of a supergiant contracts, heats up, and begins fusing into still heavier elements. • The ashes of one set of nuclear reactions become the fuel for the next set. • Each stage of fusion gives off energy. • Finally, even iron builds up. • Layers of elements of progressively lower mass surround the iron core, somewhat resembling the shells of an onion. • But when iron fuses into heavier elements, it takes up energy instead of giving it off. • No new energy is released to make enough pressure to hold up the star against the force of gravity pulling in; thus, the iron doesn’t fuse.

24. Core-Collapse Supernovae • Instead, the mass of the iron core increases as nuclear fusion of lighter elements takes place, and its temperature increases. • Eventually the temperature becomes so high that the iron begins to break down (disintegrate) into smaller units like helium nuclei. • This breakdown soaks up energy and reduces the pressure. • The core can no longer counterbalance gravity, and it collapses. • The core’s density becomes so high that electrons are squeezed into the nuclei. • They react with the protons there to produce neutrons and neutrinos. • Additional neutrinos are emitted spontaneously at the exceedingly high temperature (10 to 100 billion kelvins) of the collapsing core. • All of these neutrinos escape within a few seconds, carrying large amounts of energy.

25. Core-Collapse Supernovae • The collapsing core of neutrons overshoots its equilibrium size and rebounds outward, like someone jumping on a trampoline. • The rebounding core collides with the inward-falling surrounding layers and propels them outward, greatly assisted by the plentiful neutrinos (only a very tiny fraction of which actually interact with the gas). • The star explodes, achieving within one day a stupendous optical luminosity rivaling the brightness of a billion normal stars. • It has become a supernova (see figure on next slide), and it will continue to shine for several years, gradually fading away. • So much energy is available that very heavy elements, including those heavier than iron, form in the ejected layers. • The core remains as a compact sphere of neutrons called a neutron star. • There is even some evidence that occasionally, the neutron star further collapses to form a black hole.

26. Core-Collapse Supernovae

27. Core-Collapse Supernovae • Such supernovae (the plural of supernova), known as Type II (they show hydrogen lines in their spectra, unlike Type I supernovae), mark the violent death of heavyweight stars that have retained at least part of their outer layer of hydrogen. • Since the fundamental physical mechanism is the collapse of the iron core, they are also one type of core-collapse supernova.

28. Observing Supernovae • Only in the 1920s was it realized that some of the “novae”—apparently new stars—that had been seen in other galaxies (see figure) were really much more luminous than ordinary novae seen in our own Milky Way Galaxy. • These supernovae are very different kinds of objects. • Whereas novae are small eruptions involving only a tiny fraction of a star’s mass, supernovae involve entire stars. • A supernova may appear about as bright as the entire galaxy it is in.

29. Observing Supernovae • Unfortunately, we have seen very few supernovae in our own Galaxy, and none since the invention of the telescope. • The most recent ones definitely noticed were observed by Kepler in 1604 and Tycho in 1572. • A relatively nearby supernova might appear as bright as the full moon, and be visible night and day. • Since studies in other large galaxies show that supernovae erupt every 30 to 50 years on the average, we appear to be due, although a few supernovae have probably occurred in distant, obscured parts of our Galaxy.

30. Observing Supernovae • Photography of the sky has revealed some two dozen regions of gas in our Galaxy that are supernova remnants, the gas spread out by the explosion of a supernova (see figure, left). • The most studied supernova remnant is the Crab Nebula in the constellation Taurus (see figures, right). • The explosion was noticed widely in China, Japan, and Korea in a.d. 1054; there is still debate as to why Europeans did not see it.

31. Observing Supernovae • If we compare photographs of the Crab taken decades apart, we can measure the speed at which its filaments are expanding. • Tracing them back shows that they were together at one point, at about the time the bright “guest star” was seen in the sky by the observers in Asia, confirming the identification. • The rapid speed of expansion—thousands of kilometers per second—also confirms that the Crab Nebula comes from an explosive event.

32. 13.2c Observing Supernovae • The Chandra X-ray Observatory is giving us high-resolution x-ray images of supernova remnants (see figures). • The x-rays reveal exceptionally hot gas produced by the collision of the supernova with gas surrounding it.

33. 13.2d Supernovae and Us • The heavy elements that are formed and thrown out by both core-collapse supernovae and white-dwarf supernovae are necessary for life as we know it. • Directly or indirectly, supernovae are the only known source of most heavy elements, especially those past iron (Fe) on the Periodic Table of the Elements. • They are spread through space and are incorporated in stars and planets that form later on. • Specifically, the Sun and our Solar System were made from the debris of many previous generations of stars. • So we humans, who depend on heavy elements for our existence, are here because of supernovae and this process of recycling material. • Think about it: The carbon in your cells, the oxygen that you breathe, the calcium in your bones, and the iron in your blood are just a few examples of the elements produced long ago by stars and their explosions. (Other examples are the silver, gold, and platinum in jewelry—but these are not vital for the existence of life!) • Thus, as the late Carl Sagan was fond of saying, “We are made of star stuff [or stardust].”

34. Supernova 1987A! • An astronomer’s delight, a supernova quite bright but at a safe distance, appeared in 1987. • On February 24 of that year, Ian Shelton, then of the University of Toronto, was photographing the Large Magellanic Cloud, a small galaxy 170,000 light-years away, with a telescope in Chile. • Fortunately, he chose to develop his photograph that night.

35. Supernova 1987A! • When he looked at the photograph still in the darkroom, he saw a bright star where no such star belonged (see figures). • He went outside, looked up, and again saw the star in the Large Magellanic Cloud, this time with his naked eye. • He had discovered the nearest supernova to Earth seen since Kepler saw one in 1604. • By the next night, the news was all over the world, and all the telescopes that could see Supernova 1987A (the first supernova found in 1987) were focused on it. • Some of these telescopes, as well as the Hubble Space Telescope, continue to observe the supernova on a regular basis to this day.

36. Supernova 1987A! • Hubble’s high resolution shows clear views of an inner ring of material produced prior to the supernova, slowly expanding around the supernova (see figure, left). • The supernova debris is in the process of meeting up with the ring, and the collision should cause the supernova debris and the ring to brighten substantially over the next few years. • Already, we see many individual “hot spots” brightening in the ring of material (see figure, below). • Two outer rings are also visible.

37. Supernova 1987A! • One exciting thing about such a close supernova is that we even know which star had erupted! • Pre-explosion photographs showed that a blue supergiant star had been where the supernova now is (see figure).

38. Supernova 1987A! • The Chandra X-ray Observatory image (see figure) shows hot gas, millions of kelvins, matching the optical bright spots. • The optical and x-ray spots result from a collision of the shock waves with the fingers of cool gas. • Scientists expect the ring to brighten still more.

39. Neutron Stars • The collapsed core in a core-collapse supernova is a very compact object, generally a neutron star consisting mainly of neutrons, as already mentioned in Section 13.2a. • They are like a single, giant atomic nucleus, without the protons (see figure). • Neutron stars have measured masses of about 1.4 Suns, but some might exist with up to 2 or 3 solar masses; astronomers don’t understand the limit for neutron stars as accurately as they know that 1.4 solar masses is the limit for white dwarfs.

40. Neutron Stars • Neutron stars are only about 20 or 30 km across (see figure); a teaspoonful would weigh a billion tons. • At such high densities, the neutrons resist being further compressed; they become “degenerate.” • A pressure (“neutron-degeneracy pressure”) is created, which counterbalances the inward force of gravity. • When an object contracts, its magnetic field is compressed. • As the magnetic-field lines come together, the field gets stronger. • A neutron star is so much smaller than the Sun that its magnetic field should be about a trillion times stronger. • When neutron stars were first discussed theoretically in the 1930s, the chances of observing one seemed hopeless. • But we currently can detect signs of them in several independent and surprising ways, as we now discuss.

41. The Discovery of Pulsars • Recall that the light from stars twinkles in the sky because the stars are point-like objects, with the Earth’s atmospheric turbulence bending the light rays. • Similarly, point-like radio sources (radio sources that are so small or so far away that they have no apparent length or breadth) fluctuate in brightness on timescales of a second because of variations in the density of electrons in interplanetary space. • In 1967, a special radio telescope was built to study this radio twinkling; previously, radio astronomers had mostly ignored and blurred out the effect to study the objects themselves.

42. The Discovery of Pulsars • In 1967 Jocelyn Bell (now Jocelyn Bell Burnell) was a graduate student working with Professor Antony Hewish’s special radio telescope (see figure). • As the sky swept over the telescope, which pointed in a fixed direction, she noticed that the signal occasionally wavered a lot in the middle of the night, when radio twinkling was usually low. • Her observations eventually showed that the position of the source of the signals remained fixed with respect to the stars rather than constant in terrestrial time (for example, always occurring at exactly midnight). • This timing implied that the phenomenon was celestial rather than terrestrial or solar.

43. 13.3b The Discovery of Pulsars • Bell and Hewish found that the signal, when spread out, was a set of regularly spaced pulses, with one pulse every 1.3373011 seconds (see figure). • The source was briefly called LGM, for “Little Green Men,” because such a signal might come from an extraterrestrial civilization! • But soon Bell located three other sources, pulsing with regular periods of 0.253065, 1.187911, and 1.2737635 seconds, respectively. • Though they could be LGM2, LGM3, and LGM4, it seemed unlikely that extraterrestrials would have put out four such beacons at widely spaced locations in our Galaxy. • The objects were named pulsars—to indicate that they gave out pulses of radio waves—and announced to an astonished world. • It was immediately apparent that they were an important discovery, but what were they?

44. What Are Pulsars?

45. The Crab, Pulsars, and Supernovae • Several months after the first pulsars had been discovered, strong bursts of radio energy were found to be coming from the direction of the Crab Nebula. • Observers detected that the Crab pulsed 30 times per second, almost ten times more rapidly than the fastest other pulsar then known. • This very rapid pulsation definitively excluded white dwarfs from the list of possible explanations. • The discovery of a pulsar in the Crab Nebula made the theory that pulsars were neutron stars look more plausible, since neutron stars should exist in supernova remnants like this one. • And the case was clinched when it was discovered that the clock in the Crab pulsar was not precise—it was slowing down slightly. • The energy given off as the pulsar slowed down was precisely the amount of energy needed to keep the Crab Nebula shining. • The source of the Crab Nebula’s energy had been discovered!

46. The Crab, Pulsars, and Supernovae • Astronomers soon found, to their surprise, that an optically visible star in the center of the Crab Nebula could be seen apparently to turn on and off 30 times per second. • Actually the star only appears “on” when its beamed light is pointing toward us as it sweeps around. • Long photographic exposures had always hidden this fact, though the star had been thought to be the remaining core because of its spectrum, which oddly doesn’t show any emission or absorption lines. • Later, similar observations of the star’s blinking on and off in x-rays were also found (see figure).

47. The Crab, Pulsars, and Supernovae • Even more recently, the high-resolution observations by the Hubble Space Telescope and Chandra X-ray Observatory of the Crab Nebula revealed interesting structure near its core (see figure).

48. A Pulsar with a Planet • Earlier in this book, in Chapters 2 and 9, we described the discovery of planets around other stars. • But these extra-solar planets were not the first to be discovered. • In 1991, the first ones were discovered by observing a pulsar. • The detections were from observations of a pulsar that pulses very rapidly—162 times each second. • The arrival time of the pulsar’s radio pulses varied slightly (see figure), indicating that something is orbiting the pulsar and pulling it slightly back and forth.

49. A Pulsar with a Planet • Alex Wolszczan, now at Penn State, has concluded that the variations in the pulse-arrival time are caused by three planets in orbit around the pulsar. • These planets are 0.19, 0.36, and 0.47 A.U. from the pulsar, within about the same distance that Mercury is from the Sun. • They revolve in 25.3-, 66.5-, and 98.2-day periods, respectively. • The system is 2000 light-years from us, too faint for us to detect optically.

50. A Pulsar with a Planet • The presence of the planets was conclusively verified when they interacted gravitationally as they passed by each other. • The two most massive planets are calculated from the observations to be somewhat larger than Earth, each containing about 4 times its mass. • The innermost planet is much less massive than Earth. • The existence of a fourth planet farther out in the system is possible but uncertain. • Astronomers think that neutron stars are formed in supernova explosions, so any original planets almost certainly didn’t survive the explosion. • Most likely, the planets formed after the supernova explosion, from a disk of material in orbit around the neutron star remnant. • These pulsar planets are not the ones on which we expect life will have arisen!