The Deaths of Stars As you read this chapter, take a moment to be astonished and proud that the human race knows how stars die. Finding the masses of stars is difficult, and understanding the invisible matter between the stars takes ingenuity, but we humans have used what we know about stars and what we know about the physics of energy and matter to solve one of nature’s deepest mysteries. We know how stars die. The previous unit made heavy use of theory in describing the internal structure of stars. We tested that theory against observations of clusters of stars and variable stars. In this chapter, we have more direct observational evidence to help us understand how stars die. We still need theory, but images and spectra of dying stars give us landmarks to guide our theoretical explorations. This unit leads us onward, not only to discuss the remains of dead stars—neutron stars and black holes—but it also leads us into discussions of the formation of our galaxy, the origin of the universe, the formation of planets, and the origin of life. Our world is made of atoms that were born in the deaths of stars. This unit of endings is also a unitof beginnings.

Outline I. Lower-Main-Sequence Stars A. Red Dwarfs B. Sunlike Stars C. Mass Loss from Sunlike Stars D. Planetary Nebulae E. White Dwarfs II. The Evolution of Binary Stars A. Mass Transfer B. Recycled Stellar Evolution C. Accretion Disks D. Nova Explosions E. The End of Earth

Outline (continued) III. The Deaths of Massive Stars A. Nuclear Fusion in Massive Stars B. The Iron Core C. The Supernova Deaths of Massive Stars D. Types of Supernovae E. Observations of Supernovae F. The Great Supernova of 1987 G. Local Supernovae and Life on Earth

Less massive stars will die in a less dramatic event, called a nova The End of a Star’s Life When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die. High-mass stars will die first, in a gigantic explosion, called a supernova. Less massive stars will die in a less dramatic event, called a nova

Red Dwarfs Stars with less than ~ 0.4 solar masses are completely convective. Mass  Hydrogen and helium remain well mixed throughout the entire star.  No phase of shell “burning” with expansion to giant. Star not hot enough to ignite He burning.

Sunlike Stars Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core. Mass  Expansion to red giant during H burning shell phase  Ignition of He burning in the He core  Formation of a degenerate C,O core

The more massive the star, the stronger its stellar wind. Mass Loss From Stars Stars like our sun are constantly losing mass in a stellar wind ( solar wind). The more massive the star, the stronger its stellar wind. Far-infrared WR 124

The Final Breaths of Sun-Like Stars: Planetary Nebulae Remnants of stars with ~ 1 – a few Msun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts)  Less than 10,000 years old Have nothing to do with planets! The Helix Nebula

The Formation of Planetary Nebulae Two-stage process: Slow wind from a red giant blows away cool, outer layers of the star The Ring Nebula in Lyra Fast wind from hot, inner layers of the star overtakes the slow wind and excites it => Planetary Nebula

The Dumbbell Nebula in Hydrogen and Oxygen Line Emission

Planetary Nebulae Often asymmetric, possibly due to Stellar rotation Magnetic fields Dust disks around the stars The Butterfly Nebula

The Remnants of Sun-Like Stars: White Dwarfs Sunlike stars build up a Carbon-Oxygen (C,O) core, which does not ignite Carbon fusion. He-burning shell keeps dumping C and O onto the core. C,O core collapses and the matter becomes degenerate.  Formation of a White Dwarf

White Dwarfs White Dwarfs: Mass ~ Msun Temp. ~ 25,000 K Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! Chunk of WD material the size of a beach ball would outweigh an ocean liner! White Dwarfs: Mass ~ Msun Temp. ~ 25,000 K Luminosity ~ 0.01 Lsun

White Dwarfs (2) Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.

The Chandrasekhar Limit The more massive a white dwarf, the smaller it is.  Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity. WDs with more than ~ 1.4 solar masses can not exist!

Mass Transfer in Binary Stars In a binary system, each star controls a finite region of space, bounded by the Roche Lobes (or Roche surfaces). Lagrange points = points of stability, where matter can remain without being pulled towards one of the stars. Matter can flow over from one star to another through the Inner Lagrange Point L1.

Recycled Stellar Evolution Mass transfer in a binary system can significantly alter the stars’ masses and affect their stellar evolution.

White Dwarfs in Binary Systems X-ray emission T ~ 106 K Binary consisting of WD + MS or Red Giant star => WD accretes matter from the companion Angular momentum conservation => accreted matter forms a disk, called accretion disk. Matter in the accretion disk heats up to ~ 1 million K => X-ray emission => “X-ray binary”.

Nova Explosions Hydrogen accreted through the accretion disk accumulates on the surface of the WD Very hot, dense layer of non-fusing hydrogen on the WD surface Nova Cygni 1975 Explosive onset of H fusion Nova explosion

Recurrent Novae T Pyxidis In many cases, the mass transfer cycle resumes after a nova explosion. R Aquarii  Cycle of repeating explosions every few years – decades.

The Fate of Our Sun and the End of Earth Sun will expand to a Red giant in ~ 5 billion years Expands to ~ Earth’s radius Earth will then be incinerated! Sun may form a planetary nebula (but uncertain) Sun’s C,O core will become a white dwarf

The Deaths of Massive Stars: Supernovae Final stages of fusion in high-mass stars (> 8 Msun), leading to the formation of an iron core, happen extremely rapidly: Si burning lasts only for ~ 1 day. Iron core ultimately collapses, triggering an explosion that destroys the star: A Supernova

Observations of Supernovae Supernovae can easily be seen in distant galaxies.

Type I and II Supernovae Core collapse of a massive star: Type II Supernova If an accreting White Dwarf exceeds the Chandrasekhar mass limit, it collapses, triggering a Type Ia Supernova. Type I: No hydrogen lines in the spectrum Type II: Hydrogen lines in the spectrum

Remnant of a supernova observed in a.d. 1054 Supernova Remnants X-rays The Crab Nebula: Remnant of a supernova observed in a.d. 1054 Cassiopeia A Optical The Veil Nebula The Cygnus Loop

Synchrotron Emission and Cosmic-Ray Acceleration The shocks of supernova remnants accelerate protons and electrons to extremely high, relativistic energies. “Cosmic Rays” In magnetic fields, these relativistic electrons emit Synchrotron Radiation.

The Famous Supernova of 1987: SN 1987A Before At maximum Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987

The Remnant of SN 1987A Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays.

Local Supernovae and Life on Earth Nearby supernovae (< 50 light years) could kill many life forms on Earth through gamma radiation and high-energy particles. At this time, no star capable of producing a supernova is < 50 ly away. Most massive star known (~ 100 solar masses) is ~ 25,000 ly from Earth.

New Terms nova supernova thermal pulse planetary nebula compact object black dwarf Chandrasekhar limit Roche lobe Roche surface Lagrangian point accretion disk supernova (type I) supernova (type II) carbon deflagration supernova remnant synchrotron radiation  

Quiz Questions 1. What event marks the end of every star's main sequence life? a. The end of hydrogen fusion in the core. b. The beginning of the CNO cycle. c. The beginning of the triple-alpha process. d. The formation of a planetary nebula. e. Both a and c above.

Quiz Questions 2. Why can't the lowest-mass stars become giants? a. They never get hot enough for the triple-alpha process. b. Their gravity is too weak to stop them from expanding beyond the giant phase. c. They live so long that none has ever left the main sequence. d. The rate of hydrogen-shell fusion is too slow to cause the star to expand. e. They are fully connective, and never develop a hydrogen shell fusion zone.

Quiz Questions 3. Why do we suspect that all white dwarfs observed in our galaxy were produced by the death of medium-mass stars? a. The range of white dwarf masses is narrow. b. High-mass stars do not produce white dwarfs. c. Both a and b above.

Quiz Questions 4. What observational evidence do we have that stars are losing mass? a. The solar wind. b. Stellar emission lines at ultraviolet and X-ray wavelengths. c. Some absorption lines in the spectra of giant stars are blue shifted. d. Both a and b above. e. All of the above.

Quiz Questions 5. What type of spectrum does the gas in a planetary nebula produce? a. A continuous spectrum. b. An emission line spectrum. c. An absorption line spectrum. d. An emission line spectrum superimposed on a continuous spectrum. e. All of the above.

Quiz Questions 6. Why are the stars found inside planetary nebulae only at temperatures above 25,000 K? a. These stars are fusing hydrogen at their surface. b. These stars have at least two active layers of fusion. c. These stars have multiple concentric layers of active fusion. d. We cannot see the interior stars that are below this temperature, as they are too dim. e. Planetary nebulae glow due to the ionization of low-density gas by a hot interior star.

Quiz Questions 7. What happens to white dwarfs as they age? a. Their surface temperature decreases. b. Their luminosity decreases. c. Their size decreases. d. Both a and b above. e. All of the above.

Quiz Questions **8. Why have no black dwarfs yet been observed in our galaxy? a. They can only be detected by their gravitational influence on a binary companion. b. They are too dim for our present-day telescopes to detect. c. Astronomers are not motivated to search for such objects. d. They are all too distant (in theory) to be detected. e. Our galaxy is too young for any to have formed.

Quiz Questions 9. What unusual property do all higher-mass white dwarfs have? a. They are cooler than lower-mass white dwarfs. b. They are smaller than lower-mass white dwarfs. c. They are less dense than lower-mass white dwarfs. d. They are less luminous than lower-mass white dwarfs. e. All of the above.

Quiz Questions 10. What prevents gravity from shrinking a white dwarf to a smaller size? a. Helium core fusion. b. Helium shell fusion. c. Hydrogen core fusion. d. Hydrogen shell fusion. e. Degenerate electrons.

Quiz Questions 11. Which stars have high rates of mass loss due to intense stellar winds? a. High-mass stars. b. Newly forming stars. c. Stars approaching death. d. Both a and b above. e. All of the above.

Quiz Questions 12. What happens to a star when it becomes a giant if it has a close binary companion? a. Radiation from the giant's surface can ionize the companion's gases. b. Radiation from the companion's surface can vaporize the giant. c. Matter can be transferred from the companion to the giant d. Matter can be transferred from the giant to the companion. e. The giant can explode as a nova or supernova.

Quiz Questions 13. What can happen to the white dwarf in a close binary system when it accretes matter from the companion giant star? a. The white dwarf can become a main sequence star once again. b. The white dwarf can ignite the new matter and flare up as a nova. c. The white dwarf can accrete too much matter and detonate as a supernova type Ia. d. Both a and b above. e. Both b and c above.

Quiz Questions **14. What might be evidence that some close binary pairs have merged to become a single giant star? Remember conservation principles! a. Two sets of spectral lines, one from each star, have been observed for some giants. b. Alternating radial motion of a giant is revealed by an alternating Doppler shift. c. Some giants are between luminosity classes. d. Some giants are pulsating variable stars. e. Some giant stars have rapid rotation.

Quiz Questions 15. Which type of star eventually develops several concentric zones of active shell fusion? a. Low-mass stars. b. Medium-mass stars. c. High-mass stars. d. White dwarfs. e. Neutron stars.

Quiz Questions 16. Which of the following trends accurately represents the characteristics of the several different fusion zones inside a late-stage high-mass star going from the outer to inner-most zone? b. Mass of individual nuclei increases. c. Fusion lifetime decreases. a. Temperature decreases. d. Both a and b above. e. All of the above.

Quiz Questions 17. Why can't massive stars generate energy from iron fusion? a. The temperature at their centers never gets high enough. b. The density at their centers is too low. c. Iron fusion consumes energy. d. Not enough iron is present. e. Both a and b above.

Quiz Questions 18. Which of the following statements accurately describe some observed properties of type Ia and type II supernovae? a. Type Ia supernovae have hydrogen lines in their spectra. b. Type II supernovae have hydrogen lines in their spectra. c. Type Ia supernovae are more luminous. d. Both a and c above. e. Both b and c above.

Quiz Questions 19. Which type of supernova leaves NO core remnant? a. Type Ia supernovae. b. Type Ib supernovae. c. Type II supernovae. d. Both a and b above. e. All of the above.

Quiz Questions 20. Why do old supernova remnants emit X-rays? a. Electrons accelerated by magnetic fields produce radiation. b. The expanding hot gas collides with the interstellar medium. c. Short-lived unstable isotopes of nickel and cobalt emit X-rays. d. The remnant gas is excited by the neutrino burst. e. Radiation from the central black hole excites the gas.

Answers 1. a 2. e 3. b 4. e 5. b 6. e 7. d 8. e 9. b 10. e 11. e 12. d 13. e 14. e 15. c 16. d 17. c 18. b 19. a 20. b