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11. Dead Stars Astronomy 110: SURVEY OF ASTRONOMY 11. Dead Stars 1. White Dwarfs and Supernovae 2. Neutron Stars & Black Holes Low-mass stars fight gravity to a standstill by becoming white dwarfs — degenerate spheres of ashes left over from nuclear burning. If they gain too much mass, however, these ashes can re-ignite, producing a titanic explosion. High-mass stars may make a last stand as neutron stars — degenerate spheres of neutrons. But at slightly higher masses, gravity triumphs and the result is a black hole — an object with a gravitational field so strong that not even light can escape. 1. WHITE DWARFS AND SUPERNOVAE a. Properties of White Dwarfs b. White Dwarfs in Binary Systems c. Supernovae and Remnants White Dwarfs in a Globular Cluster Hubble Space Telescope Finds Stellar Graveyard The Companion of Sirius Sirius weaves in its path; has a companion (Bessel 1844). 1800 1810 1820 1830 1840 1850 1860 1870 “Father, Sirius is a double star!” (Clark 1862). P = 50 yr, a = 19.6 au a3 = M + M ≃ 3 M⊙ P2 A B MA = 2 M⊙, MB = 1 M⊙ Why is the Companion So Faint? LB ≈ 0.0001 LA Because it’s so small! DB = 12000 km < D⊕ ≃ 6 3 ρB 2 × 10 g/cm The Dog Star, Sirius, and its Tiny Companion Origin and Nature A white dwarf is the degenerate carbon/oxygen core left after a double-shell red giant ejects its outer layers. Degeneracy Pressure Electrons are both particles and waves. h λ = The wavelength λ of an electron is me v e Rules for electrons in a box: 1. Waves must fit evenly. 2. Each wave must be different. (Note: the same rules apply to electron orbits around nuclei.) Degeneracy Pressure Electrons are both particles and waves. h λ = The wavelength λ of an electron is me v e Rules for electrons in a box: 1. Waves must fit evenly. 2. Each wave must be different. Suppose we compress the box: • all λ decrease ⇒ all v increase! • energy cost implies pressure Sizes of White Dwarfs Planet Earth 1 M⊙ White Dwarf 1.3 M⊙ White Dwarf surface gravity: 1 G surface gravity: 3.8×105 G surface gravity: ~2×106 G The higher a white dwarf’s mass, the smaller its radius! This trend continues to lower masses; Jupiter is about as large as degenerate objects get. White Dwarf Structure Visible surface: normal gas ~50 km thick; pure H or He Interior: degenerate matter typically C/O mixture Center: degenerate matter C/O nuclei in crystal form Galaxy's Largest Diamond Star gradually crystalizes from inside out as it cools. White Dwarfs in Binary Systems A white dwarf orbiting another star can become active when the other star becomes a red giant... Animation of Interacting Stars Accretion Disks Mira: "Wonderful" Star Reveals its Hot Nature Because it has angular momentum, the transferred gas orbits around the dwarf, forming an accretion disk. Friction in the disk moves angular momentum outward and mass inward. The disk becomes incandescent. Classical Novae H and He from the companion build up on the white dwarf’s surface. When enough has accumulated, the H burns violently, producing a thermonuclear explosion. Explosions from White Dwarf Star RS Oph Classical Novae: RS Ophiuci Classical Novae: RS Ophiuci 16 Feb, 2006 Recurrent Nova RS Ophiuci Explosions repeat every ~20 yr; about 10% of accreted mass is retained. Current mass: MWD ≃ 1.35 M⊙. The Limits of Degeneracy c We can add electrons if they have smaller wavelengths, λ. Smaller wavelengths imply higher velocities, v. elocity As v gets near the speed of light, v c, electrons behave like photons. Light (radiation) pressure makes stars unstable, so no white dwarf can weigh more than ~1.4 M⊙. 0 White Dwarf Supernovae If a white dwarf’s mass reaches 1.4 M⊙, carbon ignites in its degenerate center, and a thermonuclear explosion completely destroys the star. Illustration of Mira System White Dwarf Supernova Simulation 3-D Simulation of Type Ia Supernova Origin of the Elements Explosion makes ~0.8 M⊙ of Fe-group elements. Iron-Group Elements Most of the iron in the universe is made by white-dwarf supernovae. Supernovae Compared Two different scenarios produce titanic explosions: Massive Star SN White Dwarf SN Evolved star with initial White dwarf in binary mass > 8 M⊙. with nearby giant star. Degenerate Fe core Degenerate C/O star reaches 1.4 M⊙. reaches 1.4 M⊙. Gravitational collapse Nuclear burning of C/O 2 2 yields ~ 0.2 M⊙c ; yields ~ 0.002 M⊙c . 99% escapes as neutrinos. Supernovae Light Curves Ni56 → Co56: half-life 6 days Co56 → Fe56: half-life 77 days Massive-star and white-dwarf supernovae reach similar peak luminosities and fade gradually with time. Supernovae Remnants Inner remnant Outer shock wave (iron lines) (hydrogen lines) X-ray Visible White-Dwarf Supernova Remnant DEM L71: Supernova Origin Revealed Debris from supernova explosions expand at thousands of km/s in all directions, slamming into interstellar gas. Remnant of Tycho’s Supernova (1572) supernova relatavistic debris electrons Shock velocity 3000 km/s ~ 0.01 c Tycho's Supernova Remnant Provides Shocking Evidence for Cosmic Rays Remnants accelerate cosmic rays to near light-speed. The Crab Nebula (1054) The Crab Nebula from Hubble The Veil Nebula The Veil Nebula Unveiled 2. NEUTRON STARS AND BLACK HOLES a. Neutron Stars and Pulsars b. A Brief Introduction to Black Holes Nearest Known Neutron Star Hubble Sees a Neutron Star Alone in Space Origin and Nature Animation of Star Collapse A neutron star is the degenerate sphere of neutrons left after the iron core of a massive star collapses. Birth of a Neutron Star In the core, nuclei are smashed into protons & neutrons; the protons combine with electrons to make neutrons & neutrinos. At birth, the temperature of a neutron star is ~1011 K, but neutrino emission cools it to ‘only’ 106 to 107 K. Sizes of Neutron Stars Google Maps: Oahu Sizes of Neutron Stars ~20 km surface gravity: ~1011 G density: ρ ≈ 1015 g/cm3 Artist's impression of a neutron star Why Are Neutron Stars So Small? White dwarfs are supported by electron degeneracy; the electron wavelength is h λ = me v Neutron stars are supported by neutron degeneracy; the neutron wavelength is h λ = mn v Now, mn = 1840 me, so we expect neutron stars to be about 1840 (say, 2000) times smaller than white dwarfs. Why Do Neutron Stars Spin So Fast? Conservation of angular momentum: before collapsing, the star’s core probably rotates once every few hours. Collapse by a factor of x decreases the rotation period by a factor of x2. The core collapses by roughly a factor of 1000, so it spins about 10002 = 106 times more often. Final rotation period is a few hundredth’s of a second! Pulsars A pulsar is a spinning neutron star with a magnetic field tilted at an angle to its rotation axis. Pulsars Particles accelerated by the spinning field create two beams of radiation aligned with the magnetic axis. As the pulsar turns, these beams sweep through space. Discovery of Pulsars • Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. • The pulses were coming from a spinning neutron star —a pulsar. Copyright © 2009 Pearson Education, Inc. As Small As Why Pulsars Must Be Neutron Stars Circumference of Neutron Star = 2π (radius) ~ 60 km Spin Rate of Fast Pulsars ~ 1000 cycles per second Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS Anything else would be torn to pieces! Copyright © 2009 Pearson Education, Inc. The Crab Nebula Pulsar The Crab Nebula from Hubble The Crab Nebula Pulsar The Crab Nebula and Pulsar Crab Nebula: a Dead Star Creates Celestial Havoc Neutron Stars in Binary Systems X-Ray Bursts • Matter accreting onto a neutron star can eventually become hot enough for helium to fuse. • The sudden onset of fusion produces a burst of X rays. Copyright © 2009 Pearson Education, Inc. Degeneracy’s Limits, Again! c Just as with white dwarfs, there’s a maximum allowed mass for a neutron star, roughly 3 M⊙. elocity A neutron star which somehow v gains more mass presumably collapses to form a black hole. 0 A Brief Introduction to Black Holes Black Hole Images A Brief Introduction to Black Holes A black hole is an object with a gravitational field so strong that not even light can escape. Black Hole Images Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases. B. It decreases. C. It stays the same. Hint: Copyright © 2009 Pearson Education, Inc. Escape Velocity initial kinetic final gravitational = energy potential energy (escape velocity)2 G × (mass) = 2 (radius) Copyright © 2009 Pearson Education, Inc. The Schwarzschild Radius Let’s insert the speed of light, c, into the escape-velocity equation: c2 G M = 2 R The result is a relationship between the mass, M, and radius, R, of a black hole. Solving for R, we get 2 G M R = c2 R is called the Schwarzschild Radius. Any object of mass M becomes a black hole if its radius is less than or equal to than R, because light is unable to escape. Sizes of Black Holes 18 km M = 3 M⊙ Google Maps: Oahu A black hole’s mass strongly warps space and time in the vicinity of the event horizon. Note: ‘event horizon’ is another term for the Schwarzschild Radius. These diagrams show how space becomes warped near a massive object or black hole.
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