The Death of Stars. White Dwarfs, Neutron Stars and Black Holes. White Dwarfs

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1 The Death of Stars White Dwarfs, Neutron Stars and Black Holes White Dwarfs Formed when stars like our Sun reach the end of their life When the Sun s fuel is spent, it will collapse. Don t worry, that will occur in 5 billion years! No nuclear burning - shine by residual heat Will cool and grow dimmer Stars with Mass approximately 0.5 M Sun Have effective fuel mixing by convection Can become WD without ejecting shell Black Dwarfs are the final state

2 Structure of White Dwarfs Increasing their mass makes them shrink? Density about 10 6 gm/cm 3 16 tons/cubic inch At such compressions, material does not behave like ordinary gas - degenerate Degenerate gas is much less compressible Pressure does not depend on temperature Adding material raises gravitational attraction of body on itself BUT... Structure of White Dwarfs Interior pressure supporting star rises less Star can increase degeneracy pressure by becoming smaller This makes a larger fraction of the electron population degenerate The higher the mass, the smaller the WD If mass becomes too large pressure insufficient to support star even if all its electrons are degenerate; Star collapses Maximum possible WD mass Chandrasekhar Limit approx 1.4 x Mass of the Sun

3 Nova (plural Novae) White Dwarf with a companion (red giant) star may attract matter from companion New matter is rich in hydrogen Matter accumulates on surface of WD Eventually reaches hydrogen fusion temperature Burns explosively Nova Explosion seen as a new or brightened star Star survives; nova outbursts may recur White Dwarf Companion Star Copyright 1999 The McGraw-Hill Companies

4 A White Dwarf Exploding as a Nova The hydrogen that falls onto the surface of a white dwarf from its companion may suddenly fuse into helium, creating an explosion that makes the star brighten. Copyright 1999 The McGraw-Hill Companies Nucleosynthesis Formation of heavy elements from lighter ones High-mass stars have hotter cores than lowmass stars Core in very massive stars hot enough to fuse elements heavier than helium and carbon, such as Oxygen - Neon - Silicon - Iron Reactions produce energy to keep star supported

5 End of Nucleosynthesis Core eventually becomes iron, but iron cannot fuse and release energy Iron core thus is end of nucleosynthesis in a high-mass star Iron core gets crushed by gravity; its density rises Protons and electrons in core forced so close they merge --- neutrons Core becomes tiny (10 km) ball of neutrons Supernova Explosions Core Collapse Outer parts of star fall in on tiny core - triggers massive explosion Explosion blows off outer layers at 10,000 km/sec Ejected debris called Supernova Remnant Remnant core becomes neutron star (if core less than about 3 M Sun ) or black hole (if mass of core is more than about 3 M Sun ) Type I Supernova If too much mass accumulates, WD mass exceeds Chandrasekhar Limit and star collapses Collapse compresses and heats WD interior Triggers nuclear burning of carbon and oxygen Produces silicon ( 28 Si) and nickel ( 56 Ni) Star explodes due to sudden release of energy: Supernova Explosion - Type I Radioactive decays add energy 56 Ni > 56 Co > 56 Fe Star is completely destroyed!

6 Type II Supernova and Neutron Stars One possible remnant of Type II supernova; Forms when massive star s iron core collapses and triggers supernova explosion Radius = 10 km (6 miles) Mass = 1 to 3 M Sun Collapse to tiny radius increases rotation rate (Conservation of Angular Momentum) Collapse also amplifies the magnetic field of the star by sweeping it in The Crab Nebula - A prominent supernova remnant, the result of the great A.D supernova. A white dwarf exploding as a type 1 supernova. If too much hydrogen from a companion accumulates on the white dwarf, it may raise its mass to a value above the Chandrasekhar limit and explode. Copyright 1999 The McGraw-Hill Companies. Formation of a neutron star or black hole by the collapse of the iron core of a massive star. Copyright 1999 The McGraw-Hill Companies.

7 Neutron Star/Pulsar Size Comparison with NYC Jocelyn Bell Discoverer of Pulsars Received her Ph.D. in Radio Astronomy at Cambridge where she was involved in the discovery of Pulsars. She is Professor of Physics and Chair of the Department of Physics in Then Open University. Neutron Stars as Pulsars Combination of fast rotation and high magnetic field - radiation beams from poles Beams sweep cross sky If one points at Earth, we see burst of radiation each time star spins - pulses Spinning neutron star detectable as pulsar Old ones produce only radio waves Young pulsars produce radio + visible light (Crab Nebula pulsar: 30 flashes/second)

8 Copyright 1999 The McGraw-Hill Companies. A pulsar's pulses are like the flashes of a lighthouse as its lamp rotates. Copyright 1999 The McGraw-Hill Companies. Pulsars and the Discovery of Neutron Stars Pulsar signals recorded from a radio telescope. (Courtesy M. I. Large, University of Sydney, R. N. Manchester, Australia Telescope, CSIRO, and Joseph H. Taylor, Princeton University.)

9 Copyright 1999 The McGraw-Hill Companies. Pulsars Gradual slowdown due to energy loss Occasional speeding up of rotation Structure: Gaseous atmosphere: 1 mm thick (dime) Solid [iron?] crust: a few hundred meters Superfluid neutron interior Slowing crust recouples with fast-rotating superfluid interior Crust cools and shrinks; speeds up Copyright 1999 The McGraw-Hill Companies.

10 Pulsars & X-Ray Binaries - Continued Intense beams in very young pulsars may propel them like rocket engines. Pulsar takes off and leaves the scene and remnants of explosion! X-Ray Binaries: Pulsar + companion Strong X-Ray emissions X-Ray Bursters: irregular bursts; thermonuclear explosions of in-fallen matter X-Ray Pulsars: thermal emission from hot spots where in-falling matter accumulates Gas falling onto a neutron star follows the magnetic field lines and makes a hotspot on the star's surface, creating x- rays. As the star rotates, we observe x-ray pulses. Copyright 1999 The McGraw-Hill Companies. Accelerating Pulsars Neutron star attracts matter from companion In-falling matter swirls around the neutron star in an accretion disk before falling onto it Adds angular momentum and causes the neutron star to spin faster. Most millisecond pulsars might have formed this way. Some have visible companions; others might have completely devoured theirs!

11 Black Hole - Introduction Objects on a waterbed make depressions analogous to the curvature of space created by a mass. According to the general theory of relativity, that curvature produces the effect of gravity. Bigger bodies make bigger depressions, and so a marble rolls in faster. However, a very big body may tear the waterbed, creating an analog of a black hole. Copyright 1999 The McGraw-Hill Companies. Black Holes - The Theory A massive star starts to collapse when it exhausts its nuclear fuel Can no longer counteract the pull of gravity. The crushing weight of the star s overlying layers implodes the core, and the star digs deeper into the fabric of space-time. Although the star remains barely visible, its light now has a difficult time climbing out of the enormous gravity of the still-collapsing core. The star passes through its event horizon and disappears from our universe, forming a singularity of infinite density. Notes from Stephens Hawking s Universe

12 Black Holes - Do they exist? A supermassive black hole with 2 billion times the mass of the Sun apparently lurks in the nearby giant galaxy M87. See Although general relativity predicted that black holes could exist, many scientists thought they were too bizarre to exist in the real universe. That s all changed. Astronomers have now detected several black holes in X-ray-emitting binary star systems, where a normal star orbits a massive yet invisible companion that theory says must be a black hole. Even more convincing evidence has come from the centers of several large galaxies, where stars move about so quickly that they must be caught in the grips of a massive object. Wormholes A beam of light traversing a path between two points in curved spacetime can take longer to complete the journey than a hypothetical spaceship taking advantage of a wormhole s shortcut connection between the two distinct regions of space-time. In 1935, Albert Einstein and Nathan Rosen realized that general relativity allows the existence of bridges, originally called Einstein-Rosen bridges but now known as wormholes. These space-time tubes act as shortcuts connecting distant regions of space-time. See

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