The Death of Stars The Death of Stars The Death of Stars The Death of Stars White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars and Black Holes and Black Holes and Black Holes 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
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The Death of StarsThe Death of StarsThe Death of StarsThe Death of Stars
White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars White Dwarfs, Neutron Stars and Black Holesand Black Holesand Black Holesand 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 heatWill cool and grow dimmerStars with Mass approximately 0.5 MSun
Have effective fuel mixing by convectionCan become WD without ejecting shell
…Black Dwarfs are the final state
Structure of White Dwarfs
Increasing their mass makes them shrink?Density about 106 gm/cm3
16 tons/cubic inchAt such compressions, material does not behave like ordinary gas - degenerate
Degenerate gas is much less compressiblePressure does not depend on temperature
Adding material raises gravitational attraction of body on itself
BUT...
Structure of White Dwarfs
Interior pressure supporting star rises lessStar can increase degeneracy pressure by becoming smaller
This makes a larger fraction of the electron population degenerate
The higher the mass, the smaller the WDIf mass becomes too large
pressure insufficient to support star even if all its electrons are degenerate; Star collapses
Maximum possible WD massChandrasekhar Limit approx 1.4 x Mass of the Sun
Nova (plural Novae)White Dwarf with a companion (red giant) star may attract matter from companion
New matter is rich in hydrogenMatter accumulates on surface of WDEventually reaches hydrogen fusion temperatureBurns explosively
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.
Formation of heavy elements from lighter onesHigh-mass stars have hotter cores than low-mass starsCore in very massive stars hot enough to fuse elements heavier than helium and carbon, such as Oxygen - Neon - Silicon - IronReactions produce energy to keep star supported
End of Nucleosynthesis
Core eventually becomes iron, but iron cannot fuse and release energyIron core thus is end of nucleosynthesisin a high-mass starIron core gets crushed by gravity; its density risesProtons and electrons in core forced so close they “merge” --- neutronsCore becomes tiny (10 km) ball of neutrons
Supernova Explosions
Core CollapseOuter parts of star fall in on tiny core - triggers massive explosionExplosion blows off outer layers at
10,000 km/secEjected debris called Supernova Remnant
Remnant core becomes neutron star (if core less than about 3 MSun) or black hole(if mass of core is more than about 3 MSun)
Type I Supernova
If too much mass accumulates, WD mass exceeds Chandrasekhar Limit and star collapsesCollapse compresses and heats WD interior
Triggers nuclear burning of carbon and oxygenProduces silicon (28Si) and nickel (56Ni)
Star explodes due to sudden release of energy: Supernova Explosion - Type IRadioactive decays add energy
56Ni —> 56Co —> 56Fe
Star is completely destroyed!
Type II Supernova and Neutron StarsType II Supernova and Neutron StarsType II Supernova and Neutron StarsType 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 MSun
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. 1054 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.
Formation of a neutron star or black hole by the collapse of the iron core of a massive star.
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
❚ 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.
❚ 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!
Black Hole Black Hole Black Hole Black Hole ---- IntroductionIntroductionIntroductionIntroduction
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.
•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 Universehttp://www.pbs.org/wnet/hawking/
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 http://www.pbs.org/wnet/hawking/
“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 space-time 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 http://www.pbs.org/wnet/hawking/
