# Stellar Remnants John Swez Physics/Geol 360 Fall Semester 2002 Slides marked with Are important for study.

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Stellar Remnants John Swez Physics/Geol 360 Fall Semester 2002 Slides marked with Are important for study

Stellar Remnants White Dwarfs (The Sun Eventually) Neutron Stars Black Holes Gasses Ejected From Stars –Planetary Nebula –Supernova Remnants (gas cloud)

Stellar Remnants Chandrasekhar M < 1.4 1983 Nobel Prize

The Pressure to Fight Gravity Still Exists even with White Dwarfs Chemical bonds / Inter-atomic Pressure –Holds atoms / molecules together on Earth –Supports gases (Ideal gas law PV=nRT) Degenerate Electron Pressure –Electrons in lowest orbits, as close as possible –Pauli Exclusion Principle prevents closer electrons Degenerate Neutron Pressure –Solid neutrons, in one nucleus - no individual atoms Singularity - gravity wins…

Degeneracy Pressure depends on the gas density not on the pressure (like an ordinary gas) When a degenerate gas is compressed it heats up but the heat does not raise its pressure If mass is added to a white dwarf its radius shrinks Too much mass makes a white dwarf collapse The limiting mass of a white dwarf is called the Chandrasekhar Limit (Nobel Prize physics 1983) The Chandrasekhar Limit is about 1.4 solar masses

When light escapes from a white dwarf it must work against gravity As the light “climbs away” from the object it looses energy When light escapes from a white dwarf it must work against gravity As the light “climbs away” from the object it looses energy It cannot slow down (light must always travel at the speed c) but it can loose energy Thus a spectral shift can occur -- a gravitational red shift The amount of red shift depends on the dwarf’s mass and size White dwarf’s can also exhibit a Zeeman Splitting indicating magnetic fields 10 4 that of sunspots

Origin of White Dwarf Stars

White Dwarfs (Isolated) Low mass stellar remnant (< 1.4 M sun ) Starts very hot (100,000K+) No source of new energy Slowly cools Very old white dwarfs become black dwarfs (age of the universe) Half our galaxy may consist of dead white dwarfs

More on White Dwarfs  May be very hot but dim because of small size  Light comes from residual heat  Core has too little mass to execute a nuclear burn  Very low mass stars evolve very slowly— none have turned into white dwarfs yet  Takes about 10 million years to cool to 20,000 K and more than the age of the unjverse to becomes a BLACK DWARF

White Dwarfs in Binary Systems Novas and Supernovas Type I The companion of a white dwarf can be a source of new hydrogen fuel. The intense gravitational field of the white dwarf “sucking” outer layers of the companion sun into itself. The gas compresses into a degenerate gas with high temperatures which eventually reach a hydrogen burn blasting the detonating hydrogen into space forming an expanding shell of hot gas. The “Nova” may erupt repeatedly. Eventually enough gas over 1.4 M solar masses yields a type I supernova and no remnant star.

The Massive Star end life Supernova  neutron star In 1934 astrophysicists Baade and Zwicky predicted neutron stars. No one took them seriously. Neutron stars are very tiny (roughly size of Terre Haute – 6 miles) with a mass between one and several solar masses. Their calculations showed that they too have an upper limit; about 2 – 3 solar masses. Up to 1967 no one thought neutron stars existed until they found pulsars.

Pulsar Pulses What would you think if you heard regular signals from space? In 1967 radio astronomers heard regular signals from space. http://www.jb.man.ac.uk/~pulsar/Education/Sounds/sounds.html

Why pulsars? Angular Momentum a possible reason Stars rotate faster as they shrink Why? Conservation of angular momentum This is why slowly rotating gas clouds become spinning stars

Neutron Stars Neutron stars are dense and small Collapse means: –rotation rate increases –magnetic field strengthened –magnetic field concentrated  Intense magnetic fields (~10 12 Solar) Magnetic Field can strip off outer star layers

Pulsars Type of neutron star Intense magnetic field sends particles and radiation off star Beams are tightly focused along magnetic poles If magnetic field is off-axis, then a pulsar results, due to lighthouse effect

Pulsar Beaming

Escape Velocity –Velocity Required to escape something’s gravity –Surface of the Earth11 km/s –Outer atmosphere of Jupiter60 km/s –Surface of the Sun620 km/s

Bending of Light by Curved Space

Black Holes Newton: If the escape velocity from an object exceeds the speed of light, you would have a ‘dark star.’ Not true! Einstein (relativity): Mass bends space and light. At high gravity, space is pinched off. This is a black hole. All mass is concentrated at an infinitely small point: the singularity

Black Holes All mass is at the singularity There is no surface BUT THERE IS AN EVENT HORIZON The Schwarschild Radius (EVENT HORIZON) is the radius where v esc = (2GM/R) 1/2 = c = speed of light How can we detect black holes ? Solution: Gravity is felt through the event horizon

“Classically, black holes are black. Quantum mechanically, black holes radiate, with a radiation known as Hawking radiation, after the British physicist Stephen Hawking who first proposed it.”Stephen Hawking Evaporation of a mini black hole Black holes get the energy to radiate Hawking radiation from their rest mass energy. So if a black hole is not accreting mass from outside, it will lose mass by Hawking radiation, and will eventually evaporate. “ From the website http://casa.colorado.edu/~ajs h/hawk.html The animation at left is a fanciful depiction of the final moments of the evaporation of a hypothetical mini black hole. In the final second of its existence, the mini black hole radiates about 1000 tonnes of rest mass energy. Such an explosion is large by human standards, but modest by astronomical standards. An evaporating black hole would be detectable from Earth only if it went off within the solar system, or at best no further away than the nearest star.

Stellar Black Holes

Cygnus X-1

Nova Outburst in Binary

Types of Supernova Explosions

Accretion Disk Around a Black Hole

Remnants in Binaries Nova –White Dwarf accretes matter from companion –Occasionally enough Hydrogen builds up to start brief nuclear fusion -> system brightens Type I Supernova –If white dwarf’s mass becomes greater than 1.4 M sun, it collapses –Star explodes as a supernova

Remnants in Binaries Rapid Rotation Pulsars –Mass dumped on pulsars can speed them up X-ray binaries –Emit huge amounts of x-rays –May be eclipsing –Only one star seen This implies a black hole –Black hole accretes material –Friction heats material as it nears a black hole

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