Chapter 13 The Bizarre Stellar Graveyard

White Dwarfs... n...are stellar remnants for low-mass stars. n...are found in the centers of planetary nebula. n...have diameters about the same as the Earth’s. n...have masses less than the Chandrasekhar mass.

Sirius B is a white dwarf star

Sirius A And Sirius B In X-ray Sirius B Sirius A

Novas and Supernovas n Nova - a stellar explosion n Supernova - a stellar explosion that marks the end of a star’s evolution n White Dwarf Supernova (Type I supernova)- occur in binary systems in which one is a white dwarf n Massive Star Supernova (Type II Supernova) - occur when a massive star’s iron core collapses

Close Binary Systems and Mass Transfer

March 1935May 1935 Nova Herculis

Diagram of nova process

A nova occurs when hydrogen fusion ignites on the surface of a white dwarf star system Nova T Pyxidis (HST)

Light Curve of typical Nova

Semidetached Binary System With White Dwarf Star (may result in a white dwarf (type I ) supernova)

Type II Supernova n The star releases more energy in a just a few minutes than it did during its entire lifetime. »Example: SN 1987A n After the explosion of a massive star, a huge glowing cloud of stellar debris - a supernova remnant - steadily expands. »Example: Crab Nebula n After a supernova the exposed core is seen as a neutron star - or if the star is more than 3 solar masses the core becomes a black hole.

The remnant of this explosion is The Crab Nebula On July 4, 1054 astronomers in China witnessed a supernova within our own galaxy.

Supernova 1987a

Type I and Type II Supernova

Supernova Light Curves

Hydrogen and Helium Burning

Carbon Burning and Helium Capture

Still heavier elements are created in the final stages of life of massive stars

Alpha Process – Helium Capture produces heavier elements up to Fe and Ni.

Elements beyond Fe and Ni involve neutron capture. Formation of Elements beyond Iron occurs very rapidly as the star approaches supernova. This forms unstable nuclei which then decay into stable nuclei of other elements

n The supernova explosion then distributes the newly formed matter throughout the interstellar space (space between the stars). n This new matter goes into the formation of interstellar debris. n The remnant core is a dense solid core of neutrons – a neutron star!

Neutron Stars n...are stellar remnants for high-mass stars. n...are found in the centers of some type II supernova remnants. n...have diameters of about 6 miles. n...have masses greater than the Chandrasekhar mass. (1.4M  n...have masses greater than the Chandrasekhar mass. (1.4M  )

Relative Sizes EarthWhite Dwarf Neutron Star

Pulsars n The first pulsar observed was originally thought to be signals from extraterrestrials. n (LGM-Little Green Men was their first designation) ~ 20 seconds of Jocelyn Bell’s data- the first pulsar discovered Period = seconds exact!

n It was later shown to be unlikely that the pulsar signal originated from extraterrestrial intelligence after many other pulsars were found all over the sky.

Pulsars n The pulsing star inside the Crab Nebula was a pulsar. n Pulsars are rotating, magnetized neutron stars.

The Crab Nebula

The Crab Pulsar Period = seconds = 33 milliseconds

Light House Model –Beams of radiation emanate from the magnetic poles. –As the neutron star rotates, the beams sweep around the sky. –If the Earth happens to lie in the path of the beams, we see a pulsar.

Rotating Neutron Star

Light House model of neutron star emission accounts for many properties of observed Pulsars

Artistic rendering of the light house model

Rotation Rates of Pulsars n The neutron stars that appear to us as pulsars rotate about once every second or less. n Before a star collapses to a neutron star it probably rotates about once every 25 days. n Why is there such a big change in rotation rate? n Answer: Conservation of Angular Momentum

Neutron –Star Binaries

Mass Limits n Low mass stars –Less than 8 M  on Main Sequence –Become White Dwarf (< 1.4 M  ) »Electron Degeneracy Pressure n High Mass Stars –Less than 100 M  on Main Sequence –Become Neutron Stars (1.4M  < M < 3M  ) »Neutron Degeneracy Pressure

Black Holes n...are stellar remnants for high-mass stars. –i.e. remnant cores with masses greater than 3 solar masses n …have a gravitational attraction that is so strong that light cannot escape from it. n …are found in some binary star systems and there may be super-massive black holes in the centers of some galaxies.

Supermassive Stars n If the stellar core has more than three solar masses after supernova, then no known force can halt the collapse Black Hole Black holes were first predicted by the General Theory of Relativity, which is theory of gravity that corrects for some of the short-falls of Newton’s Theory of Gravity.

In general Relativity, space, time and mass are all interconnected

Space-Time No mass Distortion caused by mass

Predictions of General Relativity n Advance of Mercury’s perihelion n Bending of starlight

Advance of Mercury’s Perihelion 43” per century not due to perturbations from other planets

Bending of Starlight Sun Light from star bent by the gravity of the Sun Apparent position of the star 1.75”

Schwarzschild Black Hole RsRs Singularity + Event Horizon R s = 3(Mass) Mass R s 3 M  9 km

Near a Black Hole

What Can We Know? n Mass –gravity n Charge –Electric Fields n Rotation Rate –Co-rotation

How Can We Find Them? n Look for X-ray sources –Must come from compact source »White Dwarf »Neutron Star »Black Hole –Differentiate by Mass »WD - < 1.4 M  »NS - between 1.4 and 3 M  »BH - > 3 M 

Cygnus X-1

End of Chapters

End of Section.

Nucleosynthesis Evolutionary Time Scales for a 15 M  Star

Energy Budget Energy Fusion Stages H He C Fe

Anazasi Pictographs

Supernova 1998S in NGC 3877

Supernova Remnants Tycho’s SNR

PSR

LGM? n Several more found at widely different places in the galaxy n Power of a power equals total power potential output of the Earth n No Doppler shifts PULSARS

Light Time Argument n An object which varies its light can be no larger than the distance light can travel in the shortest period of variation.

To Darken the Sun Time Delay = Radius/c 500,000 km/300,000 km/s = 1.67 sec

Only candidates: White Dwarfs, Neutron Stars

Pulse Mechanisms F Binary Stars - How quickly can two stars orbit? 3Two WD about 1 m 3Two NS about 1 s. 3Neutron Stars in orbit should emit gravity waves which should be detectable. F Oscillations - Depends only on density 3WD about ten seconds 3NS about.001 s Little variation permitted. F Rotation - Until the object begins to break up. 3WD about 1 s 3NS about.001 s with large variation.

SS 433

Synchrotron Radiation Magnetic lines of force Electron Radiation

Glitches