Chapter 13 The Bizarre Stellar Graveyard
White Dwarfs... • ...are stellar remnants for low-mass stars. • ...are found in the centers of planetary nebula. • ...have diameters about the same as the Earth’s. • ...have masses less than the Chandrasekhar mass.
Sirius A And Sirius B In X-ray Sirius A Sirius B
Novas and Supernovas • Nova - a stellar explosion • Supernova - a stellar explosion that marks the end of a star’s evolution • White Dwarf Supernova (Type I supernova)- occur in binary systems in which one is a white dwarf • Massive Star Supernova (Type II Supernova) - occur when a massive star’s iron core collapses
Nova Herculis March 1935 May 1935
Nova T Pyxidis (HST) A nova occurs when hydrogen fusion ignites on the surface of a white dwarf star system
Semidetached Binary System With White Dwarf Star (may result in a white dwarf (type I ) supernova)
Type II Supernova • The star releases more energy in a just a few minutes than it did during its entire lifetime. • Example: SN 1987A • After the explosion of a massive star, a huge glowing cloud of stellar debris - a supernova remnant - steadily expands. • Example: Crab Nebula • 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.
On July 4, 1054 astronomers in China witnessed a supernova within our own galaxy. The remnant of this explosion is The Crab Nebula
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. This forms unstable nuclei which then decay into stable nuclei of other elements Formation of Elements beyond Iron occurs very rapidly as the star approaches supernova.
The supernova explosion then distributes the newly formed matter throughout the interstellar space (space between the stars). • This new matter goes into the formation of interstellar debris. • The remnant core is a dense solid core of neutrons – a neutron star!
Neutron Stars • ...are stellar remnants for high-mass stars. • ...are found in the centers of some type II supernova remnants. • ...have diameters of about 6 miles. • ...have masses greater than the Chandrasekhar mass. (1.4M)
Relative Sizes Neutron Star Earth White Dwarf
Pulsars • The first pulsar observed was originally thought to be signals from extraterrestrials. • (LGM-Little Green Men was their first designation) Period = 1.337301 seconds exact! ~ 20 seconds of Jocelyn Bell’s data- the first pulsar discovered
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 • The pulsing star inside the Crab Nebula was a pulsar. • Pulsars are rotating, magnetized neutron stars.
The Crab Pulsar Period = 0.033 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.
Light House model of neutron star emission accounts for many properties of observed Pulsars
Rotation Rates of Pulsars • The neutron stars that appear to us as pulsars rotate about once every second or less. • Before a star collapses to a neutron star it probably rotates about once every 25 days. • Why is there such a big change in rotation rate? • Answer: Conservation of Angular Momentum
Mass Limits • Low mass stars • Less than 8 M on Main Sequence • Become White Dwarf (< 1.4 M) • Electron Degeneracy Pressure • High Mass Stars • Less than 100 M on Main Sequence • Become Neutron Stars (1.4M < M < 3M) • Neutron Degeneracy Pressure
Black Holes • ...are stellar remnants for high-mass stars. • i.e. remnant cores with masses greater than 3 solar masses • …have a gravitational attraction that is so strong that light cannot escape from it. • …are found in some binary star systems and there may be super-massive black holes in the centers of some galaxies.
Supermassive Stars • 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 • Advance of Mercury’s perihelion • Bending of starlight
Advance of Mercury’s Perihelion 43” per century not due to perturbations from other planets
Apparent position of the star Sun Light from star bent by the gravity of the Sun Bending of Starlight 1.75”
Event Horizon Rs + Singularity Schwarzschild Black Hole Rs = 3(Mass) Mass Rs 3 M 9 km 5 15 10 30
What Can We Know? • Mass • gravity • Charge • Electric Fields • Rotation Rate • Co-rotation
How Can We Find Them? • 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