Extreme Stars White Dwarfs & Neutron Stars White Dwarfs: • Remnants of low-mass stars • Supported by Electron Degeneracy Pressure • Maximum Mass ~1.4 Msun (Chandrasekhar Mass) Neutron Stars: • Remnants of some post-supernova massive stars • Supported by Neutron Degeneracy Pressure • Pulsar = rapidly spinning magnetized neutron star 1 When electron degeneracy becomes nearly relativistic, equation of state is “softer” and star gets smaller rapidly with mass The Stellar Graveyard Question: What happens to the cores of dead stars? Answer: They continue to collapse until either: • A new pressure law takes hold to halt further collapse & they settle into a new hydrostatic equilibrium. • If too massive they collapse to “a singularity” and become a Black Hole. All of these are seen as the remnants of stellar evolution. 7 Degenerate Gas Law At high density, a new gas law takes over: • Pack many electrons into a tiny volume • These electrons fill all low-energy states • Only high-energy = high-pressure states left Result is a "Degenerate Gas": • Pressure is independent of Temperature. • Compression does not lead to heating. This means that the objects could be very cold but still have enough pressure to maintain a state of Hydrostatic Equilibrium (BIG ROCKS!, well almost…) Can such objects exist? 8 White Dwarfs These are the remnant cores of stars with M < 8 Msun. • Supported against gravity by Electron Degeneracy Pressure • M<4 Msun: C-O White Dwarfs • M=4-8 Msun: O-Ne-Mg White Dwarfs Properties: • Mass < 1.4 Msun • Radius ~ Rearth (<0.02 Rsun) • Density ~ 105-6 g/cc • Escape Speed: 0.02c (2% speed of light) No nuclear fusion or gravitational contraction. It shines by residual heat. 9 Chandrasekhar Mass Mass-Radius Relation for White Dwarfs: Larger Mass = Smaller Radius Chadrasekhar Mass: • Maximum Mass for White Dwarf: Mch = 1.4 Msun • First calculated by Subrahmanyan Chandrasekhar in the 1930s. • Above this mass, electron degeneracy pressure fails & the star collapses. This prediction is upheld by observations. All white dwarfs in binary stars have masses below the Chandrasekhar Mass. 10 Evolution of White Dwarfs White dwarfs shine by leftover heat. • No energy source (no fusion, nothing) • Cools off and fades away slowly. • Forms a White Dwarf Cooling Sequence on the H-R diagram Ultimate State: A "Black Dwarf": • Old, cold white dwarf • Takes ~10 Tyr (1013 yrs) to cool off • U is not old enough for any Black Dwarfs yet. Note: Black Dwarfs ≠ "Black Holes" 12 WD cooling 13 White Dwarfs: The Other Supernovae What happens if mass transfer pushes a White Dwarf over the Chandrasekhar Mass? The internal electron degeneracy pressure is no longer able to balance Gravity, and it will begin to collapse... • As the density rises, the temperature does not increase • Eventually ignites carbon-oxygen burning at high enough density • This begins to generate heat, but no additional pressure to slow the collapse • The extra heat leads to greater fusion, which leads to greater heat... The effect is a runaway nuclear explosion: • Fusion of the light elements into Iron and Nickel • The white dwarf detonates and disrupts completely as a Type Ia Supernova. Ia SN leave no remnant, and produce most of the Iron in the U 17 18 19 20 21 22 SN1066 23 24 Supernovae Types: Classified by Spectra not “theory” • 1a—accreting white dwarf • 1b, 1b “stripped core-collapse SN” they lose a lot of their envelope first 25 26 27 SN from massive stars “core collapse” 28 29 30 Core collapse relies on “bounce” • There needs to be a “bounce” to blow off the envelope and to keep the remnant from being a BH rather than a neutron star (BH doesn’t have to bounce, but blowing envelope is harder) 31 Neutron Stars Remnant cores of massive stars: • 8 < M* < 18 Msun (??) • Leftover core of a core-bounce • supernova Support=Neutron Degeneracy Pressure: • Mass ~1.2 - ~3 Msun • Radius ~10 km • Density ~1014 g/cc • Escape Speed: 0.5-0.7c No nuclear fusion or gravitational contraction. It shines by residual heat or accretion. 32 33 Structure of a Neutron Star At densities > 2x1014 g/cc: • nuclei melt into a sea of subatomic particles. • protons & electrons combine into neutrons. Surface is cooler, forming a solid crystalline crust! Inside is exotic matter: superfluid neutrons, trace amount of superconducting protons, and stranger subatomic particle matter 34 Accidental Discovery 1967: Jocelyn Bell grad student of Anthony Hewish discovers rapidly pulsating radio sources while looking for something else They called these new, otherwise unknown objects "Pulsars" = Pulsating Radio Sources Pulsars emitted sharp, 1 millisecond-long pulses every second at an extremely repeatable rate (most accurate clocks known at the time). Anthony Hewish shared the 1974 Nobel Prize for Physics for his role in the discovery of pulsars. Jocelyn Bell, however, did not. 35 Pulsars Rapid spinning,magnetized NS Lighthouse Model: • Spinning magnetic field generates a strong electric field. • Electric field rips electrons of the surface and accelerates them along the magnetic poles. Twin beams of radiation shining out of the magnetic poles: When magnetic axis is mis- aligned with rotation axis, the beams sweep over us as it rotates; we see regular pulses 36 Pulsar Evolution Pulsars spin slower as they age and lose rotational energy Young neutron stars: • fast spinning pulsars. • found in supernova remnants (e.g., Crab pulsar) Old neutron stars: cold and hard to find—although they can be “recycled” via accretion spin up [Some neutron stars that are not also pulsars have been finally seen. Hubble Space Telescope detection of a lone neutron star.] 37 Over the Top? What if the remnant core is very massive? "Very Massive" = Mcore > ~3 Msun (original star would have had a mass M > 18 Msun) • Neutron degeneracy pressure fails. • Nothing can stop the gravitational collapse. • Collapses to zero radius and infinite density. The core becomes a Black Hole!!! We’ll talk about these exotic objects next.. 38 X-ray binaries White dwarfs have an escape velocity of 0.02c • H/proton falling and hitting surface, E ~ 200keV Neutron star escape velocity is >0.5c • H/proton falling and hitting surface, E ~ 200MeV Black holes have escape velocity of c! • But, there’s no surface to hit! Emission mechanisms in each case took some time 39 “3 out of 2 stars are Binaries” Binary stars are the norm One star is likely to evolve and create a remnant The other star will then swell to giant size and transfer material to the compact star 40 All of them can be X-ray binaries There is often enough information to gauge the masses and probe the transition masses between white dwarf, neutron star and black hole Some pulsars are in binaries 41 42 43 ~2 dozen confirmed black hole Xray sources 44 Jets observed in Xrays “microquasar” 45 Back to “no surface” black holes • Around a black hole, there is an accretion disk • Energy is emitted as friction/viscosity causes material to flow thru disk • There is a “last stable orbit”. After this particles spiral quickly into the black hole. This sets the efficiency of a black hole to ~10% 46 Efficiencies Recall that Nuclear Energy is a few MeV per proton • that’s an efficiency of 0.3% or 3 x 10-3 • Chemical reactions are an eV/proton, 10-9 • Gravitation onto the sun is 10-9 Gravity onto a Black Hole is 10-1 • there has been roughly the same amount of gravitational energy released in the Universe as nuclear 47.