ASTR 1020 1st Edition Lecture 15 Outline of Last Lecture I. Star Lives: High-Mass StarA. Life StagesB. FusionC. High-Mass StarD. Supernova ExplosionE. Close Binary StarsF. Red GiantOutline of Current Lecture I. The Stellar GraveyardA. White DwarfsB. NovaC. Neutron StarCurrent LectureI. The Stellar GraveyardA. White Dwarfs- Remaining cores of dead stars - Electron degeneracy pressure supports them against gravity - Slowly fade with time- Sirius and its hot WD companion (Component A brighter in visual wavelengths)- Remaining cores of dead stars - Electron degeneracy pressure supports them against gravity - Slowly fade with time - Sirius and its hot WD companion (Component B brighter in X-ray wavelengths)- White dwarfs with same mass as Sun are about same size as Earth - Higher mass white dwarfs are smaller -Cannot be more massive than 1.4 MSun, the Chandrasekhar limit-White Dwarfs in Close Binaries- Mass falls toward white dwarf from binary companion - Gas orbits white dwarf in an accretion disk - Friction causes heating and accretion onto white dwarfB. Nova-Temperature of accreted gas may become hot enough for hydrogen fusion -Fusion begins suddenly and explosively, causing a nova explosion-The nova star temporarily brightens -Explosion drives accreted matter out into space -If accretion makes WD larger than 1.4 solar masses, then WD may totally explode-Two kinds of Supernova - Type I = explosion of WD in binary (no H) - Type II = death explosion of massive star (H)C. Neutron Star- Neutron star: ball of neutrons left behind by a massive-star supernova (10 km radius) - Degeneracy pressure of neutrons supports it against gravity (maximum of 3 solarmasses)- Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky - The pulses were coming from a spinning neutron star—a pulsar- Pulsarso Radiation beams along a magnetic axis that is not aligned with the rotation axiso Pulsar at center of Crab Nebula pulses 30 times per second- In Close Binaries:o Hot gas in accretion disk forms X-rays: X-ray Binaries o Accretion may cause episodes of He fusion on the surface, leading to X-ray