Star Deaths Mass matters 1 Star Deaths Stars

Star Deaths Mass matters! 1

Star Deaths • Stars die when fusion reactions stop (still contains heat energy) • Mass at the time of death determines the form of the corpse • Stars lose mass before their corpse forms (winds, explosions) • Corpses last “forever” 2

Star Deaths Mass (at death) • > 0. 08 solar mass • 0. 08 to 1. 4 solar masses • 1. 4 to 3 solar masses • > 3 solar masses Corpse • Brown dwarf • White dwarf • Neutron star • Black hole! 3

White Dwarf • • Mass: bit less than the sun’s Size: about that of the earth Density: about 109 kg/m 3 Made of: electron gas (degenerategreater mass, smaller size) • Shines: by outflow of stored heat 4

Neutron Star • Mass: around 2 solar masses • Size: about 10 km (city!) • Density: about 1017 kg/m 3 (atom’s nucleus!) • Made of: neutron gas (degenerate) • Made in: supernova explosions 5

Black Holes • Mass: ANY; expect 3 to 10 solar masses from normal star death • Size: Schwarzschild radius-a few km (about 3 km for sun; directly proportional to mass) • Escape speed: equal to or greater than speed of light • Inside: mass has zero volume, infinite density—the singularity 6

Black Holes • Made of: warped spacetime (essentially permanent!) • Time is “frozen” at the Schwarzschild radius (to outside observer); never sees objects “fall in” • As black holes gain mass, they grow in size (can never shrink) 7

Black Holes • Measure mass by visible orbiting body (Newton’s version of Kepler’s 3 rd) • Most likely found in binary stars (black hole + regular massive star) • Are not cosmic vacuum cleaners (but don’t get up close!) 8

Black Holes: X-rays • Matter from companion star falls into an small accretion disk (about 10 km) around the black hole • Disk heats up to about 10, 000 K (conversion of gravitational energy) • Opaque material of disk emits xrays (blackbody) • Search in binary x-ray sources 9

Supernovas! • Peak at 10 billion solar luminosities • Lifetime about a few months 46 • Total energy about 10 joules (99% as neutrinos) • Blows off a few solar masses of material at thousands km/s (supernova remnant) 10

Supernovas! • One type: Old massive star (about 10 solar masses) with unstable iron core • Core collapses in about 1 second! • Implosion releases gravitational energy, high temps for fusion reactions • Fuses elements heavier than iron; blasted into space 11

Neutron Stars: Pulsars! • Supernova core collapse makes neutron star (usually!) • Pulsars usually found near center of supernova remnants • Pulsars emit very short pulses (ms) at very regular intervals • Rapidly-rotating, highly-magnetic neutron stars are pulsars 12