AST- 105 1st EditionLecture 19Outline of Last LectureI. SupernovaII. Neutron StarsIII. Pulsar ModelOutline of Current Lecture I. Interacting Binary StarsII. Black HolesIII. Curved SpaceIV. Event HorizonV. Photon Path Current Lecture I. Interacting Binary Starsa. Some white dwarfs and neutron stars are in close binary systems.b. Gas escaping from a normal star is attracted by gravity of a collapsed star and forms an accretions disk.c. Gas spirals inward in a disk, releasing gravitational energy.d. Gas is heated to a high temperature (T) by friction and radiates UV and X-ray rays.e. Neutron star binaries can have L= 10^5Lʘ in X-rays!II. Black Holesa. Maximum mass for a neutron star: Mmax = 3Mʘ 1. Gravity overcomes neutron pressure at higher massb. Some X-ray binaries have collapsed stars with M > 3Mʘ c. What is a black hole?1. Black Hole- object with such strong gravity that nothing can escape from its vicinity, not even light2. Predicted by Einstein’s Theory of general Relativity in 19161. Nothing can travel faster than light2. Gravity causes space to curve3. When a star becomes a black hole, it collapses to a point of infinite density – a singularity 4. Nothing escapes from a black hole.d. Where are the black holes?1. In binary star systems:1. Our galaxy contains about 20 binaries with a black hole2. At centers of galaxies:1. Supermassive black holes- 10^6 – 10^10 Mʘ2. Our galaxy has one with M = 4 x 10^6 Mʘ3. Black hole power active in galaxies and quasars through gravitational energy release.III. Curved Spacea. Curved Space- mass causes space to curve 1. High mass concentration causes surrounding space to pinch off from the rest of the universe.1. Nothing can escape from inside this region.IV. Event Horizona. The location of a black hole is surrounded by a spherical boundary called the event horizon.1. “Sphere of no escape”2. Escape speed exceeds speed of light inside an event horizon.3. Anything passing into event horizon merges into black holes1. Black hole mass and event horizon size increases as the material falls in.V. Photon Pathsa. Photon path- photons follow curved paths in curved space.1. Even the Sun bends starlight by a small amount (1.75 arcsec)b. The more collapsed a star, the more it curves the surrounding space.c. All photons traveling outward from a black hole curve
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