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CU-Boulder ASTR 1120 - Lecture Notes

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ASTR 1120General Astronomy:Stars & Galaxies ! EXT Tuesday 10/20: "!earching for distant world"#GO DIRECTLY TO THE PLANETARIUM!EXT $ursday 10/15: #IDTERM #2The Stellar GraveyardWhat’s In The Stellar Graveyard?• Lower mass stars ! white dwarfs– Gravity vs. electron degeneracy pressure• High mass stars ! neutron stars– Gravity vs. neutron degeneracy pressure• Even more massive stars (M>30-40 Msun)! blackholes– Gravity winsWhen the mass is too greatfor even neutron degeneracyto hold up…• Our supernova corecollapses to aninfinitely small point! Black Holewhen Mcore > 3 MsunBlack Holes – sort of courtesy of Albert• Einstein’s (1911)General Theory ofRelativity: gravity isreally the warping ofspacetime around anobject with much mass• Light travels in “straightlines” – and its bendingcomes from spacetimebeing curved by gravityGENERAL RELATIVITY: (in a nutshell)%asses &' (pace) m*how + curve.,pace)m*, -i. it/0urvature, &'s masse/how + move.[Image from “Spacetime and Gravity” by S. Carroll] [Image by R. Jantzen]Effects of strong gravity on light can act like lens can redshift lightTwo images of a distant quasarAB[Image credit: R. Ellis& I. Smail with HST(NASA/STScI)]Source behind the lens: EINSTEIN RING [Image credit: L. King with HST (NASA/STScI)]The archesare theresult ofgravitationallensing [Image credit: P. Natarajan with HST(NASA/STScI)]What is the theory of GeneralRelativity?A. A theory that describes gravity in terms offorces between massive bodiesB. A theory that describes gravity in terms ofspacetime curvatureC. The most general theory of the UniverseD. A theory that describes the atomE. A theory that describes how galaxies evolvedClicker QuestionWhat is the theory of GeneralRelativity?A. A theory that describes gravity in terms offorces between massive bodiesB. A theory that describes gravity in terms ofspacetime curvatureC. The most general theory of the UniverseD. A theory that describes the atomE. A theory that describes how galaxies evolvedClicker QuestionBlack Holes Solutions to the Einstein’s equations of GeneralRelativity, describing how spacetime curvesaround bodies of a certain size and mass.For a given size, the larger the mass, the larger thecurvature of spacetime.Black Hole: object whose escapevelocity is faster than the speedof light---> can’t escape!!• Event horizon (Schwarzschild radius) is the pointat which escape velocity equals speed of light ~ 3 km for each solar mass in the BH• Inside this radius not even light can escape– can fall in but never get out• We can’t see any light coming from inside !BLACK hole• NO hard surface!!– Event horizon is a “theoretical” point of no returnWarping of Space by Gravity• Gravity imposes curvature on space– light’s path through space will be“bent by gravity”– within the event horizon, it cannotclimb out of the hole• As matter approaches event horizon…– tidal forces are tremendous– object would be “spaghettified”An observercan see theback of her head!Can We Detect Black Holes?By Their Very Nature, BlackHoles Are Invisible!• But we can detect theireffects on nearby matter(stars, gas, etc.)• Astronomers look forcompact “X-ray binaries”Criteria:1. “Invisible” star in binarysystem is too massive to bewhite dwarf or neutron star– Mass > 3 MSun2. Too small in radius to be anormal starCygnus X-1: Blue supergiant (strong winds) pours H + He onto accretion disk of black holeBlue supergiantAccretion diskStellar-size black holeQuestions on Black Holes?What is a Black Hole?A. A black starB. A concentration of mass with a gravitationalfield so strong that not even light can escapeits gripC. A white dwarf painted in blackD. The compact remnant of a low-mass starE. A project that requires large effort with noreturnClicker QuestionWhat is a Black Hole?A. A black starB. A concentration of mass with a gravitationalfield so strong that not even light can escapeits gripC. A white dwarf painted in blackD. The compact remnant of a low-mass starE. A project that requires large effort with noreturnClicker Question GAMMA-RAY BURSTS:witnessing the birth of a new Black Holein the UniverseHow it all started…..mid 1960s: VELA satellite… looking for signs of nuclear tests…TYPICAL GAMMA-RAY BURSTMost GRB data gatheredby BATSE in the 1990sMain properties of GRBs:Rates: about 1 per dayDurations: from tens of milliseconds to severalhundreds of seconds, withbimodal distribution Highly variable ShortLong March 2003:a “special”, very energetic supernova(HYPERNOVA) is found coincident with the position of the (long) GRB Long GRBs areproduced by thecollapse of a massivestar into a Black Hole!!High star rotation likely neededSeveral pieces of evidence seem to indicate that……Short Gamma-Ray bursts are likely the result of amerger of two compact objects (i.e. NS-NS, NS-BH)Stay tuned as more observations come in…..[Image from http://www.laeff.esa.es/BOOTES/esp/grb/grb4.htm]What is a Gamma-Ray Burst?A. An energetic burst of gamma rays from thecenter of the Milky Way.B. An energetic burst of gamma rays fromdistant galaxies.C. The sign that a new star is born.D. A sign of extraterrestrial life.E. A burst of gamma-rays with duration between1000 seconds and 1 dayClicker QuestionWhat is a Gamma-Ray Burst?A. An energetic burst of gamma rays from thecenter of the Milky Way.B. An energetic burst of gamma rays fromdistant galaxies.C. The sign that a new star is born.D. A sign of extraterrestrial life.E. A burst of gamma-rays with duration between1000 seconds and 1 dayClicker QuestionWhat is the likely origin of a long-duration Gamma-Ray Burst?A. The merger of two neutron stars.B. The merger of a neutron star and ablack hole.C. The collapse of a massive, rapidlyrotating star.D. The collapse of a low-mass star.E. The merger of two neutron stars or aneutron star and a black hole.Clicker QuestionWhat is the likely origin of a long-duration Gamma-Ray Burst?A. The merger of two neutron stars.B. The merger of a neutron star and ablack hole.C. The collapse of a massive, rapidlyrotating star.D. The collapse of a low-mass star.E. The merger of two neutron stars or aneutron star and a black hole.Clicker QuestionWhat is the likely origin of a short-duration Gamma-Ray Burst?A. The merger of two neutron stars.B. The merger of a neutron star and ablack hole.C. The collapse of a massive, rapidlyrotating star.D. The collapse of a low-mass star.E. The merger


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