AST-A105 1nd EditionExam # 3 Study Guide Lectures: 14-19Lecture 14 (March 11)Be familiar with how stars form:Star Formation → stars form in cold, dense interstellar clouds called dark nebula.Dark Nebula are dense clouds of gas and dust. Dust absorbs visible light and hides the stars behind it. Clouds can collapse because they are goingin and out of hydrostatic equilibrium. Gravity pushes in.Pressure pushes out.When gravity pressure is too great, cloud collapses. When central temperature reaches 10^7K, a protostar is formed and hydrogen fusion begins to take place in the core. As hydrogen fusion takes place, a star is born and placed on the main sequence.Protostars are surrounded by gas and dust clouds from which they form.Lecture 15 (March 13)Understand the characteristics of Reflection Nebulae and Interstellar Reddening.Reflection Nebulae- a bluish haze seen around hot, young stars.- Reflection Nebulae is due to interstellar dust.- Interstellar dust reflects blue light more effectively that red light.- A blue haze is reflected in star light.- Stars behind interstellar dust appear redder than their true color.Interstellar Reddening- dust clouds between star and observer scatter more blue light that red light out of a beam.- Stars appear redder than their true color.Lecture 16 (March 25)Know the Evolution of a 1 Mʘ Star.Main Sequence EvolutionNuclear fusion in the core concerts H to HeStar leaves main sequence when H is used up in the coreSun spends 10^10 years in the main sequenceTrack of star next moves to higher luminosity (L) and lower temperature (T)More massive stars “burn” through H fasterRed Giant PhaseH fusion ends in core, but continues in shell around the coreGravity causes core to contract and heatEnergy output from shell increasesOuter region of star expands and coolsHelium flash- when Tcentral = 10^8K, He fusion begins in the coreHorizontal Branch PhaseHe fuses to C in coreCore expands and coolsEnvelope contracts and becomes bluerL becomes higher than for the SunSecond Red Giant Phase No He left in coreCore is now carbon (C) and oxygen (O)No nuclear fusion occurs in coreCore contracts and heats; envelope expands and coolsPlanetary Nebula PhaseRed supergiant envelope pulsates unstablyEnvelope is ejected and core is exposedStructure: expanding spherical gas shells around small, hot star; looks like a bright ring on skyNOT related to the planets, although it may look like a planet with a small telescopeWhite Dwarf PhaseHot, dense core becomes white dwarf = collapsed star about size of EarthNo nuclear burningStructure: density=10^9 kg/m^3= 10^6 x density of waterElectrons are packed as closely together as possibleDoes NOT contract furtherT and L decrease in timeLecture 17 (March 27)Be familiar with both Open Star Clusters and Globular Star Clusters.Also know about Equinoxes.Open Star Clustero Found in the disk of the Galaxy Ex. The Pleiadeso Stars are young and recently formed.o Most stars are near main sequence.o The most massive stars are just leaving the main sequence.o Age is approximately 2 x 10^7 years.Globular Star Clustero Found in the bulge/halo of our Galaxyo Contain only old stars.o Age is approximately greater than 10^10 years.o Almost no stars are present on upper main sequence These stars have moved on to the next phase.Lecture 18 (April 1)Be familiar with Supernovas, Supernova Blasts, Neutron Stars.A supernova is the death of a massive star. It occurs when gravity overcomes outward electron pressure in iron core, causing a rapid collapse. Electrons and protons combine to form neutrons. Neutrinos (very low mass particles) are produced. The pulse of neutrinos is emitted. Neutron pressure stops collapsing when core is 20 km (12 mi) across because the pressure is then balancing the gravity in the core. A Supernova Blast occurs when energy released by core collapse causes a shock wave totravel out and the rest of the star to explode outward.Neutron Stars are the ultra-collapsed stellar core produced by a supernova.Lecture 19 (April 3)Be familiar with Black Holes.Event horizonSingularityBlack HoleAn object with such strong gravity that nothing can escape from its vicinity, not even lightPredicted by Einstein’s Theory of general Relativity in 1916:Nothing can travel faster than lightGravity causes space to curveWhen a star becomes a black hole, it collapses to a point of infinite density – a singularityAnything that passes into the event horizon merges into the black holeBlack hole mass and event horizon size increase as material falls inNothing escapes from a black hole.Lecture 20 (April 8)Be familiar with the Milky Way Galaxy.Planet Earth is located inside the Milky Way Galaxy. It is a large spiral galaxy containing approximately 10^11 stars. Our Sun is located in the disk, which is frisbee shaped containing young and old stars, gas, and dust in a spiral pattern. When we see the Milky Way band in our night sky, we are looking at the disk of our galaxy. See notes from Lecture 20 for more