AST 115 1st Edition Lecture 25 Outline of Last Lecture I. Measuring Masses with Visual Binariesa. Newton’s Revision of Kepler’s 3rd Law and ExampleII. Measuring Diameters with Eclipsing BinariesIII. Normal Ranges in Stellar Parallaxa. Diameterb. Massc. Luminosityd. Surface TemperatureIV. Intro to Stellar Evolutiona. “Golden Rule”b. Stages of Stellar EvolutionIntro to Chapter 11: The Lives of StarsI. The Interstellar Mediuma. Gasb. DustII. Intro to Types of Nebulasa. BrightOutline of Current Lecture I. Types of Nebulas Continueda. Reflectionb. Darkc. H II regionsd.H2 regionsII. Giant Molecular CloudsIII. Triggers of Star FormationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. GMC collisionsb. Supernova shockwavec. H II explosiond. Rotation of the GalaxyIV. Process of Star FormationV. Theoretical Evolutionary TracksVI. Main Sequence and Giant Starsa. Stability of Main Sequence Starsb. Main Sequence Lifetimesc. Brown DwarfsCurrent Lecture Types of Nebulas Continuedo Reflection = Blue light from light reflected off of microscopic dust particles around hot stars.o Dark = Black clouds of dust. Interstellar dust dims and reddens starlight.o Cold hydrogen gas (not emitting light). Detectable at 21 cm (1420 MHz) radio. - Also called H I regions.o Cold Molecular hydrogen gas (not emitting light). Detectable at infrared wavelengths.- Also called H2 regions. Giant Molecular Clouds (GMC’s)o Invisible to optical telescopes, some 6,000 GMCs have been discovered in the disk of our Galaxy.o Typical GMC:- About 300 LY across- 10 ˚K- 200,000 M⊙ o Example of a nearby GMC = Orion Giant.- Molecular Cloud associated with the three stars in the belt of the mythological Hunter.o GMCs supply raw materials from which new stars form. Triggers of Star Formationo To begin gravitational contraction, a portion of an interstellar gas and dust cloud (“cloud core”) must reach critical density.- Critical density is about 1 million molecules per cubic centimeter.o Possible Triggers:- GMC collisions: movement of gases and dust within a cloud, or when clouds collide, causes localized spots to reach critical density.- Supernova shockwave: explosion of a nearby massive star causes compression of spots in the cloud to trigger contraction.- H II explosion: expanding hot gas of an H II region can compress gas and dust also.- Rotation of the Galaxy: spiral density waves are set up in the rotating disk that compress the gas and dust to critical density. Process of Star Formationo Once triggered, the cloud core will undergo gravitational contraction.o A 1/10 LY diameter portion of interstellar gas and dust cloud begins gravitational contraction.o The contraction supplies energy to heat the center: a protostar.o The spin of the cloud produces the “accretion” disk from which planets may form.o The critical temperature of hydrogen fusion is 10 million ˚K. When reached, contraction stops and the protostar turns into a true star. Theoretical Evolutionary Trackso Models are calculated that show how stellar properties change with time.o The data is then displayed on the H – R Diagram as theoretical “evolutionary tracks”. Main Sequence and Giant Starso Stability of Main Sequence Stars- A “main sequence” star is one in which hydrogen fusion takes place in its core.- The main sequence stage is the longest stage of a star’s existence. Extremely stable.- There is an exact balance between two opposing forces, creating the stability. Gravity always tries to contract stars and produces a force that is radially inwards. Gas pressure inside the star produces the radial outward force needed for the balance.- Gas pressure inside a star is due to the energy released by the fusion process.- Simple Interior Structure: Core Envelope- The core is the region where temperature is greater than 10,000,000 ˚K (hot enough for fusion to occur)o Main Sequence LifetimesMass (solar masses) Life (years)25 3,000,00015 15,000,0003 500,000,0001 10,000,000,000½ 200,000,000,0001/10 1,000,000,000,000o Brown Dwarfs- If the amount of mass in a contracting “cloud core” is less than 0.1 solar mass, it never reaches hydrogen fusion temperature (107 ˚K).- Instead, the object will shine brightly in the infrared for a brief time as it continues to contract, but will soon cool again.- Such failed stars are called “brown dwarfs”.- Range of masses of brown dwarfs is about 0.01 to 0.08 solar masses. Jupiter, at 0.001 solar mass, is about as massive as a planet can be before it becomes a brown
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