AST 115 1st Edition Lecture 26 Outline of Last Lecture I. Types of Nebulas Continueda. Reflectionb. Darkc. H II regionsd.H2 regionsII. Giant Molecular CloudsIII. Triggers of Star Formationa. 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 DwarfsOutline of Current Lecture These 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.Post Main Sequence EvolutionI. Very Low Mass StarsII. Low Mass StarsIII. Medium Mass StarsIV. High Mass StarsV. Types of Star Clustersa. Openb. Globularc. “Associations”VI. Using Star Clusters to Test EvolutionLate Stage EvolutionI. Three Categories of Variable Starsa. Eclipsing Variablesb. Erupting Variablesc. Pulsating VariablesCurrent LecturePost Main Sequence Evolution Very Low Mass Stars (0.08 to 0.4 M⊙)o Cool, red stars like Barnard’so Very long main sequence lifetimes because…- The core temperature is just high enough for hydrogen fusion.- Completely convective interiors (fresh supply of hydrogen moves into the core until the entire star is completely changed to helium)o All that the very low mass stars can do is gradually cool downo Very low mass stars are the most common type of star in our Galaxy. 85% of the stars in our Galaxy are very low mass stars. Low Mass Stars (0.4 to 2 M⊙)o This is the future predicted for our Sun → to become a low mass star.o Eventually all the hydrogen in the core of the Sun is converted into helium.o With no fusion heat to provide gas pressure, gravity starts to shrink the core.o Gravitational contraction released energy that starts hydrogen “shell” fusion at the core envelope boundary.o The hydrogen → helium shell fusion gives energy to the envelope, which drives a tremendous expansion into a “red giant” star.o When the shrinking core heats to 108 ˚K, a second level of fusion starts (He → C). This is called the “Helium Flash”.o The Core stops shrinking because now there is gas pressure to support it. The envelope then shrinks back to almost normal.Repeat Performanceo Process is repeated when the helium core is changed into carbon.o This time, the shrinking core ignites a helium shell.o The helium “burning” shell is unstable.o Thermal pulses drive the envelope away from the star in episodes.o The expanding envelope produces a “Planetary Nebula”, like the “Ring Nebula”.o On the H-R Diagram, theoretical evolutionary tracks differ according to a star’s mass.o Low mass stars cannot achieve a high enough temperature for any more fusion.o But the core contracts until it is no longer able to do so. Medium Mass Stars (2 to 8 M⊙)o No “helium flash” after the first red giant expansion.o Helium fusion begins smoothly.o Lose significant amounts of mass as red giants.o Also produce “planetary nebulas”. High Mass Stars (8 to 25 M⊙)o These stars undergo a different post-main sequence. Types of Star Clusterso Open- Consist of 100s to 1,000s of stars- Don’t have symmetric shapes- Range from young to old- Found mainly in the “Milky Way” in the sky- Example: The Pleiadeso Globular- Consist of 10,000s to 100,000s of stars- Always symmetrical (round)- Are all quite old- Found in all directions of the sky- Example: M13 in Herculeso “Associations”- Consist of 10s of stars (few stars)- No symmetry- Very young- Found only in the Milky Way in the sky- Example: the belt stars in Orion Using Star Clusters to Test Evolutiono All of the stars in a given cluster are assumed to have about the same:- Distance from the Earth (or Sun)- Age- Chemical compositiono The property that is not the same: MASSo The “golden rule” of stellar evolution gives us a panorama of stages of stellar evolution in a given cluster.o Observed properties of stars in a cluster match theoretical evolution.o The cluster “turn – off” point is the place where Main Sequence stars are evolving away to the Red Giant Region.o This point indicates the age of the star cluster.o Turn – off point is down and right on the Main Sequence for older star clusters.Late Stage Evolution The 3 Categories of Variable Stars:o Eclipsing Variables → what looks like a single star changing brightness is actually two getting in each other’s wayo Eruptive Variables → some of these get brighter by ejecting hot material (novas, supernovas), others get dimmer by ejecting particles which shade their light.o Pulsating Variables → these single stars expand and contract rhythmically due to structural instability.- RR Lyrae Variables Regular periods of about a 0.5 day All have (average) MV about 0 Distance indicators- Cepheid Variables Named after prototype Regular periods from 1 day to 1 month The longer the period, the more luminous the star is. Range is -3 ˃ MV ˃
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