AST 115 1st Edition Lecture 27 Outline of Last Lecture 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 VariablesOutline of Current Lecture I. White dwarf starsa. PropertiesII. Stellar explosions: observations of phenomenaa. Planetary nebulab. Novac. SupernovaIII. Post-main sequence for high mass and very high mass starsIV. Stellar explosions: explanations and phenomenaThese 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. White dwarf and planetary nebulab. Novac. Supernova Type Iad. Supernova Type IIe. Supernova 1987aV. Supernova remnants and the Crab NebulaVI. PulsarsVII. Black HolesCurrent LectureIntroduction to Chapter 12 – The Deaths and Remnants of Stars White Dwarf Starso Example: Sirius Bo White dwarf properties:- Diameter: about 10,000 km (about 6,000 miles)- Mass: less than 1.4 solar masses- Average density: 20 tons/¿3  Stellar explosions: Observations of Phenomenao Planetary nebula- Bubble of hot gas- Hot, low luminosity with a blue-white star at the center- About ½ LY across- Gas expanding at about 100 km/sec- Example of a planetary nebula: The Hellixo Nova- A star or stellar system that brightens suddenly by about 100,000 times its original value.- After months, it gradually fades back to obscurity.- Gas from the explosion expanding at about 1,000 km/seco Supernova- A star or stellar system that brightens suddenly by about 100 million times its original value.- After months, it gradually fades back to obscurity.- Gas from explosion expanding at about 10,000 km/sec- Types of supernovas:Peak SpectrumType Ia M = -19 No hydrogen linesType II M = -17 Hydrogen lines present Post Main Sequence for High and Very High Mass Stars o 8 ˂ mass ˂ 25 solar masses = higho 8 ˂ mass ˃ 25 solar masses = very higho Formation of Red Supergiant Stars:- High (and very high) mass stars can achieve additional levels of fusion because their greater mass allows ever-higher temperatures to be attained in the contracting core.o Evolution Scenario for a 25 solar mass star:- High mass makes higher core temperatures and fusion runs faster- Carbon is fused into Neon in just 600 years- The last level is Silicon fusing into Iron in just 1 day. Stellar Explosions: Explanations and Phenomenao White Dwarf and Planetary Nebula- A low mass star loses its envelope as a planetary nebula after becoming a red giant for the second time.- The core is left behind as a “white dwarf”- Nebula fades after about 100,000 years- The white dwarf is done with fusion. But the electrons in it have been squeezed into a “degenerate gas” and support the white dwarf against gravity indefinitely.- The white dwarf will remain the same size, but will slowly cool over billions of years to become a “black dwarf”.o Nova- This can be explained as a white dwarf in a close binary system with a red giant.- Hydrogen from the red giant overflows its “Roche Lobe” and falls into an accretion disk.- The hydrogen eventually falls onto the surface of the white dwarf, where it heats and explodes from H fusion.- Both stars survive.o Supernova Type Ia- Same as nova except… White dwarf is just under the Chandrasekhar mass limit of 1.4 solar masses.- Accretion of H from companion causes the entire white dwarf to gravitationally collapse and undergo a huge explosion.o Supernova Type II- A high mass star reaches the end of its evolution, Iron Core suffers gravitational collapse = huge explosion of the entire star. Emission of tremendous energy and huge numbers of neutrinos.o Supernova 1987a- Located in a nearby small galaxy known as the Large Megallanic Cloud (LMC), the object reached about 4th magnitude.- Because the LMC is about 160,000 LY from Earth, the actual explosion occurred in about 158,000 BC.- The last naked-eye supernova before this was Kepler’s Supernova of 1604 (in our Galaxy).- SN1987a was remarkable because… We knew which star exploded It was the first nearby (but outside of the Galaxy) supernova studied with modern telescopes and equipment. Neutrinos from the explosion were detected nearby simultaneously with the light signal from the event. Supernova Remnants and the Crab Nebulao These shells of debris from the destruction of entire stars gradually cool and disperse after tens of thousands of years.o The Crab Nebula is about 6,000 LY awayo At the same location in the sky in 1054 AD, Chinese astronomers noted the appearance of what they called a “guest star”.o This implosion from that supernova apparently compressed the star’s core to form a neutron star. Pulsarso In the 30’s, scientists postulated the existence of highly compressed stars with the properties…- Diameter: 25 km (15 miles)- Mass: 1.4 to 3 solar masses- Average density: 20 billion o In the 1960’s, astronomers detected a new kind of object (Pulsars).- Discovered because of extremely regular radio emission arriving in the form of pulses at the rate of about 30 per a second, these were called Pulsating Radio Sources, or Pulsars.- Hundreds have been detected.- A Pulsar is a rapidly rotating neutron star.o The Crab Nebula is a SNR and the pulsar must be the neutron star formed in the explosion some 1,000 years ago. Black Holeso (1914) General Theory of Relativity:- Curvature of spacetime. Mass causes this, e.g. near the Sun; 1919 Solar Eclipse data was consistent with the General Theory of Relativity. Formation of a Black Holeo If a very massive star ( ˃ 25 solar masses) ends its life as a supernova explosion, the implosion force is so great that the core is squeezed to even greater density than that of a neutron star.o The density is so high that it severely distorts the fabric of spacetime.o No known force can withstand the collapse.o This collapsing core disappears from sight as a “black hole” forms. Black Hole Propertieso The core of the massive star collapses past the...- Event Horizon = Mathematical surface at which escape velocity equals thespeed of light.o Nothing, not even light, can escape from within the event horizon.o Singularity = mathematical point to which the imploded core of a massive star is