ASTR 1514 1st Edition Lecture 29 Evolution of High-Mass StarsChapter 13.1-13.2- High Mass stars have masses greater than 8 solar masses- High Mass stars fuse hydrogen differently - CNO (Carbon-nitrogen-oxygen) cycle – uses carbon and other nuclei as catalysts - CNO Cycle- Require a higher temperature since there is a greater repulsive force between a proton and a carbon nucleus - Our sun creates about 1.7% of Helium by this process, but in heavier stars, it is much more important- Once Hydrogen is exhausted from the core, the star leaves the main sequence, expands and cools. (Move right on the H-R diagram): Super Giants- Helium does not build up as ash in the center of these stars, so there is no degenerate corein massive stars- Heavy elements can burn in a high mass star- The more massive a star the heavier elements than can fuse - Fusion shells build up like layers of an onionThese 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.- The higher the temperature and density, the heaver the element that can burn- As High-Mass stars expand and cool, they can pass through the instability strip, where the combination of temperature and luminosity results in the stars pulsating. - Pulsating stars are extremely important when determining distances – a relationship between pulsation period and luminosity. - Massive have winds, the mass loss rates are large – 10-7 solar masses to 10-5 solar masses per year.- Stars with 20 solar masses could lose more than 50% of their mass- Iron cores of a star will not fuse, they are the limit. - Star fuses elements in order to produce the pressure needed to support the outer layers. 2.) Final Days of a Massive Star- Nuclear binding energy plot tells you how much energy can be extracted from fusion of various elements - Elements with higher average binding energy are more tightly bound- Iron is the most tightly bound element- The difference in total binding energy tells you how much energy is released per fusion - Ex: The binding energy of helium is much greater than for four hydrogen , so fusing four hydrogen into one helium releases a lot of energy- The triple alpha process – The difference in binding energy between carbon and three helium is less, so less energy is released. - The difference between Silicon and Iron is not much at all, so hardly any energy is produced.- Elements beyond iron have lower binding energy, these elements do not produce energy for the star to use (exothermic) rather those reactions take energy to happen (endothermic)- So Iron, the most stable element, remains the final core constituent in the star. - All of these reactions release neutrinos which carry energy out of the star- The iron core cannot participate in fusion so it is “ash”- It is then compressed and eventually becomes electron degenerate- Photodisintegration- Photons break iron nuclei apart- Reverses effect of nuclear fusion- Absorbs thermal energy- Removes pressure- Protons and electrons combine and become neutrons- Core collapses at about ¼ the speed of light- Core Rebound- The core may not collapse indefinitely. Eventually it reaches the density of an atomic nucleus - The strong force, short range attractive force, switches a repulsive force if compressed too far- Huge numbers of neutrinos are produced, and some are trapped- The rest of the star is collapsing - Core stops, the rebounds, driving a shock wave through the star- The rebound explodes the star- Shock wave takes a few hours to rip through the star- Outer layers blow off in huge explosions (type 2 supernova), leaving the core. - But these supernova fade away after about 100 daysSupernova are found- Through automated searches- These searches visit various galaxies each night looking for