Evolution of Low Mass Stars: Main Sequence and Beyond: Chapter 12.1 – 12.3High Mass Stars have a shorter life than low mass stars – They must use their fuel faster to support their large mass. Higher temperature and pressure means faster nuclear fusionStars with higher masses burn their fuel more quicklyMass of a star determines the properties of the star including: Temperature, Spectral Type, and Luminosity. - Brown Dwarf Stars- Objects between about 13 and 80 Jupiter masses- Stars so small that the core never reaches a high enough temperature to do fusion- Brown dwarfs are very cold, dim objects- They can have fusion but not hydrogen fusion - The most massive stars have 100-200 times the mass of our sun.- They produce so much light that they blow off their outer regions- R136a1 (a star 265 times the mass of our sun, with a surface temperature of 53,000K and diameter 35 times the sun’s) located in the Large Magellanic Cloud, a nearby galaxy. Life and Times of a Low Mass Star – Before The Main Sequence- 90% of stars including our sun, exist on the main sequence. - Stars spend 90% of their lives on the main sequence. - The main sequence is where they experience hydrogen fusion in their coresWhat are stars doing the other 10% of their life out of the main sequence? - Before they are stars they are proto-stars that form from molecular-cloud cores- As they collapse they heat up due to conversion of gravitational energy to thermal energy- Proto-stars are seen in the infrared.Life and Times of a Low Mass Star – On the Main Sequence- On the main sequence, the star is converting hydrogen to helium- 1 helium takes 4 hydrogen nuclei, so the number of particles in the core of the star decreases, and the pressure drops. - To compensate, the temperature in the core increases, increasing the number of fusion reactions, and increasing the luminosity and number of photons trying to come out- The star expands cooling slightly- Star moves up (higher luminosity) and right (Lower temperature) on the HR Diagram - Main sequence stars fuse hydrogen to helium in their cores- Eventually, much of the core H is converted to He- A core of He ash is built up- Helium is called “ash” because it does not, at this point, participate in fusion. Life and Times of a Low Mass Star – After the Main Sequence- There is no more hydrogen in the core and only helium “ash” is present- The core becomes electron degenerate in response to the great pressure imposed by the star’s mass- Just like atoms, Matter itself has energy levels- At low densities, the electrons in matter are spread out thinly among many energy levels- Matter is mostly empty space- At high densities, the electrons are forced into the lowest energy levels possible (two per level)- Becomes very dense – 1 cubic centimeter weighs 1,000 kilograms. - Called electron-degenerate matter. - H fusion only takes place in a shell around the 100% He core. - Since the He is not fusing, gravity begins to win over the pressure, crushing the Helium.- Energy from conversion of gravitational energy to thermal energy is produced, so the core becomes hotter.- The core also becomes denser, and becomes electron-degenerate. - Electron degenerate matter has peculiar properties- More massive the core, the smaller it is. So a core with more mass in it will have a smaller radius. - It has a fixed pressure: If the temperature is increased, the pressure doesn’t increase- It doesn’t help to support the star- The core becomes hot enough that hydrogen can burn in a shell around the degeneratehelium core- So much energy is produced that the envelope heats up and expands to become a RedGiant- Radiation is trapped, expanding the envelope dramatically. - Red Giant- Stronger gravity – higher pressure – faster nuclear burning – more energy being produced- Not all energy can escape – some goes into heating the star to swell it up- Increase in pressure and luminosity results in increase in size and decreased surface temperature: Red Giant- H-R Diagram star moves up (more luminous) and to the right (lower temperature)- The sun will become a Red Giant star in about 5 billion years- When hot and dense enough, fusion of He begins in the degenerate core- Helium fuses to carbon via the triple-alpha process- Helium fusion requires a higher temperature than hydrogen fusion, because each He nucleus contains two protons, so the repulsive electromagnetic force is strong. - Called triple-alpha because alpha particle is another name for a helium nucleus and because 3 reactions must take place. - Helium Flash- The dominant pressure in the core is the electron degeneracy pressure- Pressure does Not increase with temperature- Helium fusion begins in the core 1. Fusion heats core rapidly2. No rise in pressure3. No expansion4. No drop in temperature5. Not stable- Runaway rise in temperature - Helium fusion begins explosively- Hidden in the core- After a Helium Flash, the stars are on the horizontal branch- He fuses to Carbon in the core, while H fuses to He in a shell.- Core is no longer degenerate, so it is larger, but total energy output is less- Star is smaller and hotter. - Eventually helium is used up in the core- He fusion in an inner shell and H fusion in an outer shell: degenerate carbon core. - Helium shell burning. - Star gets more luminous and cool, and enters the asymptotic giant branch (AGB) of the H-R diagram (up and