Astronomy 100Exploring the UniverseTuesday, Wednesday, ThursdayTom [email protected] 100Exploring the UniverseTuesday, Wednesday, ThursdayTom [email protected] Dwarfs• Failed stars•Not enough mass for fusion•Minimum mass of gas need for fusion is 0.08 •Minimum mass of gas need for fusion is 0.08 solar masses (80 times the mass of Jupiter)Brown DwarfsNot enough mass for fusionMinimum mass of gas need for fusion is 0.08 Minimum mass of gas need for fusion is 0.08 solar masses (80 times the mass of Jupiter)Main Sequence•Is not an evolutionary track– Stars do not evolve on it•Stars stop on the main sequence and spend most •Stars stop on the main sequence and spend most of their lives on itMain SequenceIs not an evolutionary trackStars stop on the main sequence and spend most Stars stop on the main sequence and spend mostSun ends it time on the main sequence•When the core hydrogen is depleted, nuclear fusion stops•The core pressure can no longer resist the crush of •The core pressure can no longer resist the crush of gravity• Core shrinksSun ends it time on the main sequenceWhen the core hydrogen is depleted, nuclear The core pressure can no longer resist the crush of The core pressure can no longer resist the crush ofWhy does the star expand?•The core is made of helium•The surrounding layers are made of hydrogenWhy does the star expand?The core is made of heliumThe surrounding layers are made of hydrogenAnd ..•Gravity shrinks the inert helium core and surrounding shell of hydrogen•The shell of hydrogen becomes hot for fusion•The shell of hydrogen becomes hot for fusion• This is called hydrogen-And ..Gravity shrinks the inert helium core and surrounding shell of hydrogenThe shell of hydrogen becomes hot for fusionThe shell of hydrogen becomes hot for fusion-shell burningAnd …•The shell becomes so hot that its fusion rate is higher than the original core•This energy can not be transported fast enough to •This energy can not be transported fast enough to surface•Thermal pressure builds up and the star expandsAnd …The shell becomes so hot that its fusion rate is higher than the original coreThis energy can not be transported fast enough to This energy can not be transported fast enough to Thermal pressure builds up and the star expandsAnd ..•More helium is being created• Mass of core increases•Increases its gravitational pull•Increases its gravitational pull•Increasing the density and pressure of this regionAnd ..More helium is being createdIncreases its gravitational pullIncreases its gravitational pullIncreasing the density and pressure of this regionWhen•When helium core reaches 100 million Kelvin,•Helium can fuse into a Carbon nucleusWhenWhen helium core reaches 100 million Kelvin,Helium can fuse into a Carbon nucleusHelium Flash•The rising temperature in the core causes the helium fusion rate to rocket upward•Creates a lot of new energy •Creates a lot of new energy Helium FlashThe rising temperature in the core causes the helium fusion rate to rocket upwardCreates a lot of new energy Creates a lot of new energyHowever• The core expands•Which pushes the hydrogenoutwardsoutwards• Lowering the hydrogen-temperatureHoweverWhich pushes the hydrogen-burning shell -burning shell’sAnd•Less energy is produced• Star starts to contractAndLess energy is producedNow•In the core, Helium can fuse to become Carbon (and some Oxygen)•Star contracts•Star contracts•Helium fusion occurs in a shell surrounding the carbon core•Hydrogen shell can fuse above the Helium shell•Inner regions become hotter• Star expandsNowIn the core, Helium can fuse to become Carbon Helium fusion occurs in a shell surrounding the Hydrogen shell can fuse above the Helium shellInner regions become hotter– Triple Alpha Process–4He + 4He ↔ 8Be–8Be + 4He ↔ 12C + gamma ray + 7.367 http://upload.wikimedia.org/wikipedia/commons/8/8d/TripleC + gamma ray + 7.367 MeVhttp://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png•Some carbon fuses with He to form Oxygen•12C + 4He → 16O + gamma ray•Harder to fuse Oxygen with Helium to produce •Harder to fuse Oxygen with Helium to produce NeonSome carbon fuses with He to form OxygenO + gamma rayHarder to fuse Oxygen with Helium to produce Harder to fuse Oxygen with Helium to producePlanetary Nebulae•There is a carbon core and outer layers are ejected into space•The core is still hot and that ionizes the expanding •The core is still hot and that ionizes the expanding gasPlanetary NebulaeThere is a carbon core and outer layers are ejected The core is still hot and that ionizes the expanding The core is still hot and that ionizes the expandingPlanetary NebulaePlanetary NebulaeWhite Dwarf•The remaining core becomes a white dwarf•White dwarfs are usually composed of carbon and oxygen (can not fuse carbon)oxygen (can not fuse carbon)• Oxygen-neon-magnesium white dwarfs can also form (hot enough to fuse carbon but not neon)•Helium white dwarfs can formWhite DwarfThe remaining core becomes a white dwarfWhite dwarfs are usually composed of carbon and oxygen (can not fuse carbon)oxygen (can not fuse carbon)magnesium white dwarfs can also form (hot enough to fuse carbon but not neon)Helium white dwarfs can formHigh-Mass Stars• The importance of high-make elements heavier than carbon•You need really hot temperatures which only •You need really hot temperatures which only occur with the weight of a very highMass Stars-mass stars is that they make elements heavier than carbonYou need really hot temperatures which only You need really hot temperatures which only occur with the weight of a very high-mass starStages of High-Mass Star’s Life• Similar to low-mass star’s• Except a high-mass star can continue to fuse elementselements•When the fusion ceases, the star becomes a supernova•Supernova is a huge explosionMass Star’s Lifemass star’smass star can continue to fuse When the fusion ceases, the star becomes a Supernova is a huge explosionFusion in High•Besides fusion of Hydrogen into Helium•The high temperatures allow Carbon, Nitrogen, and Oxygen to be catalysts for converting and Oxygen to be catalysts for converting Hydrogen into HeliumFusion in High-Mass starsBesides fusion of Hydrogen into HeliumThe high temperatures allow Carbon, Nitrogen, and Oxygen to be catalysts for converting and Oxygen