Chapter 13The Life of a StarSlide 3Mass Is the KeyThe Life of Our SunSlide 6Slide 7The Life of a High-Mass StarSlide 9Slide 10The Importance of GravityInterstellar Gas CloudsInitial CollapseTo the Protostar StageProtostarsFurther CollapseHerbig-Haro ObjectsBipolar FlowsT-Tauri StarsStellar Mass LimitsA Star’s Mass Determines Its Core TemperatureStructure of High- and Low-Mass StarsSlide 23Stellar LifetimesSlide 25Leaving the Main SequenceSlide 27Slide 28Giant StarsSlide 30Degeneracy in Low-Mass Giant StarsYellow GiantsVariable StarsWhy Variable Stars PulsateThe Instability StripThe Period-Luminosity LawThe Death of Sun-like StarsDeath of a Low-Mass StarSlide 39Planetary NebulaeOld Age of Massive StarsNucleosynthesisCore Collapse of Massive StarsSupernovaeSlide 45Slide 46Supernova RemnantsSlide 48Stellar CorpsesHistory of Stellar Evolution TheoriesTesting Stellar Evolution TheoriesSlide 52Slide 53Chapter 13Stellar EvolutionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.The Life of a Star•Gravity holds a star together while the pressure of its gases supports it against gravity’s pull•A star generates its supporting pressure from energy produced in its core by the conversion of hydrogen into helium•The hydrogen cannot last forever – consequently, the star must evolve (age)•Once its fuel is exhausted, the star dies – quietly into a white dwarf or violently into a neutron star or black hole•The violent explosions of dying large stars seed interstellar space with materials for the next generation of stars and the elements vital to human lifeThe Life of a StarMass Is the Key•Stars require millions to billions of years to evolve – a time that is incredibly slow by human standards•A star’s evolution can be studied two ways:–Stellar models via computer calculations that take into account the relevant physics–Observations – different stars represent different snapshots in the life of a star•The lifeline of a star is found to depend critically on its mass•The possible endings of a star’s life naturally divide stars into two groups: low-mass stars and high-mass stars, with the division set at about 10 solar massesThe Life of Our Sun•The Sun was born out of an interstellar cloud that gravitationally collapsed over a time span of a few million years•Fusing hydrogen into helium in its core, the Sun will reside on the main sequence for 10 billion years and in the process convert 90% of its core hydrogen into heliumThe Life of Our Sun•Starved of fuel, the core will shrink and grow hotter as the outer surface expands and cools transforming the Sun into a red giant•After one billion years, the red giant’s core will be hot enough to begin fusing helium•The Sun will then transform into a pulsating yellow giantThe Life of Our Sun•As the core’s helium fuel begins to expire, the Sun will once again transform into a red giant, but only bigger than before•The high luminosity of the red giant will drive the Sun’s atmosphere into space leaving behind its bare core•The core will cool and dwindle into a white dwarfThe Life of a High-Mass Star•The early life of a high-mass star is similar to the Sun:–Collapses from an interstellar cloud and resides on the main sequence–Proceeds through these stages much faster than the Sun, spending less than 100 million years on the main sequenceThe Life of a High-Mass Star•A high-mass star then passes through the pulsating yellow giant stage before it turns into a red giant•In the red giant phase, the core begins to fuse one element into another creating elements as massive as ironThe Life of a High-Mass Star•Once iron is reached, the core is out of fuel and it collapses–The star’s heavy elements are blown into space along with its outer layers–A neutron star or black hole is left behindThe Importance of Gravity•Gravity drives stellar evolution from a star’s formation out of a cloud to its final death–The collapsing cloud will heat because of gravity–The main-sequence star will sustain itself as gravity compresses and heat the core to fusion temperatures–Gravity will sculpt the final collapse of the star into a white dwarf, neutron star, or black hole•The amount of mass (gravity) will also drive the duration of the evolutionInterstellar Gas Clouds•General Characteristics–Gas: hydrogen (71%), helium (27%), others–Dust: microscopic particles of silicates, carbon, and iron–Temperature: Around 10 KInitial Collapse•Low temperature leads to too low pressure to support cloud against gravitational collapse•Collapse may be triggered by collision with another cloud, a star explosion, or some other process•Non-uniformity, clumpy nature of gas leads to formation of smaller, warmer, and denser clumpsTo the Protostar Stage•Rotating dense clumps flatten into disk•About one million years: small, hot dense core at center of disk forms – a protostar•Stars generally form in groups – similar ageProtostars•Characteristics–Temperature: About 1500 K–Shine at infrared and radio wavelengths–Low temperature and obscuring dust prevents visible detection–May be found in “Bok globules”, dark blobs 0.2-2 lys across with masses of up 200 solar massesFurther Collapse•Gravity continues to draw material inward•Protostar heats to 7 million K in core and hydrogen fusion commences•Collapse of core ceases, but protostar continues to acquire material from disk for 106 years•In-falling material creates violent changes in brightness and ultimately a strong outflow of gasHerbig-Haro Objects•Long, thin jets squirt out from the young star, carving a cavity in the gas around the star and creating bright blobs, “Herbig-Haro objects”, where the jet hits surrounding, distant gasBipolar Flows•Jets also create bipolar flows around protostar–Easily seen at radio wavelengths–Clears away most gas and dust around protostarT-Tauri Stars•Young stars still partially immersed in interstellar matter•Vary erratically in brightness, perhaps due to magnetic activity•Intense outward gas flows from surfaces•Occupy H-R diagram just above main-sequenceStellar Mass Limits•Stars smaller than 0.1 M rarely seen since their mass is too small for their cores to initiate fusion reactions•Objects with masses between planets and are called brown dwarfs, “failed stars” extremely dim and difficult to observe•Upper mass limit of stars