HNRS 228 - Astrobiology Chapter 11Stellar InquiriesUnderstanding how stars evolve requires both observations and ideas from physicsInterstellar gas and dust is ubiquitous the GalaxySlide 5Slide 6Slide 7Slide 8Protostars form in cold, dark nebulaeiClicker QuestionProtostars develop into main-sequence starsSlide 12During the birth process, stars both gain and lose massSlide 14Slide 15Slide 16Slide 17Slide 18O and B Stars and Their Relation to H II RegionsSlide 20Supernovae can compress the interstellar medium and trigger star birthMore Stellar InquiriesA star’s lifetime on the main sequence is proportional to its mass divided by its luminositySlide 24When core hydrogen fusion ceases, a main-sequence star becomes a red giantRed GiantsSlide 27After the helium flash, a low-mass star moves from the red-giant region of the H-R diagram to the horizontal branchSlide 29Slide 30Slide 31Slide 32The cluster’s age can be estimated by the age of the main-sequence stars at the turnoff point (the upper end of the remaining main sequence)Slide 34Slide 35Slide 36Populations (generations) of starsVariable StarsSlide 39There is a direct relationship between Cepheid periods of pulsation and their luminositiesMass transfer can affect the life cycle of close binary star systemsSlide 42Slide 43Slide 44Slide 45Slide 46Pathways of Stellar Evolution GOOD TO KNOWSun-like stars go through two distinct red-giant stagesSlide 49Bringing the products of nuclear fusion to a giant star’s surfaceSlide 51Sun-like stars “die” by gently ejecting their outer layers, creating planetary nebulaeSlide 53Slide 54The burned-out core of a low-mass star cools and contracts until it becomes a white dwarfSlide 56Slide 57Slide 58Slide 59Slide 60High-mass stars create heavy elements in their coresSlide 62High-mass stars violently blow apart in a supernova explosionSlide 64Slide 65In 1987 a nearby supernova gave us a close-up look at the death of a massive starSlide 67Slide 68Neutrinos emanate from supernovae like SN 1987AWhite dwarfs in close binary systems can also become supernovaeType Ia supernovae are those produced by accreting white dwarfs in close binariesType II supernovae are created by the deaths of massive starsSlide 73Most supernovae in our Galaxy are hidden from our view by interstellar dust and gases but a supernova remnant can be detected at many wavelengths for centuries after the explosion1HNRS 228 - AstrobiologyChapter 11The Life Cycle of StarsDr. Harold Geller2Stellar Inquiries1. Why do astronomers think that stars evolve (bad use of term – this is about the birth, life and death of stars and that is NOT evolution)?2. What kind of matter exists in the spaces between the stars?3. In what kind of nebulae do new stars form?4. What steps are involved in forming a star like the Sun?5. When a star forms, why does it end up with only a fraction of the available matter?6. What do star clusters tell us about the formation of stars?7. Where in the Galaxy does star formation take place?8. How can the death of one star trigger the birth of many other stars?3Understanding how stars evolve requires both observations and ideas from physics •Because stars shine by thermonuclear reactions, they have a finite life span–They fuse lighter elements into heavier elements•When the lighter elements are depleted, there is nothing left to fuse•The theory of stellar evolution (again, not in the same sense as biological evolution, but more like life cycle development, like growing up) describes how stars form and change during that life span4Interstellar gas and dust is ubiquitous the Galaxy•Interstellar gas and dust, which make up the interstellar medium (ISM), are concentrated in the disk of the Galaxy•Clouds within the interstellar medium are called nebulae•Dark nebulae are so dense that they are opaque–They appear as dark blots against a background of distant stars•Emission nebulae, or H II regions, are glowing, ionized clouds of gas–Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars•Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow56789Protostars form in cold, dark nebulae•Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar•As a protostar grows by the gravitational accretion of gases, Kelvin-Helmholtz contraction causes it to heat and begin glowing10iClicker Question•Before a star reaches the main sequence, its energy is generated primarily by–A chemical reactions.–B gravitational contraction.–C helium fusion.–D hydrogen fusion.–E nuclear fission.11Protostars develop into main-sequence stars •A protostar’s relatively low temperature and high luminosity place it in the upper right region on an H-R diagram•Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram•When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main sequence star12iClicker Question•A new star begins to form whenA an existing star is broken apartB an interstellar cloud collapses by its own gravityC fresh material falls onto the core of a previously dead starD nuclear reactions start inside a large planetE new stars don't form anymore13During the birth process, stars both gainand lose mass•In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in the core, a protostar may eject large amounts of gas into space•Low-mass stars that vigorously eject gas are called T Tauri stars14151617iClicker Question•Once a star has reached the main sequence, its energy is generated primarily by–A chemical reactions–B gravitational contraction–C helium fusion–D hydrogen fusion–E nuclear fission18•Star-forming regions appear when a giant molecular cloud is compressed•This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms19O and B Stars and Their Relation to H II Regions•The most massive protostars rapidly become main sequence O and B stars•O and B stars emit strong ultraviolet radiation–UV ionizes hydrogen in the surrounding cloud•creating the reddish emission nebulae called H II regions•Ultraviolet radiation and stellar winds from the O and B stars at the core of an H II region