The Birth of Stars Guiding QuestionsUnderstanding how stars evolve requires both observations and ideas from physics Interstellar gas and dust is ubiquitous the GalaxySlide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Protostars form in cold, dark nebulaeSlide Number 12Protostars develop into main-sequence stars The more massive the protostar, the more rapidly it evolvesDuring the birth process, stars both gain and lose massA circumstellar accretion disk provides material that a young star ejects as jetsClumps of glowing gas called Herbig-Haro objects are sometimes found along these jets and at their endsSlide Number 18Slide Number 19Slide Number 20Young star clusters give insight into star formation and evolutionSlide Number 22Slide Number 23Slide Number 24Star birth can begin in giant molecular clouds Slide Number 26O and B Stars and Their Relation to H II RegionsSlide Number 28Supernovae can compress the interstellar medium and trigger star birthLife After the Main SequenceGuiding QuestionsA star’s lifetime on the main sequence is proportional to its mass divided by its luminosity Slide Number 33Slide Number 34When core hydrogen fusion ceases, a main-sequence star becomes a red giantRed GiantsAs stars age and become giant stars, they expand and shed matter into space Slide Number 38Slide Number 39After helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branchSlide Number 41Slide Number 42The 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 Number 44Slide Number 45As a cluster ages, the main sequence is “eaten away” from the upper left as stars of progressively smaller mass evolve into red giantsSlide Number 47Slide Number 48Populations (generations) of starsVariable StarsSlide Number 51There is a direct relationship between Cepheid periods of pulsation and their luminositiesSlide Number 53Slide Number 54Slide Number 55Slide Number 56Mass transfer can affect the evolution of close binary star systemsGas flowing from one star to the other passes across the inner Lagrangian pointSlide Number 59Slide Number 60Slide Number 61This mass transfer can affect the developmental history of the stars that make up the binary systemSlide Number 63Jargon IJargon II1The Birth of Stars2Guiding Questions1. 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– That is, they fuse lighter elements into heavier elements• When the lighter elements are depleted, there is nothing left to fuse• The theory of stellar evolution (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 glow567891011Protostars 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 glowing1213Protostars 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 star14The more massive the protostar, the more rapidly it evolves15During the birth process, stars both gain and lose mass• In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space• Low-mass stars that vigorously eject gas are often called T Tauri stars16A circumstellar accretion disk provides material that a young star ejects as jets17Clumps of glowing gas called Herbig-Haro objects are sometimes found along these jets and at their ends18192021Young star clusters give insight into star formation and evolution• Newborn stars may form an open or galactic cluster• Stars are held together in such a cluster by gravity• Occasionally a star moving more rapidly than average will escape, or “evaporate,” from such a cluster• A stellar association is a group of newborn stars that are moving apart so rapidly that their gravitational attraction for one another cannot pull them into orbit about one another22232425Star birth can begin in giant molecular cloudsThe spiral arms of our Galaxy are laced with giant molecular clouds, immense nebulae so cold that their constituent atoms can form into molecules26• 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 mechanisms27O and B Stars and Their Relation to H II Regions• The most massive protostars to form out of a dark nebula rapidly become main sequence O and B stars• They emit strong ultraviolet radiation that