1The Birth of StarsChapter TwentyGuiding Questions1. Why do astronomers think that stars evolve?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?Understanding how stars evolve requires bothobservation 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 span Interstellar 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 glow23Protostars 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 glowingProtostars evolve 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 starThe more massive the protostar, the more rapidly it evolvesDuring 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 its core, a protostar may eject large amounts of gas into space• Low-mass stars that vigorously eject gas are called T Tauri stars4A 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 endsYoung star clusters give insight into starformation 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 another5Star 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 molecules• 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 mechanismsO 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 ionizes hydrogen in the surrounding cloud, thus 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 create shock waves that move outward through the gas cloud, compressing the gas and triggering the formation of more protostars6Supernovae can compress the interstellar mediumand trigger star birthKey Words• accretion• Barnard object• bipolar outflow• Bok globule• circumstellar accretion disk• cluster (of stars)• cocoon nebula• dark nebula• dust grains• emission nebula• evolutionary track• fluorescence• giant molecular cloud• H II region• Herbig-Haro object• interstellar extinction• interstellar medium• interstellar reddening• nebula (plural nebulae)• nebulosity• OB association• open cluster• protoplanetary disk (proplyd)•protostar• recombination• reflection nebula• stationary absorption lines• stellar association• stellar evolution• supernova remnant• supersonic• T Tauri