The Lives of Stars Understanding how stars evolve requires both observation and ideas from physics Interstellar gas and dust is ubiquitous the GalaxyProtostars form in cold, dark nebulaeDuring the birth process, stars both gain and lose massProtostars evolve into main-sequence stars All main sequence stars produce energy by hydrogen fusion, not in a single step, but in a sequence of thermonuclear reactions Stars of different masses have different structures, furthermore, the more massive the star, the more rapidly it evolvesYoung star clusters give insight into star formation and evolutionSupernovae can compress the interstellar medium and trigger star birthA star’s lifetime on the main sequence is proportional to its mass divided by its luminosity When core hydrogen fusion ceases, a main-sequence star becomes a red giantRed GiantsAfter the helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branchThe cluster’s age can be estimated by the age of the main-sequence stars at the turnoff point (the upper end of the remaining As a cluster ages, the main sequence peels away from the main sequence region as stars of progressively smaller mass evolve inPopulations (generations) of starsVariable StarsThere is a direct relationship between Cepheid periods of pulsation and their luminositiesMass transfer can affect the evolution of close binary star systemsGas flowing from one star to the other passes across the inner Lagrangian pointThis mass transfer can affect the evolutionary history of the stars that make up the binary systemPathways of Stellar Evolution GOOD TO KNOWLow-mass stars go through two distinct red-giant stages Bringing the products of nuclear fusion to a giant star’s surfaceLow-mass stars die by gently ejecting their outer layers, creating planetary nebulaeThe burned-out core of a low-mass star cools and contracts until it becomes a white dwarfHigh-mass stars create heavy elements in their coresHigh-mass stars violently blow apart in supernova explosionsIn 1987 a nearby supernova gave us a close-up look at the death of a massive starNeutrinos emanate from supernovae like SN 1987A White 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 starsScientists first proposed the existence of neutron stars in the 1930sThe discovery of pulsars in the 1960s stimulated interest in neutron starsPulsars are rapidly rotating neutron stars with intense magnetic fieldsSuperfluidity and superconductivity are among the strange properties of neutron starsPulsars gradually slow down as they radiate energy into spaceThe fastest pulsars were probably created by mass transfer in close binary systemsPulsating X-ray sources are also neutron stars in close binary systemsExplosive thermonuclear processes on white dwarfs and neutron stars produce novae and burstersLike a white dwarf, a neutron star has an upper limit on its massThe Lives of StarsUnderstanding 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 spanInterstellar 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 glowProtostars 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 glowingDuring 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 starsProtostars 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 starAll main sequence stars produce energy by hydrogen fusion, not in a single step, but in a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleusStars of different masses have different structures, furthermore, the more massive the star, the more rapidly it evolvesYoung 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 another• 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 mechanismsSupernovae can compress the interstellar mediumand trigger star birthA star’s lifetime on the main sequence isproportional to its mass divided by its luminosity • The duration of a star’s main sequence lifetime depends on the amount of hydrogen in the star’s core and the rate at which the