1The 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 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 glow2Protostars 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 star3All 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 mechanisms4Supernovae 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 hydrogen is consumed• N.B. - The more massive a star, the shorter is its main-sequence lifetimeThe Sun has been a main-sequence star for about 4.56 billion years and should remain one for about another 7 billion yearsDuring a star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosityWhen core hydrogen fusion ceases, a main-sequence star becomes a red giant5Red Giants• Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star• This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star• The core shrinks and becomes hotter, while the star’s outer layers expand and cool• The result is a red giant star• When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core.• A process called the triple alpha process, converts helium to carbon and oxygen.After the helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branch• H-R diagrams and observations of star clustersreveal how red giants evolve• The age of a star cluster can be estimated by plotting its stars on an H-R diagramThe 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)6As a cluster ages, the main sequence peels away from the main sequence region as stars of progressively smaller mass evolve into red giantsPopulations (generations) of stars• Relatively young Population I stars are metal rich; ancient Population II stars are metal poor• The metals (heavy elements) in Population I stars were manufactured by thermonuclear reactions in an earlier generation of Population II stars, then ejected into space and incorporated into a later stellar generation • Some scientists now call the oldest of stars Population III starsVariable StarsWhen a star’s evolutionary track carries it through a region in the H-R diagram called the instability strip, the star becomes unstable and begins to pulsate• Cepheid variables are high-mass variable stars• RR Lyrae variables are lower-mass, metal-poor variable stars with short periods• Long-period variable stars also pulsate but in a fashion that is less well understood7There is a direct relationship between Cepheid periods of pulsation and their luminositiesMass transfer can affect the evolution of closebinary star systemsMass transfer in a close binary system occurs when one star in a close binary overflows its Roche lobeBinary StarsGas flowing from one star to the other passes across the inner Lagrangian point8This mass transfer can affect the evolutionary history of the stars that make up the binary systemPathways of Stellar EvolutionGOOD TO KNOW9Low-mass stars go through two distinctred-giant stages • A low-mass star becomes– a red giant when shell hydrogen fusion begins– a horizontal-branch star when core helium fusion begins– an asymptotic