Ch 11--Life Cycle of Stars ASTR103, GMU, Dr. Correll
What do you think? • How do stars form? • Are stars forming today? • Do stars with greater or lesser mass shine longer ASTR103, GMU, Dr. Correll
Stellar Evolution • Birth • Evolution to main sequence line (youth) • Main sequence (adulthood) • Maturation off the main sequence towards giant stars (retirement) • Death of Stars ASTR103, GMU, Dr. Correll
Origins of Star Formation • Radio telescopes (and optical) reveal interstellar medium--giant clouds of gas and dust lying between existing stars • By number: 90% Hydrogen, 9% Helium, less than 1% other • By mass: 74% H, 25% He, 1% other • Other includes, heavier elements, molecules (H2, CO, H2O, NH3, H2CO, etc) and dust • about 1 hydrogen atom per cubic centimeter of space ASTR103, GMU, Dr. Correll
Interstellar Medium (cont.) • Visible light efficiently blocked and scattered by gas and dust, limiting observing range • Pleides appear blue due to preferential scattering of blue light from thick dust in surrounding IM • Radio waves from CO molecules (giant molecular clouds, ~1000 hydrogen atoms per cubic cm) travel further through gas and dust, allowing us to observe IM to great distance ASTR103, GMU, Dr. Correll
The Horsehead Nebula ASTR103, GMU, Dr. Correll
Star Formation • Shock wave (from supernova or colliding gas clouds) causes a local region of the IM to begin gravitational collapse • If nebula too hot, gas pressure prevents further collapse (or must wait until nebula cools) • If nebula cool enough, Jeans instability allows gravity to overtake thermal energy ASTR103, GMU, Dr. Correll
Protostars • The collapsing gas and dust form a sphere • As the sphere accretes mass, and collapses the temperature raises, stalling the collapse • At this stage the sphere, now a protostar, can radiate very much thermal energy • As the protostar radiates away energy, it gradually becomes more compact with a higher central temperature and pressure • Eventually, nuclear fusion (hydrogen burning) can begin ASTR103, GMU, Dr. Correll
Pre-Main-Sequence Stars • After about 100,000 years of accretion and collapse, nuclear fusion begins--the protostar becomes a pre-main-sequence star • PMS Stars evolve toward Main Sequence • Collapse continues somewhat • Stellar structure stabilizes • Nebular cloud push away by stellar wind • Rate of evolution depends on mass--higher mass, faster evolution! ASTR103, GMU, Dr. Correll
Masses of Stars • Upper limit--about 100 MSun • Lower limit--about 0.08 MSun (brown dwarves) ASTR103, GMU, Dr. Correll
Emission Nebula • The gas in star forming regions typically glows from the hot, newborn stars • Hydrogen in nebula ionized (HII)by UV light from the stars • Occasional capture of an electron by H II causes emission of pinkish light • These areas are referred to as emission nebula ASTR103, GMU, Dr. Correll
Star Cluster Formation ASTR103, GMU, Dr. Correll
Star Clusters on an H-R Diagram • Plotting the stars in a cluster on an H-R diagram tell us how old a cluster is • massive stars reach the main sequence first • lower mass stars take 10s, 100s, 1000s of millions of years to reach the main sequence phase ASTR103, GMU, Dr. Correll
Main Sequence Stars • Zero Age Main Sequence (ZAMS) is where stars have reached an equilibrium configuration and begin their lives on the main sequence ASTR103, GMU, Dr. Correll
Giant Stars • Hydrogen burning leaves a central core of inert Helium • When hydrogen burning stops (due to insufficient temperature and pressure) then • Core of star collapses • Helium fusion may begin--slowly, or in a helium flash for massive stars • Outer layers of star expand • Thus surface of star becomes cooler but MUCH larger • Giant stars are formed ASTR103, GMU, Dr. Correll
Evolution Off the Main Sequence • As stars enter giant phase, their size increases greatly, their surface temperature drops slightly, thus their luminosities migrate upwards ASTR103, GMU, Dr. Correll
Globular Clusters • An H-R census of the Globular cluster stars reveals the age of the cluster • since the globular cluster stars are gravitationally bound close together, they are the same distance from us • use apparent magnitude • Youngest clusters are metal rich • Population I stars (such as our Sun) • Oldest clusters are metal poor • Population II stars ASTR103, GMU, Dr. Correll
Variable Stars • As stars leave the main sequence, their structure and luminosity becomes unstable and variable • but often in predictable ways--Cepheid variables ASTR103, GMU, Dr. Correll
Cepheid Variables • The fixed relationship between period and luminosity allows Cepheid variable stars to be used as distance candles! ASTR103, GMU, Dr. Correll
Close Binary Stars • Close binary stars can exchange mass, greatly altering the evolution process for the stars! ASTR103, GMU, Dr. Correll
What do you think? • How do stars form? • Stars form from gas and dust inside giant molecular clouds • Are stars forming today? • Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars • Do stars with greater or lesser mass shine longer? • More massive stars live shorter lives because the gravitational force creates higher temperatures and greater pressures which accelerate the pace of nuclear fusion in their cores ASTR103, GMU, Dr. Correll
Questions for Thought • Describe the formation of stars within a cluster and explain a method for estimating the age of the cluster. ASTR103, GMU, Dr. Correll
