chemical composition & ages cause scatter of main sequence. high mass= high luminosity **for main sequence stars only. upper right: red giant -- cool & luminous, high mass. <1% are giants, <.01% are supergiants. lower left: white dwarf -- dim & hot, low mass. high luminosity class = lower luminosity for given surface temp age of stars:main sequence = adult stars giants & supergiants = aged stars white dwarfs = dead stars.binary & multiple star systems.⅓ stars in sky. starts orbit about their center of mass. m1+m2 = a^3/p^2 * M(sun). spectroscopy binary: when stars are too close, their binary nature can be found from the change in the doppler shifts in stellar spectrum. mass of stats cannot be found from radial velocity alone unless we know the orbital inclination. eclipsing binary:representative light curves. interstellar medium:space between the stars is not complete empty, but filled with dilute gas & dust. dense interstellar birthplace of stars. dark clouds alter & absorb light from stars behind them. nebula: (latin for cloud) emission nebula: hot star excites gas near it to produce emission lines. dark nebula: dense clouds of gas & dust that block light from distant stars. reflection nebula: star light reflects off dust in a cool nebula. star illuminates a gas & dust cloud → star light is reflected by dust→ appears blue because blue light is scattered by larger angles than red light→ absorption lines from stars whose light is being reflected. birth of stars. giant, (icy) dusty gas clouds. gravitationally unstable, collapse triggered by shock waves produced by a nearby supernova explosion. cloud contraction. clouds are first in balance with gravity & pressure. part of cloud becomes compressed because of shock wave. gravity overcomes pressure & collapse begins. collapse forms dense cores in the cloud → protostar. formation of protostar→ star: matter continues to fall onto protostar from cloud → increases protostars mass, density, temp; forms a disk & a huge outflow→ hidden by dust of clouds → can be seen by infrared. collapse of molecular cloud collapse releases energy, heats cloud, grows in infrared → protostar. cloud fragments which can form a group of stars. higher mass protostars collapse rapidly because of stronger gravity. thousands of years of contraction & high surface temp, collapse is halted by thermal pressure → pre main sequence star. pre main seq star loses heat from radiation, thermal pressure drops, contraction, heats up again, radiation… repeat. eventually core temp is high enough to trigger thermonuclear fusion, equilibrium is achieved & main sequence star is born. hydrostatic equilibrium:star becomes stable when gravity & pressure are in balance.thermal equilibrium:rate at which fusions releases energy in a star’s core = rate at which a star’s surface radiates energy to space. low mass stars a more common than high mass. brown dwarfsare failed stars: mass = <.08 mass(sun), not have enough gravitation force to become pre-main sequence stars; orbs of hydrogen & helium. max mass of main sequence stars: 150-200 mass (sun)
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