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Red Stars, Blue Stars, Old Stars, New Stars Session 2 Julie Lutz University of Washington Polaris Stars Appear to Revolve Around Polaris Special Things • Polaris is a multiple star system • “I am constant as the Northern Star”-Shakespeare • Nope, Polaris A is a pulsating supergiant. Summary of Session 1 • Most stars have temps between 3000 K and 30,000 K. • Stars have wide range in luminosity. Some are 10s of 1000s of times more luminous than sun; others much less luminous. • Masses range from 0.07 to 120 times mass of sun • Diameters planet-sized to 100s x sun Energy Source? • Why does the sun shine? • Why has it been shining for the last (approximately) 4.5 billion years? (radioactive dating of meteorites, moon rocks) How did astronomers figure out the patterns of stellar evolution? • HR Diagram • Thermonuclear reactions • Star masses and sizes • Star clusters • Nebulae--gas and dust Why these differences? How do stars form and do they change? • Where to go next for info? • Look at large groups of gravitationally associated stars • Star clusters • Stars formed at approx same time Characteristics of Open Star Clusters • Irregular in shape • Few 10s to few 1000s of stars • Associated with spiral arms of Milky Way • Sometimes seen with nebulosity Open Star Clusters HR Diagram of Pleiades Open Cluster NGC 188 HR Diagram NGC 188 Globular Clusters • Spherical shape • Spherical distribution • Many stars--estimate 50,000-200,000 HR Diagram of a Globular Cluster So the study of star clusters, plus masses, energy sources, etc. • Leads us to the conclusion that mass is a very important factor in stellar evolution • The more massive a star is, the faster it will evolve (due to faster thermonuclear reaction rates in core) • Main sequence lifetime of sun is 12 billion years; 50xsun only about 50,000 years But Lots of Questions Remain! • We’ve been looking at already formed stars-stable balance between energy released from thermonuclear reactions pushing outward balancing gravity pushing inward. • Next question: How do stars form and get to the point of starting thermonuclear reactions? Where and How Do (Did) Stars Form? • Look for places where there are young stars (perhaps massive stars that don’t last very long on the main sequence. • Think about what material stars formed from. Four Massive Stars Provide Energy to Make the Nebula Glow Young Star Clusters are Found in Many Clouds of Gas and Dust Dark Patches in Nebulae-Globules • Nebulae are composed of gas (atoms and molecules) and dust. • Globules are places where there is a concentration of dust. • Can’t see into these dust regions in optical. Space Telescopes Helped • Hubble Space Telescope: optical and ultraviolet • “Pillars of Creation” image from HST 1995 • Sites of star formation Spitzer Space Telescope • Looks at the infrared with cameras and spectrographs. • Infrared radiation is “heat: can see through the dust much better than in optical Star Formation • Solar system formation provides inspiration Factors in Star Formation • Stars form in “molecular clouds”, a variety of nebulae • Gravitational collapse plays a role, as does a triggering mechanism like a supernova, magnetic fields and angular momentum Molecular Cloud • Lots of molecules • Simple--CN, CO, molecular hydrogen, water • Complex--alcohols, formaldehyde, amino acids • Dust--graphite, silicates, olivine Supernova • Explosion of massive star releases huge amounts of energy, produces shocks • Rare: About 1 SN per 100 years in galaxy like ours. • Crab Nebula (1054 AD)--X-ray image Magnetic Fields • Blue spots are x-rays from protostars explained by acceleration of gas in magnetic fields • Weak magnetic fields in molecular clouds are “concentrated” during collapse. Star Formation Phase • During first part of collapse process, protostar is way too cool to be put on HR diagram. • Eventually it gets hot enough to show up in optical. Star Formation--How Long Does It Take? • Depends on mass • More massive stars collapse the fastest (1000s of years from start to MS) • Less massive (100,000s years from start to MS) Star Formation Phase Ends on Main Sequence • Eventually conditions in the stellar interior become hot and dense enough that thermonuclear reactions start. • They counteract the inward push of gravity. Summary of Session • Star formation occurs in clouds of interstellar gas and dust. • Mechanisms involved include gravitational collapse, shocks, magnetic fields, molecular chemistry, conservation of angular momentum. • Formation phase ends when star arrives on main sequence (hydrogen to helium fusion).