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Star Formation Life begins in a dense region of gas and dust Molecular Clouds In these nebulae much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds. The largest such formations are called giant molecular clouds (GMC). GMCs typically have diameters of 100 light-years, masses of up to 6 million solar masses, and an average interior temperature of 10 K. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula, 1,300 ly away. Orion Nebula Eagle Nebula GMC Collapse GMCs can begin to collapse, where denser areas begin to gravitationally attract more gas and dust. The collapse can also be triggered: - Supernova shock wave - Collision of galaxies The collapse continues until the temperature and pressure climb high enough to stop the collapse. The central core of this collapse become a “protostar”. Stars form in clusters of 100 to 1000. The Interstellar Medium • Today stars form in a complex environment • Hydrogen and helium are the predominant components of the ISM, but it is enriched with heavier elements from earlier stars (created in stellar fusion and supernova explosions). • Stellar formation also must deal with the presence of energetic stars, magnetic fields and supernovae explosions. Protostar Becomes A Star • Once sufficient temperature (>= 10 million degrees K) and pressure is reached in the core of the protostar, nuclear fusion begins and the protostar has now officially become a star • 2/3 of the stars form in multiple star systems • Stars range from 0.08 to ~100 solar masses • <0.08 solar mass = brown dwarf Radiation from hot, young stars has blown gas away from the center of the Rosetta Nebula Hertzsprung-Russell Diagrams • H-R diagrams are graphs that plot the luminosity (i.e. the total energy output of a star in watts) versus surface temperature •When looking at a group of stars, the data points do not fall randomly. Main-Sequence Lifetimes Future of Our Sun Lifetime of a Star Depends on its Mass • Mass is essentially the sole determining factor in how long a star lives •The more massive a star, the greater the temp and press in its core and the greater the rate of hydrogen fusion •25 solar mass star: 3 million year lifetime •1 solar mass star: 10 billion year lifetime •0.5 solar mass star: 200 billion year lifetime Post MainSequence Evolution The Instability Strip Period-Luminosity Relation of Cepheid Variable Stars Cepheid Variables Help Measure Large Distances • Cepheid variable stars have a well defined relationship between their energy output (i.e. luminosity) and their period of getting brighter, then dimmer, then brighter again. •Cepheid variable are very bright (102 – 104 solar luminosity), so can be seen from a long distance Thank you! You’ve been a stellar audience! The End