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Chapter 11 The Lives of Stars What do you think? • Where do stars come from? • Do stars with greater or lesser mass last longer? Let’s consider the starforming regions around Orion Stars form out of enormous volumes of dust and gas • Interstellar medium – H2 (mostly), CO, H2O, NH3, H2CO – Most is concentrated in giant molecular clouds Supernova explosions in cold, dark nebulae trigger the birth of stars. Stars form in large groups called “open clusters” or “galactic clusters” When a protostar ceases to accumulate mass, it, becomes a premain-sequence star. It’s life path is forever determined by its initial mass H II regions harbor young star clusters An OB association is where O and B class stars are producing ionizing radiation which makes an HII nebula glow. Star formation and glowing HII regions in the Great Orion Nebula Plotting all the stars from a cluster on an H-R diagram reveals its age Plotting all the stars from a cluster on an H-R diagram reveals its age Stars spend most of their life cycle on the main sequence • Main sequence stars are in hydrostatic equilibrium – outward thermal pressure is exactly balanced by the inward force of gravity • Main sequence stars are those stars fusing hydrogen into helium in their cores • Zero-age main sequence (ZAMS) is the location where a pre-main-sequence star fusing hydrogen in its core first becomes a stable object The more massive a star, the faster it goes through its main sequence phase When core hydrogen fusion ceases, a main-sequence star becomes a giant • When hydrogen in the core is no longer fusing into helium, the star can no longer support its weight • The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion. • This extra internal heat causes the outer layers to expand into a giant star. Helium fusion begins at the center of a giant • While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million kelvins) for helium to begin to fuse into carbon and oxygen – core helium fusion – 3 He C + energy and C + He O + energy – occurs rapidly - called the Helium Flash Some Laws of Physics are important here • Pauli exclusion principal – two identical particles cannot exist in the same place at the same time – this effect in stars is called electron degeneracy pressure and is not dependent on temperature – the star is supported by the fact that the electrons cannot get any closer together As stars evolve, they move on the H-R diagram their exact track depends on their initial mass Globular clusters are bound groups of hundreds of thousands of old stars at the edge of the galaxy A composite HR Diagram showing various star clusters Variable Stars • Change brightness because their diameter is fluctuating – (big/bright to small/dim and back again) • RR Lyrae variables (periods less than 24 hours) • Cepheid variables (periods between 1 & 100 days) • Mira variables (periods greater than 100 days) Cepheids enable astronomers to estimate vast distances • This period-luminosity relationship is important because if an astronomer can find a Cepheid and measure its period, she can determine its luminosity and absolute magnitude. • Comparing the absolute and apparent magnitudes allows for the distance to be calculated. What did you think? • Where do stars come from? Stars form from gas and dust inside giant molecular clouds • Do stars with greater or lesser mass last longer? Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars. Self-Check 1: Describe the physical properties and visual appearances of objects associated with pre-main-sequence stellar evolution. 2: Identify the defining characteristic of main-sequence stars and compare the relative lifetimes on the main sequence for stars of different mass. 3: List the names of nuclear fusion reactions and indicate the classes of stars in which each reaction is thought to be active. 4: Identify the physical property normally thought to control the life cycles of stars and planets. 5: Explain how observations of open and globular star clusters contribute to the testing and extension of current theoretical models for stellar evolution. 6: Identify the stages of stellar evolution in which mass loss is significant. 8: Compare and contrast RR Lyrae and Cepheid variable stars in terms of period, population membership, luminosity, and evolutionary status. 9: Describe how the identification of Cepheid variables can be used to determine the distance to a star cluster.