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summary_ch12.pdf

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Department
Astronomy
Course
AST201H5
Professor
J B Lester
Semester
Fall

Description
AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 12 Summary th Essential Cosmic Evolution, 5 Edition 1. Star birth - continuing today • There are very massive (thousands of M sun), very cold (10 - 30 K) clouds of interstellar gas/dust • Clouds contract and fragment into star-size masses (0.1 - 100 M sun • Fragmentation produces a few large masses, numerous medium masses and huge numbers of low masses • Gas heats from released stored gravitational energy • Gas becomes opaque, trapping the heat → protostar • Core gets hot enough for fusion to begin • Masses > 100 M sun create too much energy → disintegrate • Masses < 0.1 M suncannot create the conditions for fusion • Stars > 8 M sun - “high-mass” stars, and star < 8 M sun- “low-mass” stars live out their lives in significantly different ways • Star birth is violent as the star sheds angular momentum → jets and powerful stellar winds • About 50% of protostars split into double stars to share the angular momentum 2. Life of a low-mass star (the vast majority of stars) • Create energy by fusion of 4( H) → He - same as the Sun • Rate of fusion decreases going from the centre of the core to the boundary of the core because of decreasing pressure and temperature • Energy leaves the core as radiation • When the centre of the core has converted all of its H to He, the outer core still has plenty of H remaining → gradual adjustment • Centre pressure < gravity → shrinks gradually • Surface gradually expands → becomes red giant star 4 12 • Centre conditions become extreme enough to fuse 3( He) → C • Core expands, surface shrinks → “horizontal branch” star continued on page 2 AST201H5S – Chapter 12 Summary
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