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Astronomy 114 Lecture 17: How Stars Form Martin D. Weinberg [email protected] UMass/Astronomy Department A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—1/17 Announcements Quiz #1 redux due today PS #4 due on Friday A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—2/17 Announcements Quiz #1 redux due today PS #4 due on Friday Today: How stars form, role of Angular Momentum The Birth of Stars, Chap. 20 Stellar Evolution, Chap. 21 A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—2/17 HR diagrams of star clusters A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—3/17 HR diagrams of star clusters A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—3/17 Stellar Evolution: preview Stars are not permanent Thermonuclear reactions (fusion) that energize the star use up the hydrogen A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—4/17 Stellar Evolution: preview Stars are not permanent Thermonuclear reactions (fusion) that energize the star use up the hydrogen Life time of a star: Proportional to the mass Inversely proportional to the luminosity A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—4/17 Stellar Evolution: preview Life time of a star: Proportional to the mass Inversely proportional to the luminosity A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—4/17 Birth of stars Interstellar gas and dust are common in the disk of the Galaxy Interstellar medium (ISM) Diffuse gas (atomic H and molecular hydrogen H2 ) Dust (formed in the envelopes of stars) Clouds of denser than average accumulations of dust and gas are seen as nebula A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—5/17 Type of nebula Emission nebula Reflection nebula Dark nebula A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—6/17 Interstellar reddening and extinction (1/4) Dust grains preferentially absorb and scatter short wavelengths (blue) Light appears to “redden” A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—7/17 Interstellar reddening and extinction (2/4) Left cluster is more distant More dust between us and this cluster Emission is reddened A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—8/17 Interstellar reddening and extinction (3/4) 99% gas 74% hydrogen, 25% helium 1% dust Density: approx 1 atom per cubic cm Density of air: 3 × 1019 molecules per cubic cm A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—9/17 Interstellar reddening and extinction (4/4) Blue wavelengths from Sun are preferentially scattered but molecules, dust and turbulence Blue sky! A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—10/17 Interstellar reddening and extinction (4/4) Blue wavelengths from Sun are preferentially scattered but molecules, dust and turbulence Blue sky! Red sunset! A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—10/17 The Galaxy in the Infrared (1/2) A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—11/17 The Galaxy in the Infrared (1/2) A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—11/17 The Galaxy in the Infrared (2/2) A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—12/17 Protostars Basic steps in forming a star: 1. Fragmentation of a giant molecular cloud into clumps 2. Collapse of the clumps due to their own gravity (free-fall) 3. Contraction of the star through conversion of the gravitational energy through slow falling into heat (Kelvin-Helmholtz contraction) 4. Fusion reaction begins in center (ignition of core Hydrogen burning) A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—13/17 What causes the collapse? (1/4) 1. Accretion Clumps collide New mass overcomes internal pressure A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—14/17 What causes the collapse? (1/4) 1. Accretion Clumps collide New mass overcomes internal pressure 2. Gravity and radiation pressure Pressure of star light and gravity overcomes internal pressure A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—14/17 What causes the collapse? (2/4) 3. Supernova blast wave Exploding star compresses cloud, increasing density Gravity overcomes pressure A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—15/17 What causes the collapse? (3/4) Shell from supernova explosion plowing into ISM A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—16/17 What causes the collapse? (4/4) Collapsed clumps of dense gas in the Carina nebula A114: Lecture 17—14 Mar 2004 Read: Ch. 20, 21 Astronomy 114—17/17