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Download ASTR2050 Spring 2005 • In this class we will cover: Brief review
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ASTR2050 Spring 2005 Lecture 10am 1 March 2005 Please turn in your homework now! In this class we will cover: • The Main Sequence: Brief review • Stellar evolution off the Main Sequence • Cepheid variable stars: Measuring distance 1 The Main Sequence: Brief review 40M! 1M! 2 0.2M! Properties of the Main Sequence by mass M • Position determined 3 • Luminosity L∝M (approximately) 2 M/L∝1/M • Mass is fuel, so1 lifetime 4 • Power from 4 H→ He + energy 2 4 Model: Central pressure P ≈GM /R • C Ideal Gas: Central Temp T =mP /ρk C C • 3 Example: Model calculation for The Sun Central pressure is about 1011 atmospheres Central temperature is about 15 Million K Most of the radiation is generated inside 20% of the solar radius Compare Kutner Fig.9.11 and associated discussion 4 Evolution off the Main Sequence First: “Low mass” stars (M < 5M!) Hydrogen in core is used up. Temperature in core is not high enough to burn helium. Hydrogen burns in “shell”. Helium core is “degenerate”. H burning He core is very dense H envelope 5 Red giant phase for “low mass” stars Helium “flash” occurs when core contracts enough. H burning He burning H envelope 6 Different behavior for “high mass” stars (M > 10M!) Helium can burn before becoming degenerate Core continues to shrink, gets hotter, and burns heavier and heavier elements. Eventually iron (Fe) reached and it can burn no more... 7 Example: A Globular Cluster Very old, so none of the high mass stars are left Note: Brighest stars are “red”8 Cepheid Variable Stars Some relatively massive stars evolve through the “instability strip”: • Type I (classical) cepheids • Type II (W Vir) cepheids • RR Lyrae stars See Kutner Fig.10.4 9 Interlude: Naming stars Ordinary stars Greek letter (in order of brightness) then constellation e.g. α-Orionis is brightest star in Orion (aka Betelgeuse) δ-Cephei is fourth brightest star in Cepheus Variable stars Listed in order of discovery, starting with “R”, then “S” and on through “Z”, then “RR..RZ...SS...SZ...ZZ”, and then “AA...AZ...” (but no J’s) ending at “...QZ” which is a total of 334 variable star names. After that, “V335...” e.g. W-Virginis, RR-Lyrae, etc... 10 Cepheid Pulsation The star oscillates in both size and in temperature. It is most bright at high temperature, whichis near minimum radius. Short period 11 Long period Cepheid Mechanism: The + He valve Surface of the star ... and then falls back in. Helium zone traps radiation Radiation pushes out, but the star’s surface overshoots equilibrium... Temperature falls and helium zone breaks 12 Period-Luminosity Relation Depth of the helium zone is very sensitive to mass This is an interesting physical phenomenon, but it also makes cepheids a very useful tool! 13 Measuring distance The Period tells you the Absolute Magnitude! You will see this first hand in studio lab this Friday. You will use cepheids to find the distance to galaxy M81. 14