Universe. Chapter 19. Stellar Evolution: On and After the Main Sequence 8/13/2015. By reading this chapter, you will learn

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1 Roger Freedman Robert Geller William Kaufmann III Universe Tenth Edition Chapter 19 Stellar Evolution: On and After the Main Sequence By reading this chapter, you will learn 19 1 How a main sequence star changes as it converts hydrogen to helium 19 2 What happens to a star when it runs out of hydrogen fuel 19 3 How aging stars can initiate a second stage of thermonuclear fusion 19 4 How H R diagrams for star clusters reveal the later stages in the evolution of stars 19 5 The two kinds of stellar populations and their significance 19 6 Why some aging stars pulsate and vary in luminosity 19 7 How stars in a binary system can evolve very differently from single, isolated stars 19 1: During a star s main sequence lifetime, it expands and becomes more luminous The Zero Age Sun and Today s Sun 1

2 Changes in the Sun s Chemical Composition Main Sequence Stars of Different Masses Main Sequence Stars of Different Masses 2

3 Main Sequence Stars of Different Masses Our Sun is Partially Convective A Fully Convective Red Dwarf 3

4 19 2: When core hydrogen fusion ceases, a mainsequence star like the Sun becomes a red giant A Mass Loss Star 4

5 Timeline ZAMS Main Sequence Stages in the Evolution of the Sun 19 3: Fusion of helium into carbon and oxygen begins at the center of a red giant Stages in the Evolution of a Star with More than 0.4 Solar Masses 5

6 Helium Fusion in a Red Giant 19 4: H R diagrams and observations of star clusters reveal how red giants evolve H R Diagrams of Stellar Evolution On and Off the Main Sequence 6

7 Mass/ MS Lifetime Line 0.085M sol 0.80M sol 1.5M sol ~3M sol Brown Dwarf Red Dwarf Yellow Dwarf Giants 12 Tyr 20Gyr 5Gyr 10Myr The Evolution of a Theoretical Cluster 7

8 The Evolution of a Theoretical Cluster The Evolution of a Theoretical Cluster The Evolution of a Theoretical Cluster 8

9 Theoretical Cluster Time lapse Brief Interlude for Terms: ZAMS Zero Age Main Sequence Turnoff point Open Cluster Globular Cluster Population I stars Population II stars Mass/Temperature Line 0.085M sol 0.80M sol 1.5M sol Brown Dwarf Red Dwarf Yellow Dwarf Giants 400K 3000K 6000K 10,000K 30,000K Coolest BD yet discovered, Jupiter masses! 9

10 Two Clusters: which is younger? Two Clusters 10

11 A Globular Cluster Age of a Globular Cluster An H R Diagram for Open Star Clusters 11

12 19 5: Stellar evolution has produced two distinct populations of stars Spectra of a Metal Poor Star and a Metal Rich Star Spectra of a Metal Poor Star and a Metal Rich Star Where did the metals come from? Where can you find these populations? Mixed? Separated? 19 6: Many mature stars pulsate Mira A Long Period Variable Star 12

13 Variable Stars on the H R Diagram Henrietta Leavitt and Cepheid Variables Another of Pickering s group a straight line can readily be drawn... showing that there is a simple relation between the brightness of the variables and their periods... Actually, all stars are variable The Sun varies 0.07% Analogy for Cepheid Variability 13

14 δ Cephei A Pulsating Star Close ups follow δ Cephei A Pulsating Star δ Cephei A Pulsating Star 14

15 δ Cephei A Pulsating Star δ Cephei A Pulsating Star Period Luminosity Relation for Cepheids 15

16 19 7: Mass transfer can affect the evolution of stars in a close binary system Close Binary Star Systems Close Binary Star Systems Close Binary Star Systems 16

17 Close Binary Star Systems Three Eclipsing Binaries Three Eclipsing Binaries 17

18 Key Ideas The Main Sequence Lifetime: The duration of a star s mainsequence lifetime depends on the amount of hydrogen available to be consumed in the star s core and the rate at which this hydrogen is consumed. The more massive a star, the shorter its main sequence lifetime. The Sun has been a main sequence star for about 4.56 billion years and should remain one for about another 7 billion years. Key Ideas During a star s main sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity. If a star s mass is greater than about 0.4 M, only the hydrogen present in the core can undergo thermonuclear fusion during the star s main sequence lifetime. If the star is a red dwarf with a mass less than about 0.4 M, over time convection brings all of the star s hydrogen to the core where it can undergo fusion. Key Ideas Becoming a Red Giant: Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a mainsequence star with mass greater than about 0.4 M. This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star. The core shrinks and becomes hotter, while the star s outer layers expand and cool. The result is a red giant star. As a star becomes a red giant, its evolutionary track moves rapidly from the main sequence to the red giant region of the H R diagram. The more massive the star, the more rapidly this evolution takes place. 18

19 Key Ideas Helium Fusion: When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core. This process, also called the triple alpha process, converts helium to carbon and oxygen. In a more massive red giant, helium fusion begins gradually; in a less massive red giant, it begins suddenly, in a process called the helium flash. After the helium flash, a low mass star moves quickly from the redgiant region of the H R diagram to the horizontal branch. Key Ideas Star Clusters and Stellar Populations: The age of a star cluster can be estimated by plotting its stars on an H R diagram. The cluster s age is equal to the age of the main sequence stars at the turnoff point (the upper end of the remaining main sequence). Key Ideas As a cluster ages, the main sequence is eaten away from the upper left as stars of progressively smaller mass evolve into red giants. Relatively young Population I stars are metal rich; ancient Population II stars are metal poor. The metals (heavy elements) in Population I stars were manufactured by thermonuclear reactions in an earlier generation of Population II stars, then ejected into space and incorporated into a later stellar generation. 19

20 Key Ideas Pulsating Variable Stars: When a star s evolutionary track carries it through a region in the H R diagram called the instability strip, the star becomes unstable and begins to pulsate. Cepheid variables are high mass pulsating variables. There is a direct relationship between their periods of pulsation and their luminosities. RR Lyrae variables are low mass, metal poor pulsating variables with short periods. Long period variable stars also pulsate but in a fashion that is less well understood. Key Ideas Close Binary Systems: Mass transfer in a close binary system occurs when one star in a close binary overflows its Roche lobe. Gas flowing from one star to the other passes across the inner Lagrangian point. This mass transfer can affect the evolutionary history of the stars that make up the binary system. 20