Universe Tenth Edition Chapter 19 Stellar Evolution: On and After the Main Sequence Roger Freedman Robert Geller William Kaufmann III

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 By reading this chapter, you will learn

Changes in the Sun’s Chemical Composition

The Zero-Age Sun and Today’s Sun 19-1: During a star’s main-sequence lifetime, it expands and becomes more luminous

A Fully Convective Red Dwarf

19-2: When core hydrogen fusion ceases, a main- sequence star like the Sun becomes a red giant

A Mass-Loss Star

Stages in the Evolution of a Star with More than 0.4 Solar Masses 19-3: Fusion of helium into carbon and oxygen begins at the center of a red giant

Helium Fusion in a Red Giant

Degenerate Electrons

Stages in the Evolution of the Sun

H-R Diagrams of Stellar Evolution On and Off the Main Sequence 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

The Evolution of a Theoretical Cluster

Two Open Clusters

A Globular Cluster

Age of a Globular Cluster

An H-R Diagram for Open Star Clusters

Spectra of a Metal-Poor Star and a Metal-Rich Star 19-5: Stellar evolution has produced two distinct populations of stars

Spectra of a Metal-Poor Star and a Metal-Rich Star

Mira – A Long-Period Variable Star 19-6: Many mature stars pulsate

Variable Stars on the H-R Diagram

δ-Cephei – A Pulsating Star

Analogy for Cepheid Variability

Period-Luminosity Relation for Cepheids

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

Close Binary Star Systems

Three Eclipsing Binaries

Key Ideas The Main-Sequence Lifetime: The duration of a star ’ s main- sequence 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 main- sequence 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.

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 red- giant 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.

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.