1. 4 August 2005AST 2010: Chapter 211 Stars: From Adolescence to Old Age

2. 4 August 2005AST 2010: Chapter 212 Mass Determines Life Stages The mass of a star determines the stages it goes through and how long it lasts in each stage Massive stars evolve faster than small stars Higher mass requires higher pressure to balance it, so that hydrostatic equilibrium is maintained Higher pressure in turn is produced by higher temperature The higher the temperature inside a star, the faster it uses up its hydrogen fuel Although massive stars have more fuel, they burn it so quickly that their lifetimes are much shorter than those of low-mass stars This also explains why the most massive main-sequence stars are the most luminous

3. 4 August 2005AST 2010: Chapter 213 Lifetime of Main-Sequence Stars

4. 4 August 2005AST 2010: Chapter 214 Stellar Collapse On the main sequence, a star’s inward gravity is balanced by the outward pressure The pressure is due to the nuclear fusion in the core All the hydrogen in the core eventually gets used up in the fusion into helium In other words, the fusion of hydrogen inside the core eventually stops The core now contains only helium The star is no longer on the main-sequence Then gravity takes over and the (helium) core shrinks The energy of the inward-falling material in the core is converted to heat The heat flows outward, raising the temperature of the hydrogen just outside the core

5. 4 August 2005AST 2010: Chapter 215 Shell Burning The shell layer outside the core becomes hot enough for hydrogen fusion to start This fusion in the layer just outside the core is called shell burning The helium core continues to contract, creating more heat in the shell around it, leading to more fusion This causes the star’s luminosity to increase beyond its main sequence value With all the new energy pouring outward, the star’s size expands significantly This causes the star’s outer layers to cool down Thus the star becomes a red giant Its surface gets redder as it gets cooler It is very luminous because of its huge surface area and increased luminosity

6. 4 August 2005AST 2010: Chapter 216 Comparing the sizes of the Sun, the giant star Delta Bo Ö tis (orange sphere), and the supergiant Xi Cygni (red sphere)

7. 4 August 2005AST 2010: Chapter 217 Comparing the Sun to a Supergiant

8. 4 August 2005AST 2010: Chapter 218 End of Life on Earth … When the Sun becomes a red giant, it will swallow Mercury, Venus, and perhaps the Earth, too Or conditions on the Earth’s surface will become impossible for life to exist

9. 4 August 2005AST 2010: Chapter 219 Time to Reach Giant Stage Theoretical calculations suggest that the time for a main-sequence star to reach the giant stage is short for a high-mass star as low as 10 million (=10 7 ) years long for a low-mass star up to 10 billion (=10 10 ) years

10. 4 August 2005AST 2010: Chapter 2110 Characteristics of Star Clusters We saw that stars tend to form in clusters The stars in a cluster have different masses but about the same age The different stars in a cluster provide a test for theories of stellar evolution Three types of clusters are globular clusters, containing very old stars open clusters, containing young to middle-aged stars stellar associations, containing very young stars

11. 4 August 2005AST 2010: Chapter 2111 Testing Theory: Relatively Young Stars A comparison of the prediction for the stars of a hypothetical 3-million-year-old cluster (left) with measurements of the stars in cluster NGC 2264 (right) The theory is roughly consistent with observation

12. 4 August 2005AST 2010: Chapter 2112 Testing Theory: An Older Cluster A comparison of the prediction for a hypothetical 4.24-billion-year-old cluster (left) with measurements of stars in the globular cluster 47 Tucanae (right) The theory appears to be roughly consistent with observation

13. 4 August 2005AST 2010: Chapter 2113 Further Aging: Helium Fusion As a star becomes a red giant, its (helium) core continues to shrink, causing its temperature to continue increasing When the core temperature reaches 100 million K, the helium nuclei can fuse to form carbon nuclei through a process called the triple-alpha process In this reaction, three helium nuclei are fused into a single carbon nucleus As the triple-alpha process begins, the entire core is ignited in a quick burst called the helium flash After this, the star becomes stable, its surface temperature increases, and its luminosity and size decreases At this stage, carbon nuclei sometimes fuse with helium nuclei to form oxygen nuclei But this new period of stability does not last very long As the helium is quickly used up in the fusion into carbon and oxygen, gravity will once more take over The situation is analogous to the end of the main sequence

14. 4 August 2005AST 2010: Chapter 2114 Near Death of Stars Like the Sun Then the star becomes a red giant again, but only briefly The core now contains only carbon and oxygen At this stage, a star similar in mass to the Sun has exhausted its inner resources and will soon begin to die The star’s luminosity may pulsate for a time due to its pressure and gravity being out of sync

15. 4 August 2005AST 2010: Chapter 2115 Planetary Nebulae When stars become giants, they begin to shed their outer layers exposing hot inner layers losing a substantial fraction of their mass into space The shells of gas ejected by such stars are called planetary nebulae They looked like planets in early telescopes, but have nothing to do with planets The nebulae are glowing because the gas is heated by the ultraviolet radiation of the dying central stars

16. 4 August 2005AST 2010: Chapter 2116 Images of Planetary Nebulae

17. 4 August 2005AST 2010: Chapter 2117 Dying Process of Massive Stars At the end of the helium-fusion stage, a star with a mass greater than about 8 solar masses has not yet exhausted its inner resources Such a star is massive enough to cause more contraction that can trigger other kinds of fusion in its center Carbon can fuse into still more oxygen as well as neon, sodium, magnesium, and finally silicon After each of the possible sources of nuclear fuel is exhausted, the star contracts until it reaches a temperature high enough to lead to the fusion of still heavier nuclei Depending on the star’s mass, this continues until the star has used up all of its energy supplies

18. 4 August 2005AST 2010: Chapter 2118 Creation of Chemical Elements The creation of heavier elements from lighter ones by nuclear fusion is called nucleosynthesis Theoretical calculations suggest that all elements up to iron can be built up by nucleosynthesis Stars like our Sun produce elements up to carbon and oxygen Very massive stars can produce elements up to iron Elements heavier than iron are believed to be produced in the supernova explosions of very massive stars