Understanding Convection in Stars' Interiors

1. Units to cover: 62, 63, 64

2. Homework: Unit 60: Problems 12, 16, 18, 19 Unit 61 Problems 11, 12, 17, 18, 20 Unit 62 Problems 17, 18, 19 Unit 63, Problems 17, 19

3. Convection occurs in the interiors of stars whenever energy transport away from the core becomes too slow Radiation carries away energy in regions where the photons are not readily absorbed by stellar gas Close to the cores of massive stars, there is enough material to impede the flow of energy through radiation In less massive stars like the Sun, cooler upper layers of the Sun’s interior absorb radiation, so convection kicks in The lowest-mass stars are fully convective, and are well mixed in the interior. Internal Structure of Stars - Convection

4. The Main-Sequence Lifetime of a Star • The length of time a star spends fusing hydrogen into helium is called its main sequence lifetime • Stars spend most of their lives on the main sequence • Lifetime depends on the star’s mass and luminosity • More luminous stars burn their energy more rapidly than less luminous stars. • High-mass stars are more luminous than low-mass stars • High mass stars are therefore shorter-lived! • Cooler, smaller red stars have been around for a long time • Hot, blue stars are relatively young.

5. Two Young Star Clusters How do we know these clusters are young?

6. Stellar Evolution on the Main Sequence

7. A Reminder of a Star’s Internal Processes • The balance of forces in the interior of a star is delicate, though stable for millions or billions of years. • A star acts like it has a thermostat • If internal temperature decreases, internal pressure decreases, and the star collapses a little, raising the temperature • When hydrogen in the core is exhausted, the thermostat breaks…

8. Evolution to red giant phase • The star is expanding and cooling, so its luminosity increases while its temperature decreases • Position on the HR diagram shifts up and to the right…

9. Evolutionary tracks of giant stars

10. CNO cycle happens A. In protostars as they are not hot enough B. In the stars similar to our Sun C. In high mass stars with very hot core D. In fully convective low mass stars

11. When a star leaves the main sequence and expands towards the red giant region, what is happening inside the star? • a. Hydrogen burning is taking place in a spherical shell just outside the core; the core itself is almost pure helium. • b. Helium is being converted into carbon and oxygen in the core. • c. Helium burning is taking place in a spherical shell just outside the core. • d. hydrogen burning is taking place in a spherical shell, while the core has not yet started thermonuclear reactions and still mostly hydrogen.

12. Normally, the core of a star is not hot enough to fuse helium Electrostatic repulsion of the two charged nuclei keeps them apart The core of a red giant star is very dense, and can get to very high temperatures If the temperature is high enough, helium fuses into Beryllium, and then fuses with another helium nuclei to form carbon. Helium Fusion

13. A (temporary) new lease on life • The triple-alpha process provides a new energy source for giant stars • Their temperatures increase temporarily, until the helium runs out • The stars cool, and expand once again • The end is near…

14. Light Curves • To characterize the variability of a star, scientists measure the brightness, and plot it as a function of time. • Light Curves • Different kinds of variability • Irregular Variable • Novae (death) • T Tauri stars (birth) • Pulsating Variable • Periodic changes in brightness

15. Yellow Giants and Pulsating Stars • If you plot the positions of variable stars on the HR diagram, many of them fall in the “instability strip” • Most have surface temperatures of ~5000K, so appear yellow • Most are giants (Yellow Giants) • Instability comes from partial absorption of radiation in the interior of the star • Helium absorbs radiation, and the outer layers of the star get pushed away from core • As the star expands, the density decreases, letting photons escape • Outer layers head back inward toward core • Repeat • RR Lyrae and Cepheid variables are useful for finding distances to the stars, as the star’s period is proportional to its luminosity.

16. The Valve Mechanism

17. A Cepheid variable is • a. a low mass red giant that varies in size and brightness in an irregular way • b. a big planet • c. a high-mass giant or supergiant star that pulsates regularly in size and brightness • d. a variable emission nebula near a young star

18. The Life-path of the Sun

19. As a red giant expands, it cools Outer layers cool enough for carbon flakes to form Flakes are pushed outward by radiation pressure Flakes drag stellar gas outward with them This drag creates a high-speed stellar wind! Flakes and gas form a planetary nebula Formation of Planetary Nebula

20. The Hourglass Nebula

21. White Dwarf Stars • At the center of the planetary nebula lies the core of the star, a white dwarf • Degenerate material • Incredibly dense • Initially the surface temperature is around 25,000 K • Cools slowly, until it fades from sight.

22. Figure 64.05e

23. Our Sun will end its life by becoming • A. a molecular cloud • B. a pulsar • C. a white dwarf • D. a black hole

24. A Roche lobe can be seen as a sphere of gravitational influence around a star Red Giant stars can fill their Roche lobes In a binary star system, the Roche lobes of the two stars can touch, and mass can pass between them. If a white dwarf is in orbit around a red giant companion star, it can pull material off the companion and into an accretion disk around itself Material in the accretion disk eventually falls to the surface of the white dwarf Mass Transfer and Novae

25. Novae • If enough material accumulates on the white dwarf’s surface, fusion can be triggered, causing a massive explosion • This explosion is called a nova • If this process happens repeatedly, we have a recurrent nova.