Units to cover

1. Units to cover • 65, 66, 67

2. Evolutionary tracks of giant stars

3. 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

4. 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.

5. 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

6. 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…

7. 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

8. 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.

9. The Valve Mechanism

10. 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

11. Cepheid and RR Lyrae Variables The Period-Luminosity Relation

12. Periods of Variable Stars

13. In terms of nuclear reactions, what is the next stage of a star's life after the end of hydrogen burning in the core? a. Hydrogen burning in a thin shell around the core b. Helium burning in the core c. carbon burning d. death

14. What makes a red giant star so large? a. The star has many times more mass than the Sun. b. The helium-rich core expanded, pushing the outer layers of the star outward. c. Red giants are rapid rotators, and the centrifugal forces pushes the surface of the star outwards. d. The hydrogen-burning shell is heating the envelope and making it expand.

15. 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

16. The Fate of Sunlike Stars The Sun’s Lifetime: 10 billion years on the main sequence Once the hydrogen is consumed, it will enter the red giant phase Helium burning begins, starting the yellow giant phase Once helium is consumed, core contracts and outer envelope expands, beginning the red supergiant phase Core begins to cool and the outer envelope expands again, forming a planetary nebula The core remains as a white dwarf

17. The Life-path of the Sun

18. Formation of Planetary Nebula 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

19. The Hourglass Nebula

20. 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.

21. Figure 64.05e

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

23. Mass Transfer and Novae 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

24. 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.

25. A Post-nova expansion

26. The Chandrasekhar Limit and Supernovae If mass is added to a white dwarf, its gravity increases If the white dwarf mass exceeds 1.4 solar masses (the Chandrasekhar Limit), the end of the white dwarf is near. The additional gravity squeezes the degenerate material in the white dwarf, causing it to compress by a small amount This compression causes the temperature to soar, and this allows carbon and oxygen to begin to fuse into silicon The energy released by this fusion blows the star apart in a Type 1a supernova

27. Type 1a Supernova – Another standard candle! The light output from a Type 1a supernova follows a very predictable curve Initial brightness increase followed by a slowly decaying “tail” All Type 1a supernova have similar peak luminosities, and so can be used to measure the distance to the clusters or galaxies that contain them!

28. Formation of Heavy Elements Hydrogen and a little helium were formed shortly after the Big Bang All other elements were formed inside stars! Low-mass stars create carbon and oxygen in their cores at the end of their lifespan, thanks to the higher temperatures and pressures present in a red giant star High-mass stars produce heavier elements like silicon, magnesium, etc., by nuclear fusion in their cores Temperatures are much higher Pressures are much greater Highest-mass elements (heavier than iron) must be created in supernovae, the death of high-mass stars

29. The Lifespan of a Massive Star

30. Layers of Fusion Reactions As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core Carbon is left behind until it too starts to fuse into heavier elements. A nested shell-like structure forms. Once iron forms in the core, the end is near…

31. Core Collapse of Massive Stars Iron cannot be fused into any heavier element, so it collects at the center of the star Gravity pulls the core of the star to a size smaller than the Earth’s diameter! The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “bounce” The “bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction. The star then explodes. This is a supernova!

32. Before and After – a Supernova