Chapter 18

1. Chapter 18 Evolution from the Main Sequence Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

2. Chapter 18 Evolution from the Main Sequence During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium. Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

3. Chapter 18 Evolution from the Main Sequence Again to follow the post-main-sequence evolution of a star we will resort to the stage method. Not every star adheres to this sequence but it serves to describe the steps that many stars take Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

4. Chapter 18 Even while on the main sequence, Stage 7, the composition of a star’s core is changing. Eventually, as hydrogen in the core is consumed, the Star leaves the main sequence, Stage 8. Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly.Medium-mass stars struggle.High-mass stars go out with a bang! Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

5. Chapter 18 Evolution from the Main Sequence When the fuel in the core is used up the fusion ceases. The result is a contraction of the Star and the formation of a new fusion furnace in a shell around the helium core. This is Stage 8 Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

6. Chapter 18 Evolution from the Main Sequence Stage 9: The Red-Giant Branch. The now much larger surface of the furnace causes outer layers of the star to expand and cool. It is now a red giant, extending out beyond the orbit of Mercury. Despite its cooler surface temperature, its luminosity increases enormously due to its large size. Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

7. Chapter 18 Evolution from the Main Sequence Stage 10: Helium fusion. Once the core temperature has risen to 100,000,000 K, the helium in the core can fuse, through a three-alpha process: UVU Survey of Astronomy Astro1010-lee.com The 8Be nucleus is highly unstable, and will decay in about 10–12 sec unless an alpha particle fuses with it first. This is why high temperatures and densities are necessary. twlee2016@gmail.com

8. Chapter 18 Evolution from the Main Sequence The helium flash: The pressure within the helium core is almost totally due to “electron degeneracy” – two electrons cannot be in the same quantum state, so the core cannot contract beyond a certain point. This pressure is almost independent of temperature so when the helium starts fusing, the pressure cannot adjust and the core explodes completely disrupting the surrounding shell furnace. Helium begins to fuse extremely rapidly; within hours to days the enormous energy output is over, but the star is now on its way to White Dwarf, Stage 13 Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

9. Chapter 18 Evolution from the Main Sequence Stage 10. Disruption of the hydrogen furnace throws the star out of equilibrium and it starts to shrink, but it has much heat to dissipate from the Helium Flash. The result is the surface gets smaller as the surface temperature gets higher, causing movement across the graph toward the blue while maintaining nearly the same brightness. Astro1010-lee.com UVU Survey of Astronomy

10. Chapter 18 Evolution from the Main Sequence Stages 11 and 12 depend very much on the mass of the star. From .5 to 1.4 solar masses the transition from the horizontal branch White Dwarf goes smoothly. From 1.4 to about 5.5 solar masses they must shed the extras mass to get down to the Chandrasakar limit of 1.4 solar masses, then they can transition to White Dwarf. Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

11. Chapter 18 Evolution from the Main Sequence Horizontal Branch, Stage 10. The Helium Flash has disrupted the nuclear shell and the star is dying. There is much heat to be shed before the star can become a White Dwarf. The star shrinks by gravity but remains bright by the surface getting hotter. Stars over 1.4 Solar masses also must rid themselves of the extra mass. Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

12. Chapter 18 Evolution from the Main Sequence The Instability Strip, still Stage 10 .. As the dying star moves along the horizontal branch it encounters a region, discovered by Hertsprung, called the Instability Strip. The star becomes a variable star changing brightness slightly in a very few days. Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

13. Chapter 18 Evolution from the Main Sequence It was later shown that there are two families of Cephieds Astro1010-lee.com UVU Survey of Astronomy Cephieds Absolute Magnitude Henrietta Levitt discovered a direct relationship of Period to Luminosity of the Cephied Variables and the RR Lyra Variables RR Lyras Period twlee2016@gmail.com

14. Chapter 18 Evolution from the Main Sequence Born July 4, 1868 Lancaster, Massachusetts Died December 12, 1921 (aged 53) Cambridge, Massachusetts Institution: Harvard University Alma mater: Radcliffe College, Oberlin College Known for period-luminosity relationship of Cepheid stars . Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

15. Chapter 18 Evolution from the Main Sequence Some stars with more than about 5.5 solar masses have a different problem. The Helium flash becomes a permanent nuclear furnace. The Helium core fuses helium to carbon and the shield furnace continues to fuse Hydrogen to Helium and the star is now in a some what stable state. Many stars go into a new Red Giant condition for a period. This is the Asymptotic Giant Branch, Stage 11 Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

16. Chapter 18 In Class Quiz Upon leaving the Main Sequence on their way to death: Low-mass stars go quietly. Medium-mass stars struggle. High-mass stars go out with a _________! Astro1010-lee.com UVU Survey of Astronomy twlee2016@gmail.com

17. Chapter 18 Evolution from the Main Sequence Astro1010-lee.com End Chapter 18 UVU Survey of Astronomy twlee2016@gmail.com