1. Stellar Evolution: The Live and Death of a Star Star ch. 20

2. Standards Understand the scale and contents of the universe, including stars Describe how stars are powered by fusion, how luminosity and temperature indicate their age, and how stellar processes create heavier and stable elements that are found throughout the universe.

3. As a star begins to run out of fuel & die, its properties change greatly. They travel evolutionary paths that take them far from the main sequence. Their ultimate fate depends on their mass.

4. Leaving the Main Sequence main sequenceMost stars spend most of their life on the main sequence. M The coolest M – type stars burn so slowly not one has yet left the main sequence. OB The most massive O & B – type stars evolve from main sequence after only a few tens of millions of years  Most high mass stars that ever existed perished long ago

5. Structural Change hydrogen gravitypressure leavesAs hydrogen is consumed, balance between gravity and pressure begins to shift, both internal structure and outward appearance begin to change, and the star leaves the main sequence. massThe end of a star’s life depends critically on its mass. gently Low mass stars die gently catastrophically High mass stars die catastrophically The dividing line between the two is about 8 times the mass of the sun

6. Evolution of a Sun-like Star changesA solar mass star does not experience sudden, large-scale changes in properties. temperature luminosity Its average surface temperature remains constant, while luminosity increases very slowly over time 10 billion sun-like After about 10 billion years of steady core hydrogen burning, a sun-like star begins to run out of fuel (like a car cruising down the highway at a constant 70 mph for many hours, only to have engine suddenly cough & sputter as the gas gauge reaches empty).

7. The Sub-Giant Branch Composition of the star’s interior changes: helium It has increased helium and decreased hydrogen. center The helium content increases fastest in the center depleted When hydrogen becomes depleted in the center fusion moves to higher layers in the core

8. The Sub-Giant Branch helium An inner core of non-burning helium starts to grow pressure contract The gas pressure weakens in the helium core and gravity causes the inner core to begin to contract

9. Hydrogen Shell-Burning Stage shell ashHydrogen burns at a furious rate in a shell surrounding the non-burning inner core of helium “ash”

10. Hydrogen Shell-Burning Stage fasterThe hydrogen shell generates energy faster than the original main sequence fusion, & energy production continues to increase as the helium core continues to shrink brighterThe star’s response is to get brighter

11. Hydrogen Shell-Burning Stage temperature luminosityAfter a lengthy stay on the main sequence, the star’s temperature and luminosity begin to change subgiantThe star evolves to the right on the H-R diagram to the subgiant branch

12. The Red Giant Branch The star is now far from the main sequence and no longer in stable equilibrium unbalanced shrinking The helium core is unbalanced and shrinking increased The rest of the core is unbalanced & fusing at an increased rate

13. The Red Giant Branch expanding cooling Gas pressure exerted by enhanced hydrogen burning forces star’s non- burning outer layers to increase in radius, and the overlying layers are expanding and cooling red giant Star is on its way to becoming a red giant 100 This change takes around 100 million years The red giant has a luminosity many hundreds of times the luminosity of the sun and its radius is around 100 solar radii

14. Helium Fusion heliumA few hundred million years after a solar- mass star leaves the main sequence helium begins to burn in the core carbonThe helium fuses into carbon and the central fires reignite

15. Helium Flash At the highest densities in the core, gas enters a new state of matter governed by the laws of quantum mechanics (deals with behavior of matter on subatomic scales) In this state, the Pauli exclusion principle prohibits electrons in the core from being squeezed too close together, known as electron degeneracy The pressure associated with the contact of electrons is called electron degeneracy pressure

16. Helium Flash explosiveIn the core’s degenerate state, helium burning becomes unstable with explosive consequences expansion stabilization When burning starts and temperature increases, there is no corresponding rise in pressure, no expansion of gas & no stabilization of core helium flash The rapid temperature rise results in runaway explosion called the helium flash The helium burns ferociously for a few hours, then equilibrium is eventually reached and stable core fuses helium into carbon

17. Back to the Giant Branch consumedWhatever helium exists in the core is rapidly consumed (lasts a few tens of millions of years after helium flash) As helium fuses to form carbon, a new carbon-rich inner core forms, surrounded by helium burning, hydrogen burning and non-burning shells giantsupergiantThe non-burning layer expands and star becomes red giant or red supergiant

18. Core of Carbon Ash

19. Death of a Low-Mass Star The inner carbon core becomes too cool for further nuclear burning and continues to contract The fires go out carbon Before the core attains the temperature necessary to fuse carbon, its density reaches a point where core can no longer be compressed 1000 At this density, a cubic centimeter of core matter would weigh 1000 kg on Earth: a ton of matter compressed into a volume the size of a grape

20. Planetary Nebulae radiation instabilitiesDriven by increasing radiation and instabilities in the core and outer layers, all of the star’s outer envelope is ejected into space in less than a few million years at a speed of a few 10’s of km/s

21. Planetary Nebulae carbon ash dustgas solar systemThe star now has two distinct parts: a core of carbon ash (a.k.a. white dwarf) and an expanding cloud of dust and cool gas spread over a volume roughly the size of our solar system planetary nebula This is a planetary nebula (they have no association with planets)

22. Planetary Nebulae It continues to spread out over time, and eventually disperses into interstellar space, enriching it with atoms of helium, carbon, oxygen & heavier elements These elements eventually get swept up into nebulae (see ch. 18) and formed into new stars and planets

25. White and Black Dwarfs planetary nebulaThe carbon core at the center of the planetary nebula continues to evolve Earth The core is very small, size of Earth or smaller half Its mass is about half the mass of the sun heat It shines by stored heat, not nuclear reactions white dwarf The core’s temperature & size give it the name of white dwarf

26. White and Black Dwarfs Once a star becomes a white dwarf, its evolution is over black dwarf It eventually becomes a black dwarf – a cold, dense, burned-out ember in space that remains about the size of Earth

27. Evolution of Stars More Massive than the Sun fasterHigh-mass stars evolve much faster than low-mass stars. fuel Its ravenous fuel consumption shortens its main sequence lifetime. A solar mass star spends 10 billion years on the main sequence 100A 5 solar mass B-type star is on main sequence for about a 100 million years 20A 10 solar mass O-type star is on main sequence for about 20 million years

28. Evolution of Stars More Massive than the Sun At 8 solar masses and larger, stars can fuse carbon, oxygen and even heavier elements. These stars die in violent explosions soon after leaving main sequence (next chapter!!!)