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Lecture 22

Lecture 22. The Death of Stars. RIP. Announcements. Tonight is the last regular Lab. A signup sheet will be posted next to the door for the make-up lab next week. Please indicate which labs you are missing so that I can decide how to do the make-up. The Main Sequence. -5 -3 -1 1 3 5 7

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Lecture 22

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  1. Lecture 22 The Death of Stars RIP

  2. Announcements • Tonight is the last regular Lab. A signup sheet will be posted next to the door for the make-up lab next week. Please indicate which labs you are missing so that I can decide how to do the make-up.

  3. The Main Sequence -5 -3 -1 1 3 5 7 9 On the HR diagram, the sun starts here. 40,000 20,000 10,000 5,000 2,500

  4. Early Red Giant -5 -3 -1 1 3 5 7 9 By the time the sun first becomes a red giant, it is now here on the diagram (in the region for giants). 40,000 20,000 10,000 5,000 2,500

  5. Just Before The Helium Flash By the time the sun reaches the helium flash, it is here on the diagram. -5 -3 -1 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500

  6. The Red Giant Branch -5 -3 -1 1 3 5 7 9 This path stars follow as they become red giants is often called the giant branch of the HR diagram. 40,000 20,000 10,000 5,000 2,500

  7. A Helium-Burning Star -5 -3 -1 1 3 5 7 9 After the helium flash, the sun becomes, smaller, warmer, and dimmer than before. 40,000 20,000 10,000 5,000 2,500

  8. The Horizontal Branch Once a solar-type star begins helium burning, it ends up somewhere along this horizontal line on the HR diagram. -5 -3 -1 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500

  9. The Horizontal Branch For this reason, helium-burning solar-type stars are called horizontal branch stars. -5 -3 -1 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500

  10. The Horizontal Branch • The core helium burning phase is sometimes called “the second main sequence” because of similarities to the hydrogen burning phase: • Energy is again produced in the core (but using a different fuel). • Pressure-Temperature thermostat is very effective again: star’s size/temperature stays very stable.

  11. The End Of The Reprieve • Important Differences: • Helium burning doesn’t last as long. • Helium fusion is not as efficient as hydrogen fusion: produces less energy per kg of nuclear fuel. • Sun is still 40 times brighter than today. • Starts to run out of helium in only about 250 million years.

  12. The Second Ascension • As helium fuel runs out: • Carbon core starts shrinking. • Helium burning begins in shell around carbon core. • Hydrogen burning begins in shell around helium shell. • The star is swelling into a red giant again! Called the second ascension.

  13. About How Big Will Our Sun Get? • This phase is the largest and brightest our sun will ever get. Here’s original size for comparison. L = 4,800 T = 3,000 K R = 260

  14. The Death Of Earth? • During this phase, the sun will swallow the Earth. • Probably won’t make it out to Mars.

  15. The Second Giant Branch -5 -3 -1 1 3 5 7 9 There are two giant branches on the HR diagram, side by side. 40,000 20,000 10,000 5,000 2,500

  16. The Second Giant Branch The second one is called the asymptotic giant branch. -5 -3 -1 1 3 5 7 9 So stars in their second ascension are often called AGB stars. 40,000 20,000 10,000 5,000 2,500

  17. AGB Giants • Very large, luminous, and red. • R > 200 • L = 5,000-10,000 • T ~ 3,000 K • Energy source is helium and hydrogen shell fusion. • Star has inert C, N, O core.

  18. AGB Giants • AGB Giants experience significant mass loss. • Gravity too low to hold onto distended outer layers. • Dust forms in cool outer layers; “reflects” core light, helping to push outer layers out into space. • Lose up to 1 solar mass every 100,000 years.

  19. Mass Loss In Giant Stars • Giant stars have strong stellar winds and weak surface gravity. • During the giant phase, these winds carry off a large percentage of the star’s mass.

  20. By The End Of The Giant Phase… • Up to half or more of the star’s gasses can end up as a nebula around the giant star.

  21. The Planetary Nebula • Forms in two stages: • Early in AGB stage, mass loss occurs in the form of a slow cool wind. Forms an expanding shell of gas around the star. • After expulsion of outer layers, core is exposed to space.

  22. The Planetary Nebula • Hot, fast stellar wind from core slams into cool expanding shell. • Gas glows by emission. Result is a planetary nebula.

  23. Planetary Nebulae • Mass loss not necessarily symmetric: • Cold shell may be less dense at poles. • Easier for hot wind to get through the poles. • Results in an asymmetric nebula (like an hourglass).

  24. Planetary Nebulae • Planetary Nebulae are very short lived: • Expansion of nebula rapidly cools gases. • Emission fades, nebula becomes too dim to observe after a few 10,000’s of years.

  25. The Final Collapse • Core finishes consuming all nuclear fuel. • Gravity wins! • Core collapses until electron degeneracy prevents further contraction. • What’s left of star is now about the size of Earth, but very, very hot: a white dwarf star.

  26. White Dwarf Stars • No nuclear fusion. Star is “dead.” • Electron degeneracy (From Quantum Mechanics) provides the pressure that prevents gravity from collapsing the star. • Pauli Exclusion Principle: • No two electrons can be in the same place at the same time, doing the same thing. • Electrons can exert a powerful outward pressure to keep from getting too close together!

  27. White Dwarf Stars • Heat is left over from energy released during gravitational collapse. • Star starts out very hot: 100,000 K! • No way to replace heat radiated into space. Star slowly cools down over billions of years. • End stage is black dwarf – but none have formed yet!

  28. The Structure Of A White Dwarf • Mostly a sphere of C, N, and O that is completely electron degenerate. • Atmosphere of hydrogen and helium. • Carbon center may crystallize to form a giant diamond!

  29. Daily Quiz 22 – Question 1 What prevents gravity from shrinking a white dwarf to a smaller size? • Helium core fusion. • Helium shell fusion. • Hydrogen core fusion • Degenerate electrons (electromagnetic force).

  30. White Dwarf Sizes • Higher mass results in smaller, denser white dwarf. • Upper mass limit of 1.44 solar masses. • Called the Chandrasekar limit. • Above this mass, gravity overcomes electron degeneracy. • The white dwarf collapses!

  31. White Dwarf Sizes

  32. Novae! • Occur in binary systems. • One star is “normal” (often a giant or supergiant). • Other star is a white dwarf.

  33. Novae! • Companion star loses mass to the white dwarf. • Forms an accretion disk that deposits hydrogen onto the dwarf’s surface. • Hydrogen crushed to degeneracy. • Pressure and temperature increase as more hydrogen is added. • “Kindling point” is reached, and …

  34. Novae! • Surface of dwarf is consumed in a thermonuclear explosion! • Light output jumps to 10,000’s to 100,000’s of times normal! • Hydrogen layer is ejected from white dwarf. • White dwarf is not “damaged” • Process begins again. • Most nova recur!

  35. And Now: Supernovae! A much bigger class of stellar explosion is called a Supernova

  36. Supernovae have two types: • Supernovae classed by spectrum: • Type I • Spectrum shows no hydrogen lines. • Some Type I SN’s just as bright as Type II: called Type Ib. • Remaining Type I SN’s soar to 4 billion times solar luminosity, then fade quickly: called Type Ia. • Type II • Spectrum shows hydrogen lines. • Caused by core collapse in massive star. Hydrogen lines from exploding outer layers of star.

  37. Type Ia Supernova • Some supernova are exploding white dwarfs. • How do you blow up a white dwarf? • Start with a star system similar to setup for a nova: • White dwarf drawing material from companion star.

  38. Blowing Up White Dwarfs • BUT white dwarf is very close to Chandrasekar Limit (1.44 solar masses). • Matter “stolen” from companion star drives mass above Chandrasekar Limit before a nova can occur.

  39. Blowing Up White Dwarfs • White dwarf collapses. Internal temperature reaches kindling point for Carbon before dwarf reaches neutron degeneracy. • Gas still electron degenerate – no pressure/temperature thermostat: • Runaway fusion – all carbon fused all at once! • Resulting thermonuclear explosion totally blasts the white dwarf apart! Result is a Type Ia Supernova!

  40. Daily Quiz 22 – Question 2 What can happen to the white dwarf in a close binary system when it accretes matter from the companion giant star? • The white dwarf can become a main sequence star once again. • The white dwarf can ignite the new matter and flare up as a nova. • The white dwarf can accrete too much matter and detonate as a supernova type Ia. • Either the white dwarf can ignite the new matter and flare up as a nova, or the white dwarf can accrete too much matter and detonate as a supernova type Ia.

  41. And Now: Type Ib and II Supernovae! The Times Listed Are For An M=25 Star

  42. The Supergiants • Core runs low on H fuel. Collapses and ignites He. • He burning creates C, N, and O. • Ignites H to He burning shell around core. • Star’s luminosity increases. Swells in size.

  43. Countdown to Disaster • After 7 million years: • H to He fusion in core ends. • He to C, N, O fusion in core begins. • H to He burning shell forms. • Star becomes supergiant.

  44. Countdown to Disaster • 500,000 years later: • He in core exhausted. • Core collapses, heats up to 800 million K. • C, N, O burning begins, producing Ne and Mg. • 600 years later: • Core C, N, O supply used up. • Core collapses, heats up to 1.5 billion K. • Ne and Mg burning begins, producing Si.

  45. Countdown to Disaster • Six months later: • Core supply of Ne and Mg used up. • Core collapses, heats to 3 billion K. • Si fusion begins, producing Fe. • Now there’s a problem! Remember, we can’t fuse iron into heavier elements and make energy!

  46. Countdown to Disaster • One day after Silicon fusion begins: • Si is running low in the core. • Heat/Pressure from Si fusion cannot support Fe core. • Fe core begins to collapse. Core heats up. • Fe cannot be fused into heavy elements (and still release energy)!

  47. Countdown to Disaster • Only milliseconds to go: • Temperature in Fe core soars above 100 billion K! • Two nuclear reactions can occur at this temperature: • Neutronization – protons and electrons react to form neutrons. • Photodisintegration – photons hit Fe nuclei and shatter them into He nuclei!

  48. Countdown to Disaster • Both reactions require energy! Core rapidly cools down! • Loss of heat/pressure speeds up collapse! • Result is a catastrophic, runaway collapse of the Fe core!

  49. The Fuse is Lit! • 500 km Fe core collapses to 10 km across. • Reaches same density as nuclear matter. • Core collapse stops abruptly as core becomes unimaginably rigid. • Outer layers of star slam into now rigid core at extreme speeds. • Shockwave forms, rocketing outward through the star!

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