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Life story of a star Micro-world Macro-world Lecture 20.

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Presentation on theme: "Life story of a star Micro-world Macro-world Lecture 20."— Presentation transcript:

1 Life story of a star Micro-world Macro-world Lecture 20

2 Our favorite star: The Sun R ๏ = 696,000 km R ๏ = 696,000 km (109 x R earth ) (109 x R earth ) M= 2x10 30 kg M= 2x10 30 kg ( 3x10 5 x M earth ) ( 3x10 5 x M earth ) Rotation period:Rotation period: 25 days(equator) 25 days(equator) 30 days (poles) 30 days (poles)Composition: 70% Hydrogen 70% Hydrogen 28% Helium 28% Helium

3 Life Cycle of Stars Recycling Supernovae produce - heavy elements - neutron stars - black holes Martin Rees - Our Cosmic Habitat

4 vertical scale L = Brightness 4 Main star types

5 Stars close to the Sun Sun

6 Solar fusion processes MeV MeV MeV

7 Neutrinos come directly from solar core

8 Superkamiokande

9 Sun as seen by a neutrino detector

10 What happens when the Hydrogen is all used up?

11 Evolution of a Star (Sun) Red Giant

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13 Main Sequence Evolution Core starts with same fraction of hydrogen as whole star Fusion changes H  He Core gradually shrinks and Sun gets hotter and more luminous

14 Evolution of the Sun Fusion changes H  He Core depletes of H Eventually there is not enough H to maintain energy generation in the core Core starts to collapse

15 The Sun will become a Red Giant

16 The Sun 5 Billion years from now Earth

17 The Sun Engulfs the Inner Planets

18 Red Giant Phase He core –No nuclear fusion –Gravitational contraction produces energy H layer –Nuclear fusion Envelope –Expands because of increased energy production –Cools because of increased surface area

19 Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon Helium fusion

20 Helium Flash He core –Eventually the core gets hot enough to fuse Helium into Carbon. –This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. –We don’t see this flash because it’s buried inside the Sun. H layer Envelope

21 Red Giant after Helium Ignition He burning core –Fusion burns He into C, O He rich core –No fusion H burning shell –Fusion burns H into He Envelope –Expands because of increased energy production

22 What happens when the star’s core runs out of helium? – The star explodes – Carbon fusion begins – The core starts cooling off – Helium fuses in a shell around the core

23 Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate

24 Sun looses mass via winds Creates a “planetary nebula” Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen  a “white dwarf star”

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26 Planetary nebula

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29 Hourglass nebula

30 White dwarf Star burns up rest of hydrogen Nothing remains but degenerate core of Oxygen and Carbon “White dwarf” cools but does not contract because core is degenerate No energy from fusion, no energy from gravitational contraction White dwarf slowly fades away…

31 Time line for Sun’s evolution

32 Sirius Comet Hale- Bop Orion Constellation ( Nebula) Brightest Star – Sirius A – (Sirius B is a white dwarf) Sirius B Betelgeuse (Red Giant)

33 1.This is a Hubble Space Telescope image - the first direct picture of the surface of a star other than the Sun. Hubble Space Telescope imagepicture 2. While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger. If placed at the center of our Solar System, it would extend past the orbit of Jupiter.Betelgeusecooler than the Sunour Solar System 3.Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familiar constellation of Orion, the Hunter.Betelgeusebrightest starsOrion, the Hunter 4.The name Betelgeuse is Arabic in origin. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova. In this historic image, a bright hotspot is revealed on the star's surface.The namesupernovaa bright hotspot Betelgeuse is a red supergiant star about 600 light years distant

34 What happens to stars more massive than the sun?

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36 The Sun Engulfs the Inner Planets

37 The Sun becomes a White Dwarf Composition: Carbon & Oxygen

38 What about more massive stars?

39 Nuclear burning continues past Helium 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core

40 Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells

41 Fusion stops at Iron

42 Fusion versus Fission

43 Advanced reactions in stars make elements like Si, S, Ca, Fe

44 Atomic collapse  Supernova Explosion Core pressure goes away because atoms collapse: electrons combine with protons, making neutrons and neutrinos Neutrons collapse to the center, forming a neutron star

45 Core collapse Iron core grows until it is too heavy to support itself Atoms in the core collapse, density increases, normal iron nuclei are converted into neutrons with the emission of neutrinos Core collapse stops, neutron star is formed Rest of the star collapses in on the core, but bounces off the new neutron star (also pushed outwards by the neutrinos)

46 Supernova explosion

47 SN1987A Tarantula Nebula in LMC Neutrinos are detected Feb 22, 1987 Feb 23, 1987

48 Previously observed Supernova “Kepler’s Supernova” Oct 8, 1604 Kepler’s Supernova today Chosun Silok

49 Light curve from Kepler’s Supernova

50 Where do the elements in your body come from? Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets Supernova produce all of the heavier elements – Elements up to Iron can be produced by fusion – Elements heavier than Iron are produced by the neutrons and neutrinos interacting with nuclei during the supernova explosion

51 How do high-mass stars make the elements necessary for life?

52 Advanced Nuclear Burning Core temperatures in stars with >8M Sun allow fusion of elements as heavy as iron

53 In 1987 a nearby supernova gave us a close-up look at the death of a massive star

54 54 core collapse supernova mechanism Fe core inner core pre SN star 1. infalling outer core outgoing shock from rebounce proto neutron star 2. infalling outer core proto neutron star stalled shock 3. revived shock proto neutron star matter flow gets reversed - explosion 4. neutrinos neutrino heated layer

55 55 Mass and composition of the core depends on the ZAMS mass and the previous burning stages: M 0 He burning C,O 8-12 M 0 C burning O,Ne,Mg > 8-12 M 0 Si burning Fe M ZAMS Last stageCore MM Ch collapse MassResult < 0.3 M 0 H burning He How can 8-12M 0 mass star get below Chandrasekhar limit ?


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