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The Birth, Life and Death of Stars Prasad 1 U6_StarLife.

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Presentation on theme: "The Birth, Life and Death of Stars Prasad 1 U6_StarLife."— Presentation transcript:

1 The Birth, Life and Death of Stars Prasad 1 U6_StarLife

2 How can we learn about the lives of stars when little changes except on timescales much longer than all of human history? Suppose you had never seen a tree before, and you were given one minute in a forest to determine the life cycle of trees. Could you piece together the story without ever seeing a tree grow? This is about the equivalent of a human lifetime to the lifetime of the Sun. Prasad 2 U6_StarLife

3 Stellar “Forest” Prasad 3 U6_StarLife

4 Molecular cloud Cool molecular clouds gravitationally collapse to form clusters of stars Stars generate helium, carbon and iron through stellar nucleosynthesis The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion. New (dirty) molecular clouds are left behind by the supernova debris. The Stellar Cycle Prasad 4 U6_StarLife

5 Star Birth Cold gas clouds contract and form groups of stars. When O and B stars begin to shine, surrounding gas is ionized The stars in a cluster are all about the same age. Prasad5U6_StarLife

6 Cloud Collapses to Form Stars Radiation from protostars arises from the conversion of gravitational energy to heat. 6

7 Pre-Main Sequence Contraction Protostars contract until core reaches H  He fusion temperature. Low mass protostars contract more slowly. Nature makes more low-mass stars than high- mass stars. Prasad7U6_StarLife

8 Hydrogen fuel Hydrogen burning core Helium “ash” Anatomy of a Main Sequence Star shell Prasad 8U6_StarLife

9 Up the red giant branch As hydrogen in the core is being used up, it starts to contract, raising temperature in the surrounding. Eventually, hydrogen will burn only in a shell. There is less gravity from above to balance this pressure. The Sun will then swell to enormous size and luminosity, and its surface temperature will drop,  a red giant. Sun today Sun in ~5 Gyr Prasad 9U6_StarLife

10 Helium fusion at the center of a giant While the exterior layers expand, the helium core continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon While the exterior layers expand, the helium core continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion. Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion. For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion. For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion. In very massive stars, elements can be fused into Fe. In very massive stars, elements can be fused into Fe. 10U6_StarLife

11 The Sun will expand and cool again, becoming a red giant. Earth will be engulfed and vaporized within the Sun. The Sun’s core will consist mostly of carbon. Red Giants create most of the Carbon in the universe (from which organic molecules—and life—are made) Prasad 11U6_StarLife

12 H, He, C burning Since fusing atomic nuclei repel each other because of their electric charge, the order of easiest to hardest to fuse must be (1) H, He, C (2) C, He, H (3) H, C, He (4) He, C, H Carbon-triple alpha process Prasad 12U6_StarLife

13 The Sun’s Path Prasad13U6_StarLife

14 Planetary Nebula Formation When the Red Giant exhausts its He fuel –the C core collapses  white dwarf –No fusion going on inside … this is a dead star. He & H burning shells overcome gravity –the outer envelope of the star is blown outward  a planetary nebula Prasad14U6_StarLife

15 What holds the white dwarf from collapsing? As matter compresses, it becomes denser. Eventually, the electrons are forced to be too close together. A quantum mechanical law called the Pauli Exclusion Principle restricts electrons from being in the same state (i.e., keeps them from being too close together). Indistinguishable particles are not allowed to stay in the same quantum state. Prasad15U6_StarLife

16 What holds the white dwarf from collapsing? The resulting outward pressure which keeps the electrons apart is called electron degeneracy pressure – this is what balances the weight. Only if more energy drives the electrons into higher energy states, can the density increase. Adding mass can drive electrons to higher energies so star shrinks. At 1.4 solar masses—the Chandrasekhar Limit— a star with no other support will collapse, which will rapidly heat carbon to fusion temperature. Prasad16U6_StarLife

17 1 teaspoon = 1 elephant WD has a size slightly less than that of the earth. It is so dense, one teaspoon weights 15 tons! WD from an isolated star will simply cool, temperature dropping until it is no longer visible and becomes a “black dwarf”. Prasad17U6_StarLife

18 Sun’s life Prasad18U6_StarLife

19 What is a planetary nebula? (1)A large swarm of planets surrounding a star. (2)A disk of gas and dust around a young star. (3)Glowing gas in Earth’s upper atmosphere. (4)Ionized gas around a white dwarf star. Prasad19U6_StarLife

20 The lead-up to disaster In massive stars (M > 8 M sun ), elements can be fused into Fe. Iron cores do not immediately collapse due to electron degeneracy pressure. If the density continues to rise, eventually the electrons are forced to combine with the protons – resulting in neutrons. Now the electron degeneracy pressure disappears. What comes next … is core collapse. 20

21 The core implodes, but no fuel there, so it collapses until neutron degeneracy pressure kicks in. Core “bounces” when it hits neutron limit; huge neutrino release; unspent fuel outside core fuses… Outer parts of star are blasted outward. A tiny “neutron star” or a black hole remains at the center. Supernova! Type II (Core-Collapse) Prasad21U6_StarLife

22 Supernova 1987a before/after Prasad22U6_StarLife

23 Production of Heavy Elements (There is evidence that the universe began with nothing but hydrogen and helium.) To make elements heavier than iron extra energy must be provided. Supernova temperatures drive nuclei into each other at such high speeds that heavy elements can be made. Gold, Silver, etc., -- any element heavier than iron, were all made during a supernova. We were all once fuel for a stellar furnace. Parts of us were formed in a supernova! Prasad23U6_StarLife

24 All of the Heavy Elements are Made During Supernovae Prasad24U6_StarLife

25 Prasad25U6_StarLife

26 Life of a 15 solar mass star Prasad26U6_StarLife

27 Stellar Evolution in a Nutshell Mass controls the evolution of a star! 0.5 M Sun < M < 8 M Sun M > 8 M Sun M core < 3M Sun M core > 3M Sun Prasad27U6_StarLife

28 The H-R diagram 1.Which of these star is the hottest? 2.What are Sun-like stars (0.5 M sun < M < 8 M sun ) in common? 3.What about red dwarfs (0.08 M sun < M < 0.5 M sun ) ? 4.Where do stars spend most of their time? 5.Which is the faintest? the sun, an O star, a white dwarf, or a red giant? O Stars with M < 0.08 M sun  Brown dwarf (fusion never starts) Answers: 1. O star, 2. end as a WD, 3. no RG phase, lifetime longer than the age of the Universe, 4. MS, 5. WD Prasad28U6_StarLife

29 The evolution of 10,000 stars Prasad29U6_StarLife

30 If we came back in 10 billion years, the Sun will have a remaining mass about half of its current mass. Where did the other half go? It was lost in a supernova explosion It flows outward in a planetary nebula It is converted into energy by nuclear fusion The core of the Sun gravitationally collapses, absorbing the mass Prasad30U6_StarLife

31 A star cluster containing _____ would be MOST likely to be a few billion years old. (1)luminous red stars (2)hot ionized gas (3)infrared sources inside dark clouds (4)luminous blue stars Prasad31U6_StarLife


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