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1 The Birth, Life and Death of Stars Prasad 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 U6_StarLife 3 Stellar “Forest” Prasad U6_StarLife The Stellar Cycle New (dirty) molecular clouds are left behind by the supernova debris. Cool molecular clouds gravitationally collapse to form clusters of stars Molecular cloud Stars generate helium, carbon and iron through stellar nucleosynthesis Prasad 4 U6_StarLife The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion. 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. Prasad U6_StarLife 5 Cloud Collapses to Form Stars Radiation from protostars arises from the conversion of gravitational energy to heat. 6 Pre-Main Sequence Contraction • Protostars contract until core reaches HHe fusion temperature. • Low mass protostars contract more slowly. • Nature makes more low-mass stars than highmass stars. Prasad U6_StarLife 7 Anatomy of a Main Sequence Star Hydrogen burning core shell Hydrogen fuel Helium “ash” Prasad U6_StarLife 8 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 in ~5 Gyr Sun today Prasad U6_StarLife 9 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 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. In very massive stars, elements can be fused into Fe. U6_StarLife 10 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 U6_StarLife 11 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 Prasad Carbon-triple alpha process U6_StarLife 12 The Sun’s Path Prasad U6_StarLife 13 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 Prasad U6_StarLife 14 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). Prasad U6_StarLife Indistinguishable particles are not allowed to stay in the same quantum state. 15 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. Prasad U6_StarLife 16 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 1 teaspoon = 1 elephant Prasad visible and becomes U6_StarLife 17 longer a “black dwarf”. Sun’s life Prasad U6_StarLife 18 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. Prasad U6_StarLife 19 The lead-up to disaster • In massive stars (M > 8 Msun), 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 20 collapse. Supernova! Type II (Core-Collapse) • 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. Prasad U6_StarLife 21 Supernova 1987a before/after Prasad U6_StarLife 22 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. Prasad U6_StarLife Parts of us were formed in a supernova! 23 Prasad U6_StarLife 25 Life of a 15 solar mass star Prasad U6_StarLife 26 Stellar Evolution in a Nutshell 0.5 MSun < M < 8 MSun M > 8 MSun Mcore < 3MSun Prasad Mass controls the evolution of a star! U6_StarLife Mcore > 3MSun 27 The H-R diagram 1. Which of these star is the hottest? 2. What are Sun-like stars (0.5 Msun < M < 8 Msun) in common? 3. What about red dwarfs (0.08 Msun < M < 0.5 Msun) ? 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 Msun Brown dwarf (fusion never starts) Prasad 28 Answers: 1. OU6_StarLife star, 2. end as a WD, 3. no RG phase, lifetime longer than the age of the Universe, 4. MS, 5. WD The evolution of 10,000 stars Prasad U6_StarLife 29 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 Prasad U6_StarLife 30 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 Prasad U6_StarLife 31