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Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof

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Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof. Thomas H. Puzia Formation of a star Giant molecular clouds of dust and gas contract and divide. That collapse and fragmentation can be induced by compression due to shock waves. It can produce a chain reaction, forming many stars. Angular momentum (rotation) + magnetic fields halt collapse. As the collapse stops, the remaining material forms a protostellar disk and jet, which eventually becomes a protoplanetary disk out of which planets may form. Star collapse eventually ignites fusion and stellar radiation blows remaining gas far away. Star Formation Model of the collapse and fragmentation of a 500 solar mass cloud to produce a cluster containing more than 1250 stars and brown dwarfs (i.e., like Orion’s Trapezium Cluster). Protostellar Objects Some stars with strong disks show bipolar outflows or jets (so-called Herbig-Haro objects). Process of forming can be surprisingly violent. As in the case of Herbig-Haro object 34, some have jet protostars and massive flows. Carina Nebula Stellar “Births” The duration of the contraction phase depends on its protostar mass, but is relatively fast: t <107 yrs. The growing star undergoes a violent change with variations in brightness, activity of stellar winds and X-ray emission (so- called T-Tauri stage). The Sun went through such a stage at birth. The star eventually stabilizes on the main sequence. The initial position on the zero-age main sequence depends only on its total mass. The star is now an adult (like the Sun), and spends ~90%of its life on this sequence. If the object is too small, it never supports fusion and becomes a brown dwarf, simply cooling over time. Brown Dwarfs Stars that do not have enough 6 mass to reach TNUC = 10 K and reliably burn hydrogen are called brown dwarfs (BD). The BD has less than 8% of the mass of the Sun, and are fully convective. They have 500K <Tsup <3000K, and sizes of the order of RJupiter They are very faint; the first were only discovered a few years ago. They constitute the lower end of the stellar sequence, intermediate between stars and planets. However, the boundaries between stars, brown dwarfs and planets are not well defined. • The radii of main sequence dwarf stars, brown dwarfs, and giant planets are surprisingly similar ! ! ! ! Sun G M L T J Evolution of the Sun Evolution of the Sun in terms of radius and color change. ! Most of its life is spent on the main sequence with the current appearance. ! It will die as a 0.6 M! white dwarf. ! The rest of the mass is shed into the interstellar medium (0.4 M!), and may be used to form new generations of stars. Evolution of the HR Diagram The Hertzsprung-Russell diagram illustrates the different stages of evolution of stars. ! The evolution of a low-mass star like the Sun is: • main sequence (stable H core) • subgiant (contraction, H shell + He ‘ash’) • red giant (core He flash) • horizontal branch (stable He core) • asymptotic giant branch (contraction, H/He shells + CO ‘ash’ + s- process can create elements up to Bismuth83) • planetary nebula (surface instability) • white dwarf (contraction) Late evolution stars are often “puffy” and unstable, with spots, pulsations and extreme mass loss. Internal Structure on Main Sequence Different mass stars transport energy differently: • The energy transport mechanism of radiation is present in stars between 0.8 and 1.2 Solar masses • Stars less than 0.8 (Brown Dwarfs, YSO's, T-Tauri) have convecting cores • Stars more massive than 1.2 Solar masses show the pattern of energy transport of a massive star as above. Evolving Internal Structure Off Main Sequence Stars with lower mass than the Sun have distinct types of evolution: Main Sequence Red Giant Horizontal Branch Convective zone Radiative zone Fusion of H Convection zone Ash core of He Fusion of H Fusion of H Fusion of He Radiative Zone Convection Relative radii not to exact scale Which of these objects does NOT have fusion occurring in its core? A. a horizontal branch star B. a red main-sequence star C. a blue main-sequence star D. a white dwarf E. all have fusion in the core A star moves upwards and to the right on the HR diagram. What is probably happening in the core? A. It has just started to fuse a new element. B. All nuclear burning is slowing down. C. The inner core temperature is cooling. D. The inner core is collapsing and heating up, but no fusion yet. What will happen when there is no more helium to fuse in the core of a star? A. The core will cool down. B. Carbon fusion will start immediately. C. The star will explode. D. The core will start to collapse again. E. The hydrogen fusing shells will go out. Evolution of the HR Diagram The Hertzsprung-Russell diagram illustrates the different stages of evolution of stars. ! The evolution of a low-mass star like the Sun is: • main sequence (stable H core) • subgiant (contraction, H shell + He ‘ash’) • red giant (core He flash) • horizontal branch (stable He core) • asymptotic giant branch (contraction, H/He shells + CO ‘ash’ + s- process can create elements up to Bismuth83) • planetary nebula (surface instability) • white dwarf (contraction) Late evolution stars are often “puffy” and unstable, with spots, pulsations and extreme mass loss. Consequences of Late-Stage Evolution for Low-Mass Stars... The extended atmospheres of stars on the asymptotic giant branch are unstable, and the star begins to vary in size periodically. The pulsations lead to the expulsion of its outer layers and planetary nebulae are formed. The layers are gently released, with speeds v <100 km/s. They are NOT expelled explosively. The name planetary nebula that coined very early, and does NOT mean that they have planets. Consequences of Late-Stage Evolution for Low-Mass Stars... Planetary Nebulae + White Dwarf Planetary nebulae survive a few million years before the gaseous material is lost into the interstellar medium. Note: 1 km / s = 1 pc in 1 million years. Eventually, the bare nucleus is all that remains of the original star, which ends its life as a white dwarf. Most white dwarfs start at T = 10000 - 50000 K on the surface, are half the mass of the Sun, and a size similar to Earth (about 10,000 km in diameter). This implies that they are very dense: the interiors are made of degenerate gas, supported by the electron pressure. Because there is no internal power source white dwarfs slowly cool and contract, simply fading and reddening away to end their lives. There are no black dwarves yet, as age of Universe precludes these Ring WD Planetary Nebulae Hourglass Nebula Planetary Nebulae Hourglass Nebula Abell 39 Abell 39 White Dwarfs As they age, white dwarfs will contract and cool, becoming fainter and redder. Globular clusters (very old stellar populations) contain many white dwarfs, but these are quite old and have significantly contracted and cooled. Thus they are generally much fainter and redder than the Galactic disk white dwarfs in the Solar neighborhood. White Dwarfs in M15 Late-Stages for Massive Stars Si 28 He 4 The evolution of massive stars is different from that of the Sun. ! ! S 32 For example, a star 20 times more massive than the Sun ! evolves much more rapidly and violently. After burning H and He, the nucleus is so massive that fusion continue producing Ar 36 increasingly heavier elements. ! ! – Fusion of H to He produces energy (107 yrs), Ca 40 6 ! – Fusion of He to C produces energy (10 yrs), – Fusion of C to O produces energy (103 yrs), Ti 44 – Fusion of O to Ne produces energy (1 yrs), ! – Fusion of Ne to Mg produces energy (months), Cr 48 – Fusion of Mg to Si produces energy (weeks), ! – Fusion of Si to Fe produces energy (1 week). Fe 52 ! Ni 56 Fusion Shell Structure • The massive star is arranged in a layered structure with increasingly heavy elements towards the center, so-called onion structure. The H, which is lightest, floats to the surface, forming the outermost layer, while iron (Fe) which is the heaviest, sinks to the core. The core of the star reaches T = 1010 K. Consequences of Late-Stage Evolution for Massive Stars... ! Because they are unstable, they can also have violent nova (“pre- supernovas?”) Interacting Eta Carinae Binary Star Consequences of Late-Stage Evolution for Massive Stars... Supernova Explosion When Fe ashes accumulate in the nucleus, that Fe cannot produce energy by fusion. Therefore no more nuclear reactions and radiation pressure occurs from the core. Gravity easily overcomes the pressure. The star cannot bear its own mass and the interior of the star collapses...violently. This collapse generates an enormous amount of gravitational energy, causing a huge explosion called a supernova (SN). The heavy elements (He => Ni) formed by the different layers of the star are ejected by the explosion at speeds of ~10,000 km/s and returned to enrich the interstellar medium. Products of a SN 21 • Gigantic explosion (L > 1.000.000.000.000.000.000.000 L! = 10 L!) • Rapidly expanding remnant (v > 10.000 km/s); generates powerful shocks which shine at X-ray and radio wavelengths. • Explosion expels heavy elements (Fe, Ca, Na, Ni, O, C…) • Explosive nucleosynthesis makes more (Si, S, Cl, Ar, Na, K, Ca, Sc, Ti, V, Cr, Mn, Co, Fe, Ni) • R-process builds up elements heavier than Ni through rapid n-capture and radioactive decay • In some SNe, a neutron star (M > 1.4 M!) or black hole (M > 3 M!) is believed to be produced..
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