Transcript lecture21
The birth of a star Chapter 11 Questions to be addressed: 1. 2. 3. 4. Where are the birth places of stars? What are the main components of a protostar? When and how a new is born? What prevents a star from collapsing? How does a star form? • A cloud of hydrogen gas began to gravitationally collapse. • As more gas fell in, it’s potential energy was converted into thermal energy. • Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core. • Gas that continued to fall in helped to establish gravitational equilibrium with the pressure generated in the core. How can collapse occur? • No collapse if thermal pressure wins over gravity • When clouds too cold, pressure insufficient to balance gravity: collapse • During collapse (compression) temperature increases: gravitational energy converted into thermal energy The Stellar Cycle Cool molecular clouds gravitationally collapse to form clusters of stars New (dirty) molecular clouds are left behind by the supernova debris. Molecular cloud 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. Proto-stellar disk crucial: It is where planets form O Stellar Evolution in a Nutshell M < 8 MSun M > 8 MSun Mcore < 3MSun Mass controls the evolution of a star! Mcore > 3MSun A main sequence star is the one which is supported by hydrogen fusion From cloud to protostar: gravity is the key for the collapse Initial cloud with some rotation Cloud spins up as it collapse A protostar The structure of a protostar Dark band is the proto-stellar disk seen edge-on Herbig-Haro objects From a protostar to a true star • Gas is heated when it is compressed • The central part of a protostar is compressed the most, and when the temperature there reaches 10 million K, hot enough to ignite hydrogen fusion, the collapse is halted by the heated generated by the nuclear reaction • A new star is born, and its internal structure is stabilized, because the energy produced in the center matches the amount of radiation from the surface A main-sequence star can hold its structure for a very long time. Why? Thermal Pressure Gravitational Contraction 41H --> 4He + energy ( E = mc2 ) Two ways to do this fusion reaction: If M<1.1Mo: p-p chain If M>1.1 Mo: CNO cycle p-p cycle is a “direct way to fuse 4 H into 1 He CNO cycle needs the help of C, N and O (catalysts) C, N and O simply assist the reaction, but do not partecipate Final output is the same: 4 H fuse into 1 He Energy output of p-p cycle depends mildly on T: 10% Dt 46% De 50% of energy in 11% of mass Energy output of CNO has steep dependence on T: 10% Dt 340% De 50% of energy in 2% of mass In the Sun, about 500 million tons/sec are needed! Balance happens thanks to flow (transport) of radiation from center (hotter) to surface (colder) • Conduction, radiation, convection • Opacity is key to efficiency of radiation transport • p-p stars: radiative core, convective envelope • CNO stars: convective core, radiative envelope • Small stars (M<~0.4 Mo) all convective Pressure and Temperature of a Gas How does a star hold itself? This balance between weight and pressure is called hydrostatic equilibrium. The Sun's core, for example, has a temperature of about 16 million K. The Stellar Thermostat Outward thermal pressure of core is larger than inward gravitational pressure Core expands Nuclear fusion rate rises dramatically Contracting core heats up Core contracts Expanding core cools Nuclear fusion rate drops dramatically Outward thermal pressure of core drops (and becomes smaller than inward grav. pressure) Why is there a Main Sequence? • The Main Sequence is just a manifestation of the relationship between Mass and Luminosity: L ~ M3.5 • The more massive the star the larger its weight • The larger the weight, the larger the pressure • The larger the pressure, the higher the temperature • The higher the temperature, the more energetic the nuclear reaction • The more energetic the nuclear reactions, the more luminous the star • Also, the more energetic the nuclear reactions, the faster the rate at which fusion occurs • The faster the rate, the quicker the star burns its fuel, the shorter its life