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The birth of a star Chapter 11

The birth of a star Chapter 11. Questions to be addressed:. 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?.

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The birth of a star Chapter 11

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  1. The birth of a starChapter 11 Questions to be addressed: • 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?

  2. 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.

  3. 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

  4. The hottest, mostmassive stars in thecluster supernova –heavier elements areformed in the explosion. The Stellar Cycle New (dirty) molecularclouds are leftbehind by thesupernova debris. Cool molecular cloudsgravitationally collapseto form clusters of stars Molecular cloud Stars generatehelium, carbonand iron throughstellar nucleosynthesis

  5. Proto-stellar disk crucial: It is where planets form

  6. O

  7. M < 8 MSun M > 8 MSun Mcore > 3MSun Mcore < 3MSun Mass controls the evolution of a star! Stellar Evolution in a Nutshell

  8. A main sequence star is the one which is supported by hydrogen fusion

  9. From cloud to protostar: gravity is the key for the collapse Initial cloud with some rotation Cloud spins up as it collapse A protostar

  10. The structure of a protostar Dark band is the proto-stellar disk seen edge-on Herbig-Haro objects

  11. 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

  12. A main-sequence star can hold its structure for a very long time. Why? GravitationalContraction ThermalPressure

  13. 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!

  14. Balance happens thanks toflow (transport) of radiationfrom 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

  15. Pressure and Temperature of a Gas

  16. 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.

  17. The Stellar Thermostat Outward thermal pressure of coreis larger than inward gravitational pressure Core expands Nuclear fusion raterises dramatically Expanding core cools Contracting core heats up Nuclear fusion ratedrops dramatically Core contracts Outward thermal pressureof core drops (and becomessmaller than inward grav. pressure)

  18. 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

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