ASTR 1020 1st Edition Lecture 12Outline of Last Lecture I. Star BirthA. Cloud CompositionOutline of Current Lecture I. Star BirthA. Trapping of Thermal EnergyB. ProtostarC. Brown DwarfsD. Largest Stars: Radiation PressureCurrent LectureI. Star BirthA. Trapping of Thermal Energy- As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape - Thermal energy then begins to build up inside, increasing the internal pressure - Contraction slows down, and the center of the cloud fragment becomes a protostarB. Protostar- Growth- Matter from the cloud continues to fall onto the protostar until either the protostar or a neighboring star blows the surrounding gas away- Angular momentum of cloud leads to disks and jets. - Protostars have strong winds, which clear out an area around the star roughly the size of our solar system.- From Protostar to Main Sequence- Protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion- Contraction must continue until the core becomes hot enough for nuclear fusion- Contraction stops when the energy released by core fusion balances energy radiated from the surface—the star is now a main-sequence star- Assembly- Luminosity and temperature grow as matter collects into a protostar- Convective Contraction- Surface temperature near 3,000 K; energy transport by convection- Radiative Contraction- Luminosity nearly constant while radiation transports energy out- Self-Sustaining Fusion- Core temperature continues to rise until star arrives on the main sequence- Life Tracks for Different Masses- Models show that Sun required about 30 million years to go from protostar to main sequence- Higher-mass stars form faster - Lower-mass stars form more slowly- Smallest Stars: Fusion and Contraction- Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K. - Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation Depends on heat content The main form of pressure in most stars- Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in same placeParticles can’t be in same state in same place Doesn’t depend on heat contentC. Brown Dwarfs - Degeneracy pressure halts the contraction of objects with<0.08MSun before core temperature becomes hot enough for fusion- Starlike objects not massive enough to start fusion are brown dwarfs- A brown dwarf emits infrared light because of heat left over from contraction - Its luminosity gradually declines with time as it loses thermal energy- Brown Dwarfs in Orion- Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminousD. Largest Stars: Radiation Pressure- Photons exert a slight amount of pressure when they strike matter- Very massive stars are so luminous that the collective pressure of photons drivestheir matter into space- Upper Limit on a Star’s Mass - Models of stars suggest that radiation pressure limits how massive a star canbe without blowing itself apart - Observations have not found stars more massive than about 150MSun (Pistol Star)- Stars more massive than 150 MSun would blow apart- Stars less massive than 0.08 MSun can’t sustain fusion- Observations of star clusters show that star formation makes many more low-mass stars than high-mass