Chapter 16: Star Birth

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  1. Star-Forming Clouds
    stars form in dark clouds of dusty gas in interstellar space
  2. Interstellar Medium
    the clouds of gas between the stars
  3. Composition of Clouds
    we can determine the composition of interstellar gas from its absorption lines in the spectra of stars

    70% H, 28% He, 2% heavier elements (metals) in our region of the Milky Way
  4. Molecular Clouds
    • most of the matter in star-forming clouds is in the form of molecules
    • they have a temperature of 10-30 K and a density of about 300 molecules per cubic cm
    • most of what we know about molecular clouds comes from observing the emission lines of carbon monoxide
  5. Interstellar Dust
    • tiny solid particles of interstellar dust block our view of stars on the other side of a cloud
    • particles are <1 micrometer in size and are made of elements like C, O, Si, and Fe
  6. Interstellar Reddening
    • stars viewed through the edges of the cloud look redder because dust blocks (shorter-wavelength) blue light more effectively than (longer-wavelength) red light
    • long-wavelength infrared light passes through a cloud more easily than visible light
    • observations of infrared light reveal stars on the other side of the cloud
  7. Observing Newborn Stars
    • visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed
    • observing the infrared light from a cloud can reveal the newborn star embedded inside it
  8. Glowing Dust Grains
    • dust grains that absorb visible light heat up and emit infrared light of even longer wavelength
    • long-wavelength infrared light is brightest from regions where many stars are currently forming
  9. Gravity vs. Pressure
    • gravity can create stars only if it can overcome the force of thermal pressure in a cloud
    • emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons
  10. Mass of a Star-Forming Cloud
    • a typical molecular cloud (T ~ 30 K, n ~ 300 particles/cm³) must contain at least a few hundred solar masses for gravity to overcome pressure
    • emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud
  11. Resistance to Gravity
    • a cloud must have even mass to begin contracting if there are additional forces opposing gravity
    • both magnetic fields and turbulent gas motions increase resistance to gravity
  12. Fragmentation of a Cloud
    • gravity within a contracting gas cloud becomes stronger as the gas becomes denser
    • gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star
    • this simulation begins with a turbulent cloud containing 50 solar masses of gas
    • the random motions of different sections of the cloud cause it to become lumpy
    • each lump of the cloud in which gravity can overcome pressure can go on to become a star
    • a large cloud can make a whole cluster of stars
  13. Isolated Star Formation
    Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense
  14. The First Stars
    • Elements like carbon and oxygen had not yet been made when the first stars formed
    • without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today's molecular clouds
    • the first stars must therefore have been more massive than most of today's stars, for gravity to overcome pressure
  15. Simulation of the First Star
    simulations of early star formation suggest the first molecular clouds never cooled bellow 100 K, making stars of ~ 100Msun
  16. 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 protostar
  17. Growth of a Protostar
    matter from the cloud continues to fall onto the protostar until either the protostar or a neighbouring star blows the surrounding gas away
  18. How does a cloud's rotation affect star birth?
    • Evidence from the Solar System: the nebular theory of solar system formation illustrates the importance of rotation
    • Conservation of Angular Momentum: the rotation speed of the cloud from which a star forms increases as the cloud contracts. rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms
    • Flattening: collisions between particles in the cloud cause it to flatten into a disk. collisions between gas particles in cloud gradually reduce random motions. collisions between gas and particles also reduce up and down motions. spinning clouds flatten as it shrinks.
    • Formation of Jets: rotation also causes jets of matter to shoot out along the rotation axis. jets are observed coming from the centers of disks around protostars.
  19. From Protostar to Main Sequence
    • protostar looks star-like after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion
    • contraction must continue until the comes becomes hot enough for nuclear fusion
    • contraction stops when the energy released by the core fusion balances energy radiated from the surface- the star is now a main-sequence star
  20. Birth Stages on a Life Track
    life track illustrates star's surface temperature and luminosity at different moments in time
  21. Assembly of a Protostar
    luminosity and temperature grow as matter collects into a protostar
  22. Convection Contraction
    surface temperature remains near 3 000 K while convection is main energy transport mechanism
  23. Radiative Contraction
    luminosity remains nearly constant during late stages of contraction while radiation is transporting energy through star
  24. Self-Sustaining Fusion
    core temperature continues to rise until star arrives on the main sequence
  25. Tracks for Different Masses
    • models show that the sun required ~ 30 million years to go from a protostar to the main sequence
    • higher-mass stars form faster than lower-mass stars
  26. Fusion and Contraction
    • fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10
    • thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation
  27. Degeneracy Pressure
    • particles (electrons) can't be in the same state in the same place. laws of quantum mechanics prohibit it
    • doesn't depend on heat content
  28. Thermal Pressure
    • depends on heat content
    • the main form of pressure in most stars
  29. Brown Dwarfs
    • star-like objects not massive enough to start fusion
    • emits infrared light because of heat left over from contraction
    • loses thermal energy -> luminosity gradually declines with time
    • degeneracy pressure halts the contraction of objects with < 0.08Msun before core temperature becomes hot enough for fusion

    In Orion, infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous
  30. 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 drives their matter into space
  31. Upper Limit on a Star's Mass
    • models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart
    • observations have not found stars more massive than about 150Msun. 

    • stars >150Msun would blow apart
    • stars < 0.08Msun can't sustain fusion
  32. Demographics of Stars
    observations of star clusters show that star formation makes many more low-mass stars than high-mass stars
Card Set:
Chapter 16: Star Birth
2016-10-22 00:04:22
Lecture 9
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