Allison Thompson Chapter 12

Chapter 12 Star Stuff

How do stars form?
-Stars form in dark clouds of dusty interstellar space.
-Gravity can create stars only if it can overcome the force of thermal pressure in a cloud.
-Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.
-A typical molecular cloud must contain at least  a few hundred solar masses for gravity to overcome pressure.
-The cloud can prevent a pressure build up by converting thermal energy into infrared and radio photons that Escape the cloud.
-This simulation begins with a turbulent cloud containing 50 solar
-The random motions of different sections of the cloud  causes it to be lumpy .
-Each lump of the cloud in which gravity can overcome pressure can to on to be a Star.
-A large cloud can make a whole cluster of stars.
-As stars begin to form dust grains that absorb visible light heat up and emit infrared light.
-Long wavelength infrared light is brightest from regions where many stars are currently forming.
-Solar system formation is a good example of Star birth.
-Cloud heats up as gravity causes it to contract due to conservation n energy. contraction can continue if thermal energy is radiated away.
-As gravity forces a cloud to become smaller it begins to spin faster and faster due to conservation of angular momentum.
-As gravity forces a cloud to become smaller it begins to spin faster and faster.
-Collision between gas particles in cloud gradually reduce random motions.
-cloud flattens as it spins causing it to shrink.
-Rotation also causes jets of matter to shoot out along the rotation axis.
-Jets are observed coming from the centers of disks around protostars.
-a protostar contracts and heats until the core temperature is sufficient for hydrogen fusion.
-Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface.
-It takes 30milliom years for a Star like the sun.
-Gravity causes gas clouds to shrink and fragment
-Core if shrinking cloud heats up.
When core gets hot enough fusion begins and stops the shrinking.
-New star achieves long lasting state of balance.

How massive are new born stars?
-A cluster of many stars can firm out of a single cloud
-very massive stars are rare
-Low mass stars are common.
-Photons exert a slight amount of pressure when They strike matter.
-Very massive stars are so luminous that the collective pressure of photons drive their matter into space.
-Model of stars suggest that Radiation pressure limits how massive a star can be without blowing itself apart.
-Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above.
-Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from it's surface through Radiation.
-Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place. (degeneracy  pressure)
-Thermal pressure depends on heat. The main form of pressure in most stars
-Degeneracy pressure particles can't be in the same state in same place.
-Brown dwarfs degeneracy pressure halts the contractions of objects before the core temperature becomes hot enough for fusion.
-A brown dwarf emits infrared light because if heat leftover.
-Infrared observations can reveal recently formed brown dwarfs because They are still relatively warm and luminous.
-Stars more massive than 300M would blow apart.

12.2

What are the life stages of low mass stars?
-A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.
-Observation of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.
-As the core contracts H begins fusion He in a shell around the core.
-Luminosity increases because the core thermostat is broken. the increasing fusion rate in the shell does not stop the core from contracting.
-Helium fusion does not begin right away because it requires higher temperature than hydrogen fusion larger charge leads to greater repulsion.
The fusion of two helium nuclei doesn't work so helium fusion must combine three He.
-the thermostat is broken in low mass red giant because degeneracy pressure supports the core.
-helium core fusion stars neither shrink nor grow because the core thermostat is temporarily fixed.
-Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.
-observation of star clusters agree with these model.
-helium core fusion stars are found in horizontal branch on the HR diagram.
-Combining models of stars of similar age but different mass helps us to age date star clusters.

How does a low mass star die?
-After core helium fusion stops He fuses into carbon in a shell around the carbon core an h fuses into he shekk around helium layer.
-Double shell fusion ends with a gust that ejects the H and He into space as a planetary nebula.
-The core left behind becomes a white dwarf.
-Fusion progresses no further in a low mass star because the core temperature never grows hot enough for fusion of heavier elements
-Degeneracy pressure supports the white dwarf against gravity.

12.3
What are the life stage of a high mass star?
-High mass main sequence stars fuse H to He at higher rate using carbon nitrogen and oxygen as catalysts.
-A greater core temperature enables H nuclei to over come greater repulsion.
-Later life stages of high mass stars are similar to those of low mass stars

-How do high mass stars make the elements necessary for life?
- high temperature allow helium to fuse with heavier elements.
-Core temp in stars allow fusion of elements as heavy as iron.
-Advance nuclear burning. proceeds in serious of nested shells
-Iron is a dead end for fusion because nuclear reactions involving iron do not release energy
-Higher abundance of elements with even number of protons

-How does a high mass star die
Iron builds up and the core collapses.
Neutrons collapse forming a neutron star

-How does a star's mass determine it's life story?
1. Main sequence H fuses to He in core
2. Red Giant: H fuse into He around He core.
3. Helium core burning: H fuse to C core He fuse into He in shell
4.Double Burning shell: Fuses both H and He in shells
5. Planetary Nebula leaves white dwarfs.
 - Core shirnks
-Core thermostat breaks
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